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From Apache Wiki <wikidi...@apache.org>
Subject [Httpcomponents Wiki] Update of "HttpClientTutorial" by OlegKalnichevski
Date Wed, 08 Jul 2009 21:48:01 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Httpcomponents Wiki" for change notification.

The following page has been changed by OlegKalnichevski:
http://wiki.apache.org/HttpComponents/HttpClientTutorial

------------------------------------------------------------------------------
- = Preface =
+ HttpClient tutorial moved to [http://hc.apache.org/httpcomponents-client/tutorial/html/]
  
- == HttpClient scope ==
-  
-  * Client-side HTTP transport library based on HttpCore
- 
-  * Based on classic (blocking) I/O
- 
-  * Content agnostic (no content processing of any kind)
- 
- == What HttpClient is NOT ==
- 
-  * HttpClient is NOT a browser. It is a client side HTTP transport library. HttpClient's purpose is to transmit and receive HTTP messages. HttpClient will not attempt to cache content, execute javascript embedded in HTML pages, try to guess content type, or reformat request / redirect location URIs.
- 
- = Fundamentals =
- 
- == Request execution ==
- 
-   The most essential function of HttpClient is to execute HTTP methods. Execution of an HTTP method involves one or several HTTP request / HTTP response exchanges, usually handled internally by HttpClient. The user is expected to provide a request object to execute and HttpClient is expected to transmit the request to the target server return a corresponding response object, or throw an exception if execution was unsuccessful.
-   
-   Quite naturally, the main entry point of the HttpClient API is the HttpClient interface that defines the contract described above.
- 
-   Here is an example of request execution process in its simplest form:
- 
- {{{
- HttpClient httpclient = new DefaultHttpClient();
- HttpGet httpget = new HttpGet("http://localhost/");
- HttpResponse response = httpclient.execute(httpget);
- HttpEntity entity = response.getEntity();
- if (entity != null) {
-     InputStream instream = entity.getContent();
-     int l;
-     byte[] tmp = new byte[2048];
-     while ((l = instream.read(tmp)) != -1) {
-     }
- }
- }}}
-   
- === HTTP request ===
- 
-   All HTTP requests have a request line consisting a method name, a request URI and a HTTP protocol version. 
-   
-   HttpClient supports out of the box all HTTP methods defined in the HTTP/1.1 specification: GET, HEAD, POST, PUT, DELETE, TRACE and OPTIONS. There is a special class for each method type.: HttpGet, HttpHead, HttpPost, HttpPut, HttpDelete, HttpTrace, and HttpOptions.
-   
-   The Request-URI is a Uniform Resource Identifier that identifies the resource upon which to apply the request. HTTP request URIs consist of a protocol scheme, host name, optional port, resource path, optional query, and optional fragment. 
-   
- {{{
- HttpGet httpget = new HttpGet("http://www.google.com/search?hl=en&q=httpclient&btnG=Google+Search&aq=f&oq=");
- }}}
- 
-   HttpClient provides a number of utility methods to simplify creation and modification of request URIs.
- 
-   URI can be assembled programmatically:
-   
- {{{
- URI uri = URIUtils.createURI("http", "www.google.com", -1, "/search", 
-   "q=httpclient&btnG=Google+Search&aq=f&oq=", null);
- HttpGet httpget = new HttpGet(uri);
- System.out.println(httpget.getURI());
- }}}  
-   
- stdout>
- {{{
- http://www.google.com/search?q=httpclient&btnG=Google+Search&aq=f&oq=
- }}}
- 
-   Query string can also be generated from individual parameters: 
- 
- {{{
- List<NameValuePair> qparams = new ArrayList<NameValuePair>();
- qparams.add(new BasicNameValuePair("q", "httpclient"));
- qparams.add(new BasicNameValuePair("btnG", "Google Search"));
- qparams.add(new BasicNameValuePair("aq", "f"));
- qparams.add(new BasicNameValuePair("oq", null));
- URI uri = URIUtils.createURI("http", "www.google.com", -1, "/search", 
-         URLEncodedUtils.format(qparams, "UTF-8"), null);
- HttpGet httpget = new HttpGet(uri);
- System.out.println(httpget.getURI());
- }}}
- 
- stdout>
- {{{
- http://www.google.com/search?q=httpclient&btnG=Google+Search&aq=f&oq=
- }}}
- 
- === HTTP response ===
- 
-     HTTP response is a message sent by the server back to the client after having received and interpreted a request message. The first line of that message consists of the protocol version followed by a numeric status code and its associated textual phrase.
- 
- {{{
- HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, 
-     HttpStatus.SC_OK, "OK");
- 
- System.out.println(response.getProtocolVersion());
- System.out.println(response.getStatusLine().getStatusCode());
- System.out.println(response.getStatusLine().getReasonPhrase());
- System.out.println(response.getStatusLine().toString());
- }}}
- 
- stdout >
- 
- {{{
- HTTP/1.1
- 200
- OK
- HTTP/1.1 200 OK
- }}}
- 
- === Working with message headers ===
- 
-    An HTTP message can contain a number of headers describing properties of the message such as the content length, content type and so on. HttpClient provides methods to retrieve, add, remove and enumerate headers. 
- 
- {{{
- HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, 
-     HttpStatus.SC_OK, "OK");
- response.addHeader("Set-Cookie", 
-     "c1=a; path=/; domain=localhost");
- response.addHeader("Set-Cookie", 
-     "c2=b; path=\"/\", c3=c; domain=\"localhost\"");
- Header h1 = response.getFirstHeader("Set-Cookie");
- System.out.println(h1);
- Header h2 = response.getLastHeader("Set-Cookie");
- System.out.println(h2);
- Header[] hs = response.getHeaders("Set-Cookie");
- System.out.println(hs.length);
- }}}
- 
- stdout >
- 
- {{{
- Set-Cookie: c1=a; path=/; domain=localhost
- Set-Cookie: c2=b; path="/", c3=c; domain="localhost"
- 2
- }}}
- 
- The most efficient way to obtain all headers of a given type is by using the HeaderIterator interface.
- 
- {{{
- HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, 
-     HttpStatus.SC_OK, "OK");
- response.addHeader("Set-Cookie", 
-     "c1=a; path=/; domain=localhost");
- response.addHeader("Set-Cookie", 
-     "c2=b; path=\"/\", c3=c; domain=\"localhost\"");
- 
- HeaderIterator it = response.headerIterator("Set-Cookie");
- 
- while (it.hasNext()) {
-     System.out.println(it.next());
- }
- }}}
- 
- stdout >
- 
- {{{
- Set-Cookie: c1=a; path=/; domain=localhost
- Set-Cookie: c2=b; path="/", c3=c; domain="localhost"
- }}}
- 
-     It also provides convenience methods to parse HTTP messages into individual header elements.
- 
- {{{
- HttpResponse response = new BasicHttpResponse(HttpVersion.HTTP_1_1, 
-     HttpStatus.SC_OK, "OK");
- response.addHeader("Set-Cookie", 
-     "c1=a; path=/; domain=localhost");
- response.addHeader("Set-Cookie", 
-     "c2=b; path=\"/\", c3=c; domain=\"localhost\"");
- 
- HeaderElementIterator it = new BasicHeaderElementIterator(
-         response.headerIterator("Set-Cookie"));
- 
- while (it.hasNext()) {
-     HeaderElement elem = it.nextElement(); 
-     System.out.println(elem.getName() + " = " + elem.getValue());
-     NameValuePair[] params = elem.getParameters();
-     for (int i = 0; i < params.length; i++) {
-         System.out.println(" " + params[i]);
-     }
- }
- }}}
- 
- stdout >
- 
- {{{
- c1 = a
-  path=/
-  domain=localhost
- c2 = b
-  path=/
- c3 = c
-  domain=localhost
- }}}
-    
- === HTTP entity ===
-   
-   HTTP messages can carry a content entity associated with the request or response. Entities can be found in some requests and in some responses, as they are optional. Requests that use entities are referred to as entity enclosing requests. The HTTP specification defines two entity enclosing methods: POST and PUT. Responses are usually expected to enclose a content entity. There are exceptions to this rule such as responses to HEAD method and 204 No Content, 304 Not Modified, 205 Reset Content responses.
- 
- HttpClient distinguishes three kinds of entities, depending on where their content originates:
- 
-     * streamed:  The content is received from a stream, or generated on the fly. In particular, this category includes entities being received from HTTP responses. Streamed entities are generally not repeatable.
-     
-     * self-contained:  The content is in memory or obtained by means that are independent from a connection or other entity. Self-contained entities are generally repeatable. This type of entities will be mostly used for entity enclosing HTTP requests.
-     
-     * wrapping:  The content is obtained from another entity. 
- 
-    This distinction is important for connection management when streaming out content from an HTTP response. For request entities that are created by an application and only sent using HttpClient, the difference between streamed and self-contained is of little importance. In that case, it is suggested to consider non-repeatable entities as streamed, and those that are repeatable as self-contained. 
- 
- ==== Repeatable entities ====
- 
-     An entity can be repeatable, meaning its content can be read more than once. This is only possible with self contained entities (like ByteArrayEntity or StringEntity)
- 
- ==== Using HTTP entities ====
- 
-    Since an entity can represent both binary and character content, it has support for character encodings (to support the latter, ie. character content).
- 
-    The entity is created when executing a request with enclosed content or when the request was successful and the response body is used to send the result back to the client.
- 
-    To read the content from the entity, one can either retrieve the input stream via the HttpEntity#getContent() method, which returns an java.io.InputStream, or one can supply an output stream to the HttpEntity#writeTo(OutputStream) method, which will return once all content has been written to the given stream.
- 
-    When the entity has been received with an incoming message, the methods HttpEntity#getContentType() and HttpEntity#getContentLength() methods can be used for reading the common metadata such as Content-Type and Content-Length headers (if they are available). Since the Content-Type header can contain a character encoding for text mime-types like text/plain or text/html, the HttpEntity#getContentEncoding() method is used to read this information. If the headers aren't available, a length of -1 will be returned, and NULL for the content type. If the Content-Type header is available, a Header object will be returned.
- 
-    When creating an entity for a outgoing message, this meta data has to be supplied by the creator of the entity.
- 
- {{{
- StringEntity myEntity = new StringEntity("important message", 
-     "UTF-8");
- 
- System.out.println(myEntity.getContentType());
- System.out.println(myEntity.getContentLength());
- System.out.println(EntityUtils.getContentCharSet(myEntity));
- System.out.println(EntityUtils.toString(myEntity));
- System.out.println(EntityUtils.toByteArray(myEntity).length);
- }}}
- 
- stdout >
- 
- {{{
- Content-Type: text/plain; charset=UTF-8
- 17
- UTF-8
- important message
- 17
- }}}
- 
- === Ensuring release of low level resources ===
- 
-    When finished with a response entity, it's important to ensure that all entity content has been fully consumed, so that the connection could be safely returned to the connection pool and re-used by the connection manager for subsequent requests. The easiest way to do so is to call the HttpEntity#consumeContent() method to consume any available content on the stream. HttpClient will automatically release the underlying connection back to the connection manager as soon as it detects that the end of the content stream has been reached. The HttpEntity#consumeContent() method is safe to call more than once.
- 
-    There can be situations, however, when only a small portion of the entire response content needs to be retrieved and the performance penalty for consuming the remaining content and making the connection reusable is too high, one can simply terminate the request by calling HttpUriRequest#abort() method.
-    
- {{{
- HttpGet httpget = new HttpGet("http://localhost/");
- HttpResponse response = httpclient.execute(httpget);
- HttpEntity entity = response.getEntity();
- if (entity != null) {
-     InputStream instream = entity.getContent();
-     int byteOne = instream.read();
-     int byteTwo = instream.read();
-     // Do not need the rest
-     httpget.abort();
- }
- }}}
- 
-     The connection will not be reused, but all level resources held by it will be correctly deallocated.
-    
- === Consuming entity content ===
- 
-     The recommended way to consume content of an entity is by using its HttpEntity#getContent() or  HttpEntity#writeTo(OutputStream) methods. HttpClient also comes with the EntityUtils class, which exposes several static methods to more easily read the content or information from an entity. Instead of reading the java.io.InputStream directly, one can retrieve the whole content body in a string / byte array by using the methods from this class. However, the use of HttpEntity is strongly discouraged unless the response entities originate from a trusted HTTP server and are known to be of limited length.
- 
- {{{
- HttpGet httpget = new HttpGet("http://localhost/");
- HttpResponse response = httpclient.execute(httpget);
- HttpEntity entity = response.getEntity();
- if (entity != null) {
-     long len = entity.getContentLength();
-     if (len != -1 && len < 2048) {
-         System.out.println(EntityUtils.toString(entity));
-     } else {
-         // Stream content out
-     }
- }
- }}}
- 
-     In some situations it may be necessary to be able to read entity content more than once. In this case entity content must be buffered in some way, either in memory or on disk. The simplest way to accomplish that is by wrapping the original entity with the BufferedHttpEntity class. This will cause the content of the original entity to be read into a in-memory buffer. In all other ways the entity wrapper will be have the original one. 
- 
- {{{
- HttpGet httpget = new HttpGet("http://localhost/");
- HttpResponse response = httpclient.execute(httpget);
- HttpEntity entity = response.getEntity();
- if (entity != null) {
-     entity = new BufferedHttpEntity(entity);
- }
- }}}
- 
- === Producing entity content ===
- 
-     HttpClient provides several classes that can be used to efficiently stream out content though HTTP connections. Instances of those classes can be associated with entity enclosing requests such as POST and PUT in order to enclose entity content into outgoing HTTP requests. HttpClient provides several classes for most common data containers such as string, byte array, input stream, and file: StringEntity, ByteArrayEntity, InputStreamEntity, and FileEntity.
- 
- {{{
- File file = new File("somefile.txt");
- FileEntity entity = new FileEntity(file, "text/plain; charset=\"UTF-8\"");
- 
- HttpPost httppost = new HttpPost("http://localhost/action.do");
- httppost.setEntity(entity);
- }}}
-     
-     Please note InputStreamEntity is not repeatable, because it can only read from the underlying data stream once. Generally it is recommended to implement a custom HttpEntity class which is self-contained instead of using generic InputStreamEntity. FileEntity can be a good starting point.
- 
- ==== Dynamic content entities ====
- 
-     Often HTTP entities need to be generated dynamically based a particular execution context. HttpClient provides support for dynamic entities by using EntityTemplate entity class and ContentProducer interface. Content producers are objects which produce their content on demand, by writing it out to an output stream. They are expected to be able produce their content every time they are requested to do so. So entities created with EntityTemplate are generally self-contained and repeatable.
- 
- {{{
- ContentProducer cp = new ContentProducer() {
-     public void writeTo(OutputStream outstream) throws IOException {
-         Writer writer = new OutputStreamWriter(outstream, "UTF-8");
-         writer.write("<response>");
-         writer.write("  <content>");
-         writer.write("    important stuff");
-         writer.write("  </content>");
-         writer.write("</response>");
-         writer.flush();
-     }
- };
- HttpEntity entity = new EntityTemplate(cp);
- HttpPost httppost = new HttpPost("http://localhost/handler.do");
- httppost.setEntity(entity);
- }}}
- 
- ==== HTML forms ====
- 
-     Many applications frequently need to simulate the process of submitting an HTML form, for instance, in order to log in to a web application or submit input data. HttpClient provides special entity class UrlEncodedFormEntity to facilitate the process.
- 
- {{{
- List<NameValuePair> formparams = new ArrayList<NameValuePair>();
- formparams.add(new BasicNameValuePair("param1", "value1"));
- formparams.add(new BasicNameValuePair("param2", "value2"));
- UrlEncodedFormEntity entity = new UrlEncodedFormEntity(formparams, "UTF-8");
- HttpPost httppost = new HttpPost("http://localhost/handler.do");
- httppost.setEntity(entity);
- }}}
- 
-     This UrlEncodedFormEntity instance will use the so called URL encoding to encode parameters and produce the following content: 
- 
- {{{
- param1=value1&param2=value2
- }}}
- 
- ==== Content chunking ====
- 
-    Generally it is recommended to let HttpClient choose the most appropriate transfer encoding based on the properties of the HTTP message being transferred. It is possible, however, to inform HttpClient that the chunk coding is preferred by setting HttpEntity#setChunked() to true. Please note that HttpClient will use this flag as a hint only. This value well be ignored when using HTTP protocol versions that do not support chunk coding, such as HTTP/1.0. 
- 
- {{{
- StringEntity entity = new StringEntity("important message", "text/plain; charset=\"UTF-8\"");
- entity.setChunked(true);
- HttpPost httppost = new HttpPost("http://localhost/acrtion.do");
- httppost.setEntity(entity);
- }}}
- 
- === Response handlers ===
- 
-   The simplest and the most convenient way to handle responses is by using ResponseHandler interface. This method completely relieves the user from having to worry about connection management. When using a ResponseHandler HttpClient will automatically take care of ensuring release of the connection back to the connection manager regardless whether the request execution succeeds or causes an exception.  
- 
- {{{
- HttpClient httpclient = new DefaultHttpClient();
- HttpGet httpget = new HttpGet("http://localhost/");
- 
- ResponseHandler<byte[]> handler = new ResponseHandler<byte[]>() {
-     public byte[] handleResponse(
-             HttpResponse response) throws ClientProtocolException,IOException {
-         HttpEntity entity = response.getEntity();
-         if (entity != null) {
-             return EntityUtils.toByteArray(entity);
-         } else {
-             return null;
-         }
-     }
- };
- 
- byte[] response = httpclient.execute(httpget, handler);
- }}}
- 
- === HTTP execution context ===
- 
-     Originally HTTP has been designed as a stateless, response-request oriented protocol. However, real world applications often need to be able to persist state information through several logically related request-response exchanges. In order to enable applications to maintain a processing state HttpClient allows HTTP requests to be executed within a particular execution context, referred to as HTTP context. Multiple logically related requests can participate in a logical session if the same context is reused between consecutive requests. HTTP context functions similarly to java.util.Map<String, Object>. It is simply a collection of arbitrary named values. Application can populate context attributes prior to a request execution or examine the context after the execution has been completed. 
- 
-     In the course of HTTP request execution HttpClient adds the following attributes to the execution context:
- 
-     * '''http.connection''' - HttpConnection instance representing the actual connection to the target server.
- 
-     * '''http.target_host''' - HttpHost instance representing the connection target.
- 
-     * '''http.proxy_host''' - HttpHost instance representing the connection proxy, if used.
- 
-     * '''http.request''' - HttpRequest instance representing the actual HTTP request.
- 
-     * '''http.response''' - HttpResponse instance representing the actual HTTP response.
- 
-     * '''http.request_sent''' - Boolean object representing the flag indicating whether the actual request has been fully transmitted to the connection target.
-     
-     For instance, in order to determine the final redirect target, one can examine the value of the  'http.target_host' attribute after the request execution:
-     
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- 
- HttpContext localContext = new BasicHttpContext();
- HttpGet httpget = new HttpGet("http://www.google.com/");
- 
- HttpResponse response = httpclient.execute(httpget, localContext);
- 
- HttpHost target = (HttpHost) localContext.getAttribute(
-         ExecutionContext.HTTP_TARGET_HOST);
- 
- System.out.println("Final target: " + target);
- 
- HttpEntity entity = response.getEntity();
- if (entity != null) {
-     entity.consumeContent();
- }
- }}}
- 
- stdout >
- 
- {{{
- Final target: http://www.google.ch
- }}}
- 
- === Exception handling ===
- 
-     HttpClient can throw two types of exceptions: IOException in case of an I/O failure such as socket timeout or an socket reset and HttpException that signals an HTTP failure such as a violation of the HTTP protocol. Usually I/O errors are considered non-fatal and recoverable, whereas HTTP protocol errors are considered fatal and cannot be automatically recovered from.   
- 
- ==== HTTP transport safety ====
-     
-     It is important to understand that the HTTP protocol is not well suited for all types of applications. HTTP is a simple request/response oriented protocol which was initially designed to support static or dynamically generated content retrieval. It has never been intended to support transactional operations. For instance, the HTTP server will consider its part of the contract fulfilled if it succeeds in receiving and processing the request, generating a response and sending a status code back to the client. The server will make no attempts to roll back the transaction if the client fails to receive the response in its entirety due to a read timeout, a request cancellation or a system crash. If the client decides to retry the same request, the server will inevitably end up executing the same transaction more than once. In some cases this may lead to application data corruption or inconsistent application state.
- 
-     Even though HTTP has never been designed to support transactional processing, it can still be used as a transport protocol for mission critical applications provided certain conditions are met. To ensure HTTP transport layer safety the system must ensure the idempotency of HTTP methods on the application layer.
- 
- ==== Idempotent methods ====
- 
-     HTTP/1.1 specification defines idempotent method as
- 
- {{{
- Methods can also have the property of "idempotence" in that (aside from error or expiration issues) 
- the side-effects of N > 0 identical requests is the same as for a single request
- }}}
- 
-     In other words the application ought to ensure that it is prepared to deal with the implications of multiple execution of the same method. This can be achieved, for instance, by providing a unique transaction id and by other means of avoiding execution of the same logical operation.
- 
-     Please note that this problem is not specific to HttpClient. Browser based applications are subject to exactly the same issues related to HTTP methods non-idempotency.
- 
-     HttpClient assumes non-entity enclosing methods such as GET and HEAD to be idempotent and entity enclosing methods such as POST and PUT to be not.
- 
- ==== Automatic exception recovery ====
- 
-     By default HttpClient attempts to automatically recover from I/O exceptions. The default auto-recovery mechanism is limited to just a few exceptions that are known to be safe.
- 
-     * HttpClient will make no attempt to recover from any logical or HTTP protocol errors (those derived from HttpException class).
- 
-     * HttpClient will automatically retry those methods that are assumed to be idempotent.
-     
-     * HttpClient will automatically retry those methods that fail with a transport exception while the HTTP request is still being transmitted to the target server (i.e. the request has not been fully transmitted to the server).
- 
-     * HttpClient will automatically retry those methods that have been fully transmitted to the server, but the server failed to respond with an HTTP status code (the server simply drops the connection without sending anything back). In this case it is assumed that the request has not been processed by the server and the application state has not changed. If this assumption may not hold true for the web server your application is targeting it is highly recommended to provide a custom exception handler.
- 
- ====  Request retry handler ====
- 
-     In order to enable a custom exception recovery mechanism one should provide an implementation of the  HttpRequestRetryHandler interface. 
-     
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- 
- HttpRequestRetryHandler myRetryHandler = new HttpRequestRetryHandler() {
- 
-     public boolean retryRequest(
-             IOException exception, 
-             int executionCount,
-             HttpContext context) {
-         if (executionCount >= 5) {
-             // Do not retry if over max retry count
-             return false;
-         }
-         if (exception instanceof NoHttpResponseException) {
-             // Retry if the server dropped connection on us
-             return true;
-         }
-         if (exception instanceof SSLHandshakeException) {
-             // Do not retry on SSL handshake exception
-             return false;
-         }
-         HttpRequest request = (HttpRequest) context.getAttribute(
-                 ExecutionContext.HTTP_REQUEST);
-         boolean idempotent = !(request instanceof HttpEntityEnclosingRequest); 
-         if (idempotent) {
-             // Retry if the request is considered idempotent 
-             return true;
-         }
-         return false;
-     }
-     
- };
- 
- httpclient.setHttpRequestRetryHandler(myRetryHandler);
- }}}
- 
- === Aborting requests ===
- 
-     In some situations HTTP request execution fail to complete within the expected time frame due to high load on the target server or too many concurrent requests issued on the client side. In such cases it may be necessary to terminate the request prematurely and unblock the execution thread blocked in a I/O operation. HTTP requests being executed by HttpClient can be aborted at any stage of execution by invoking HttpUriRequest#abort method. This method is thread-safe and can be called from any thread. When an HTTP request is aborted its execution thread blocked in an I/O operation is guaranteed to unblock by throwing a InterruptedIOException
- 
- == HTTP protocol interceptors ==
- 
-     HTTP protocol interceptor is a routine that implements a specific aspect of the HTTP protocol. Usually protocol interceptors are expected to act upon one specific header or a group of related headers of the incoming message or populate the outgoing message with one specific header or a group of related headers. Protocol interceptors can also manipulate content entities enclosed with messages, transparent content compression / decompression being a good example. Usually this is accomplished by using the 'Decorator' pattern where a wrapper entity class is used to decorate the original entity. Several protocol interceptors can be combined to form one logical unit. 
-     
-     Protocol interceptors can collaborate by sharing information - such as a processing state - through the HTTP execution context. Protocol interceptors can use HTTP context to store a processing state for one request or several consecutive requests. 
- 
-     Usually the order in which interceptors are executed should not matter as long as they do not depend on a particular state of the execution context. If protocol interceptors have interdependencies and therefore must be executed in a particular order, they should be added to the protocol processor in the same sequence as their expected execution order.
- 
-     Protocol interceptors must be implemented as thread-safe. Similarly to servlets, protocol interceptors should not use instance variables unless access to those variables is synchronized. 
- 
-     This is an example of how local context can be used to persist a processing state between consecutive requests:
-     
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- 
- HttpContext localContext = new BasicHttpContext();
- 
- AtomicInteger count = new AtomicInteger(1);
- 
- localContext.setAttribute("count", count);
- 
- httpclient.addRequestInterceptor(new HttpRequestInterceptor() {
- 
-     public void process(
-             final HttpRequest request, 
-             final HttpContext context) throws HttpException, IOException {
-         AtomicInteger count = (AtomicInteger) context.getAttribute("count");
-         request.addHeader("Count", Integer.toString(count.getAndIncrement()));
-     }
-     
- });
- 
- HttpGet httpget = new HttpGet("http://localhost/");
- for (int i = 0; i < 10; i++) {
-     HttpResponse response = httpclient.execute(httpget, localContext);
-     
-     HttpEntity entity = response.getEntity();
-     if (entity != null) {
-         entity.consumeContent();
-     }
- }
- }}}
- 
- == HTTP parameters ==
- 
-     HttpParams interface represents a collection of immutable values that define a runtime behavior of a component. In many ways HttpParams  is similar to HttpContext. The main distinction between the two lies in their use at runtime. Both interfaces represent a collection of objects that are organized as a map of keys to object values, but serve distinct purposes:
- 
-     * HttpParams is intended to contain simple objects: integers, doubles, strings, collections and objects that remain immutable at runtime. HttpParams is expected to be used in the 'write once - ready many' mode. HttpContext is intended to contain complex objects that are very likely to mutate in the course of HTTP message processing.
-     
-     * The purpose of HttpParams is to define a behavior of other components. Usually each complex component has its own HttpParams object. The purpose of HttpContext is to represent an execution state of an HTTP process. Usually the same execution context is shared among many collaborating objects.   
- 
- === Parameter hierarchies ===
- 
-     In the course of HTTP request execution HttpParams of the HttpRequest object are linked together with HttpParams of the HttpClient instance used to execute the request. This enables parameters set at the HTTP request level take precedence over HttpParams set at the HTTP client level. The recommended practice is to set common parameters shared by all HTTP requests at the HTTP client level and selectively override specific parameters at the HTTP request level.
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- httpclient.getParams().setParameter(CoreProtocolPNames.PROTOCOL_VERSION, 
-     HttpVersion.HTTP_1_0);
- httpclient.getParams().setParameter(CoreProtocolPNames.HTTP_CONTENT_CHARSET, 
-     "UTF-8");
- 
- HttpGet httpget = new HttpGet("http://www.google.com/");
- httpget.getParams().setParameter(CoreProtocolPNames.PROTOCOL_VERSION, 
-     HttpVersion.HTTP_1_1);
- httpget.getParams().setParameter(CoreProtocolPNames.USE_EXPECT_CONTINUE, 
-     Boolean.FALSE);
- 
- httpclient.addRequestInterceptor(new HttpRequestInterceptor() {
- 
-     public void process(
-             final HttpRequest request, 
-             final HttpContext context) throws HttpException, IOException {
-         System.out.println(request.getParams().getParameter(
-                 CoreProtocolPNames.PROTOCOL_VERSION));
-         System.out.println(request.getParams().getParameter(
-                 CoreProtocolPNames.HTTP_CONTENT_CHARSET));
-         System.out.println(request.getParams().getParameter(
-                 CoreProtocolPNames.USE_EXPECT_CONTINUE));
-         System.out.println(request.getParams().getParameter(
-                 CoreProtocolPNames.STRICT_TRANSFER_ENCODING));
-     }
-     
- });
- }}}
-       
- stdout>
- {{{
- HTTP/1.1
- UTF-8
- false
- null
- }}}
- 
- === HTTP parameters beans ===
- 
-     HttpParams interface allows for a great deal of flexibility in handling configuration of components. Most importantly, new parameters can be introduced without affecting binary compatibility with older versions. However, HttpParams also has a certain disadvantage compared to regular Java beans: HttpParams cannot be assembled using a DI framework. To mitigate the limitation, HttpClient includes a number of bean classes that can used in order to initialize HttpParams objects using standard Java bean conventions. 
- 
- {{{
- HttpParams params = new BasicHttpParams();
- HttpProtocolParamBean paramsBean = new HttpProtocolParamBean(params);
- paramsBean.setVersion(HttpVersion.HTTP_1_1);
- paramsBean.setContentCharset("UTF-8");
- paramsBean.setUseExpectContinue(true);
- 
- System.out.println(params.getParameter(
-     CoreProtocolPNames.PROTOCOL_VERSION));
- System.out.println(params.getParameter(
-     CoreProtocolPNames.HTTP_CONTENT_CHARSET));
- System.out.println(params.getParameter(
-     CoreProtocolPNames.USE_EXPECT_CONTINUE));
- System.out.println(params.getParameter(
-     CoreProtocolPNames.USER_AGENT));
- }}}    
-       
- stdout>
- {{{
- HTTP/1.1
- UTF-8
- false
- null
- }}}
- 
- == HTTP request execution parameters ==
- 
-     These are parameters that can impact the process of request execution:
- 
-     * '''http.protocol.version''': defines HTTP protocol version used if not set explicitly on the request object. This parameter expects a value of type ProtocolVersion. If this parameter is not set HTTP/1.1 will be used.
- 
-     * '''http.protocol.element-charset''': defines the charset to be used for encoding HTTP protocol elements. This parameter expects a value of type java.lang. String. If this parameter is not set US-ASCII will be used.
- 
-     * '''http.protocol.content-charset''': defines the charset to be used per default for content body coding. This parameter expects a value of type java.lang.String. If this parameter is not set ISO-8859-1 will be used.   
-     
-     * '''http.useragent''': defines the content of the {{{User-Agent}}} header. This parameter expects a value of type java.lang.String. If this parameter is not set, HttpClient will automatically generate a value for it.
- 
-     * '''http.protocol.strict-transfer-encoding''': defines whether responses with an invalid {{{Transfer-Encoding}}} header should be rejected. This parameter expects a value of type java.lang.Boolean. If this parameter is not set invalid {{{Transfer-Encoding}}} values will be ignored.
- 
-     * '''http.protocol.expect-continue''': activates 'Expect: 100-Continue' handshake for the entity enclosing methods. The purpose of the {{{Expect: 100-Continue}}} handshake is to allow the client that is sending a request message with a request body to determine if the origin server is willing to accept the request (based on the request headers) before the client sends the request body. The use of the {{{Expect: 100-continue}}} handshake can result in a noticeable performance improvement for entity enclosing requests (such as POST and PUT) that require the target server's authentication. {{{Expect: 100-continue}}} handshake should be used with caution, as it may cause problems with HTTP servers and proxies that do not support HTTP/1.1 protocol. This parameter expects a value of type java.lang.Boolean. If this parameter is not set HttpClient will attempt to use the handshake. 
- 
-     * '''http.protocol.wait-for-continue''': defines the maximum period of time in milliseconds the client should spend waiting for a 100-continue response. This parameter expects a value of type java.lang.Integer. If this parameter is not set HttpClient will wait 3 seconds for a confirmation before resuming the transmission of the request body.
- 
- = Connection management =
- 
-     HttpClient has a complete control over the process of connection initialization and termination as well as I/O operations on active connections. However various aspects of connection operations can be controlled using a number of parameters:  
- 
- == Connection parameters ==
- 
-     These are parameters that can influence connection operations:
- 
-     * '''http.socket.timeout''': defines the socket timeout ({{{SO_TIMEOUT}}}) in milliseconds, which is the timeout for waiting for data or, put differently, a maximum period inactivity between two consecutive data packets). A timeout value of zero is interpreted as an infinite timeout. This parameter expects a value of type java.lang.Integer}. If this parameter is not set read operations will not time out (infinite timeout). 
- 
-     * '''http.tcp.nodelay''': determines whether Nagle's algorithm is to be used. The Nagle's algorithm tries to conserve bandwidth by minimizing the number of segments that are sent. When applications wish to decrease network latency and increase performance, they can disable Nagle's algorithm (that is enable {{{TCP_NODELAY}}}. Data will be sent earlier, at the cost of an increase in bandwidth consumption. This parameter expects a value of type java.lang.Boolean. If this parameter is not, {{{TCP_NODELAY}}} will be enabled (no delay). 
- 
-     * '''http.socket.buffer-size''': determines the size of the internal socket buffer used to buffer data while receiving / transmitting HTTP messages. This parameter expects a value of type java.lang.Integer. If this parameter is not set HttpClient will allocate 8192 byte socket buffers.
- 
-     * '''http.socket.linger''': sets {{{SO_LINGER}}} with the specified linger time in seconds. The maximum timeout value is platform specific. Value 0 implies that the option is disabled. Value -1 implies that the JRE default is used. The setting only affects the socket close operation. If this parameter is not set value -1 (JRE default) will be assumed. 
- 
-     * '''http.connection.timeout''': determines the timeout in milliseconds until a connection is established. A timeout value of zero is interpreted as an infinite timeout. This parameter expects a value of type java.lang.Integer. If this parameter is not set connect operations will not time out (infinite timeout).
- 
-     * '''http.connection.stalecheck''': determines whether stale connection check is to be used. Disabling stale connection check may result in a noticeable performance improvement (the check can cause up to 30 millisecond overhead per request) at the risk of getting an I/O error when executing a request over a connection that has been closed at the server side. This parameter expects a value of type java.lang.Boolean. For performance critical operations the check should be disabled. If this parameter is not set the stale connection will be performed before each request execution.
- 
-     * '''http.connection.max-line-length''': Determines the maximum line length limit. If set to a positive value, any HTTP line exceeding this limit will cause an IOException. A negative or zero value will effectively disable the check. This parameter expects a value of type java.lang.Integer. If this parameter is not set, no limit will be enforced.
- 
-     * '''http.connection.max-header-count''': determines the maximum HTTP header count allowed. If set to a positive value, the number of HTTP headers received from the data stream exceeding this limit will cause an IOException. A negative or zero value will effectively disable the check. This parameter expects a value of type java.long.Integer. If this parameter is not set, no limit will be enforced.
- 
-     * '''http.connection.max-status-line-garbage''': defines the maximum number of ignorable lines before we expect a HTTP response's status line. With HTTP/1.1 persistent connections, the problem arises that broken scripts could return a wrong {{{Content-Length}}} (there are more bytes sent than specified). Unfortunately, in some cases, this cannot be detected after the bad response, but only before the next one. So HttpClient must be able to skip those surplus lines this way. This parameter expects a value of type java.lang.Integer. 0 disallows all garbage/empty lines before the status line. Use java.lang.Integer#MAX_VALUE for unlimited number. If this parameter is not set unlimited number will be assumed. 
- 
- == Connection persistence ==
- 
-     The process of establishing a connection from one host to another is quite complex and involves multiple packet exchanges between two endpoints, which can be quite time consuming. The overhead of connection handshaking can be significant, especially for small HTTP messages. One can achieve a much higher data throughput if open connections can be re-used to execute multiple requests. 
- 
-     HTTP/1.1 states that HTTP connections can be re-used for multiple requests per default. HTTP/1.0 compliant endpoints can also use similar mechanism to explicitly communicate their preference to keep connection alive and use it for multiple requests. HTTP agents can also keep idle connections alive for a certain period time in case a connection to the same target host may be needed for subsequent requests. The ability to keep connections alive is usually refered to as connection persistence. HttpClient fully supports connection persistence.
-   
- == HTTP connection routing ==
- 
-     HttpClient is capable of establishing connections to the target host either directly or via a route that may involve multiple intermediate connections also referred to as hops. HttpClient differentiates connections of a route into plain, tunneled and layered. The use of multiple intermediate proxies to tunnel connections to the target host is referred to as proxy chaining.
-     
-     Plain routes are established by connecting to the target or the first and only proxy. Tunnelled routes are established by connecting to the first and tunnelling through a chain of proxies to the target. Routes without a proxy cannot be tunnelled. Layered routes are established by layering a protocol over an existing connection. Protocols can only be layered over a tunnel to the target, or over a direct connection without proxies.
- 
- === Route computation ===
- 
-     RouteInfo interface represents information about a definitive route to a target host involving one or more intermediate steps or hops. HttpRoute is a concrete implementation of RouteInfo, which cannot be changed (is immutable). HttpTracker is a mutable RouteInfo implementation used internally by HttpClient to track the remaining hops to the ultimate route target. HttpTracker can be updated after a successful execution of the next hop towards the route target. HttpRouteDirector is a helper class that can be used to compute the next step in a route. This class is used internally by HttpClient.
- 
-     HttpRoutePlanner is an interface representing a strategy to compute a complete route to a given target based on the execution context. HttpClient ships with two default HttpRoutePlanner implementation. ProxySelectorRoutePlanner is based on java.net.ProxySelector. By default, it will pick up the proxy settings of the JVM, either from system properties or from the browser running the application. DefaultHttpRoutePlanner implementation does not make use of any Java system properties, nor of system or browser proxy settings. It computes routes based exclusively on HTTP parameters described below.
- 
- === HTTP route parameters ===
- 
-     These are parameters that can influence route computation:
- 
-     * '''http.route.default-proxy''': defines a proxy host to be used by default route planners that do not make use of JRE settings. This parameter expects a value of type HttpHost. If this parameter is not set direct connections to the target will be attempted.
- 
-     * '''http.route.local-address''': defines a local address to be used by all default route planner. On machines with multiple network interfaces, this parameter can be used to select the network interface from which the connection originates. This parameter expects a value of type java.net.InetAddress. If this parameter is not set a default local address will be used automatically.
- 
-     * '''http.route.forced-route''': defines an forced route to be used by all default route planner. Instead of computing a route, the given forced route will be returned, even if it points to a completely different target host. This parameter expects a value of type HttpRoute.
- 
- === Secure HTTP connections ===
- 
-     HTTP connections can be considered secure if information transmitted between two connection endpoints cannot be read or tampered with by an unauthorized third party. The SSL/TLS protocol is the most widely used technique to ensure HTTP transport security. However, other encryption techniques could be employed as well. Usually, HTTP transport is layered over the SSL/TLS encrypted connection.
- 
- == Socket factories ==
- 
-     HTTP connections make use of a java.net.Socket object internally to handle transmission of data across the wire. They, however, rely on SocketFactory interface to create, initialize and connect sockets. This enables the users of HttpClient to provide application specific socket initialization code at runtime. PlainSocketFactory is the default factory for creating and initializing plain (unencrypted) sockets. 
-     
-     The process of creating a socket and that of connecting it to a host are decoupled, so that the socket could be closed while being blocked in the connect operation.
- 
- {{{
- PlainSocketFactory sf = PlainSocketFactory.getSocketFactory();
- Socket socket = sf.createSocket();
- 
- HttpParams params = new BasicHttpParams();
- params.setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT, 1000L);
- sf.connectSocket(socket, "locahost", 8080, null, -1, params);
- }}}
- 
- === Secure socket layering ===
- 
-     LayeredSocketFactory is an extension of SocketFactory interface. Layered socket factories are capable of creating sockets that are layered over an existing plain socket. Socket layering is used primarily for creating secure sockets through proxies. HttpClient ships with SSLSocketFactory that implements SSL/TLS layering. Please note HttpClient does not use any custom encryption functionality. It is fully reliant on standard Java Cryptography (JCE) and Secure Sockets (JSEE) extensions.
-    
- === SSL/TLS customization ===
- 
-     HttpClient makes use of SSLSocketFactory to create SSL connections. SSLSocketFactory allows for a high degree of customization. It can take an instance of javax.net.ssl.SSLContext as a parameter and use it to create custom configured SSL connections. 
- 
- {{{
- TrustManager easyTrustManager = new X509TrustManager() {
- 
- 	@Override
- 	public void checkClientTrusted(
- 			X509Certificate[] chain,
- 			String authType) throws CertificateException {
-         // Oh, I am easy!
- 	}
- 
- 	@Override
- 	public void checkServerTrusted(
- 			X509Certificate[] chain,
- 			String authType) throws CertificateException {
-         // Oh, I am easy!
- 	}
- 
- 	@Override
- 	public X509Certificate[] getAcceptedIssuers() {
- 		return null;
- 	}
- 	
- };
- 
- SSLContext sslcontext = SSLContext.getInstance("TLS");
- sslcontext.init(null, new TrustManager[] { easyTrustManager }, null);
- 
- SSLSocketFactory sf = new SSLSocketFactory(sslcontext); 
- SSLSocket socket = (SSLSocket) sf.createSocket();
- socket.setEnabledCipherSuites(new String[] { "SSL_RSA_WITH_RC4_128_MD5" });
- 
- HttpParams params = new BasicHttpParams();
- params.setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT, 1000L);
- sf.connectSocket(socket, "locahost", 443, null, -1, params);
- }}}
-     
-     Customization of SSLSocketFactory implies a certain degree of familiarity with the concepts of the SSL/TLS protocol, a detailed explanation of which is out of scope for this document. Please refer to the [http://java.sun.com/j2se/1.5.0/docs/guide/security/jsse/JSSERefGuide.html Java Secure Socket Extension] for a detailed description of javax.net.ssl.SSLContext and related tools.
- 
- === Hostname verification ===
- 
-    In addition to the trust verification and the client authentication performed on the SSL/TLS protocol level, HttpClient can optionally verify whether the target hostname matches the names stored inside the server's X.509 certificate, once the connection has been established. This verification can provide additional guarantees of authenticity of the server trust material. X509HostnameVerifier interface represents a strategy for hostname verification. HttpClient ships with three X509HostnameVerifier. Important: hostname verification should not be confused with SSL trust verification.
-    
-    * '''StrictHostnameVerifier''': The strict hostname verifier works the same way as Sun Java 1.4, Sun Java 5, Sun Java 6. It's also pretty close to IE6. This implementation appears to be compliant with RFC 2818 for dealing with wildcards. The hostname must match either the first CN, or any of the subject-alts. A wildcard can occur in the CN, and in any of the subject-alts. 
- 
-    * '''BrowserCompatHostnameVerifier''': The hostname verifier that works the same way as Curl and Firefox. The hostname must match either the first CN, or any of the subject-alts. A wildcard can occur in the CN, and in any of the subject-alts. The only difference between BrowserCompatHostnameVerifier and StrictHostnameVerifier is that a wildcard (such as "*.foo.com") with BrowserCompatHostnameVerifier matches all subdomains, including "a.b.foo.com".
- 
-    * '''AllowAllHostnameVerifier''': This hostname verifier essentially turns hostname verification off. This implementation is a no-op, and never throws the SSLException.
-    
-    Per default HttpClient uses BrowserCompatHostnameVerifier implementation. One can specify a different hostname verifier implementation if desired
-    
- {{{
- SSLSocketFactory sf = new SSLSocketFactory(SSLContext.getInstance("TLS"));
- sf.setHostnameVerifier(SSLSocketFactory.STRICT_HOSTNAME_VERIFIER);
- }}}
-    
- == Protocol schemes ==
-     
-     Scheme class represents a protocol scheme such as "http" or "https" and contains a number of protocol properties such as the default port and the socket factory to be used to creating Sockets for the given protocol. SchemeRegistry class is used to maintain a set of Schemes HttpClient can choose from when trying to establish a connection by a request URI:
-     
- {{{
- Scheme http = new Scheme("http", PlainSocketFactory.getSocketFactory(), 80);
- 
- SSLSocketFactory sf = new SSLSocketFactory(SSLContext.getInstance("TLS"));
- sf.setHostnameVerifier(SSLSocketFactory.STRICT_HOSTNAME_VERIFIER);
- Scheme https = new Scheme("https", sf, 443);
- 
- SchemeRegistry sr = new SchemeRegistry();
- sr.register(http);
- sr.register(https);
- }}}
-     
- == HttpClient proxy configuration ==
- 
-     Even though HttpClient is aware of complex routing scemes and proxy chaining, it supports only simple direct or one hop proxy connections out of the box.
- 
-     The simplest way to tell HttpClient to connect to the target host via a proxy is by setting the default proxy parameter:
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- 
- HttpHost proxy = new HttpHost("someproxy", 8080);
- httpclient.getParams().setParameter(ConnRoutePNames.DEFAULT_PROXY, proxy);
- }}}
- 
-    One can also instruct HttpClient to use standard JRE proxy selector to obtain proxy information:
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- 
- ProxySelectorRoutePlanner routePlanner = new ProxySelectorRoutePlanner(
-         httpclient.getConnectionManager().getSchemeRegistry(),
-         ProxySelector.getDefault());  
- httpclient.setRoutePlanner(routePlanner);
- }}}
- 
-    Alternatively, one can provide a custom RoutePlanner implementation in order to have a complete control over the process of HTTP route computation:
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- httpclient.setRoutePlanner(new HttpRoutePlanner() {
- 
-     public HttpRoute determineRoute(
-             HttpHost target, 
-             HttpRequest request, 
-             HttpContext context) throws HttpException {
-         return new HttpRoute(target, null,  new HttpHost("someproxy", 8080), 
-                 "https".equalsIgnoreCase(target.getSchemeName()));
-     }
-     
- });
- }}}
- 
- == HTTP connection managers ==
- 
- === Connection operators ===
- 
-     Operated connections are client side connections whose underlying socket or its state can be manipulated by an external entity, usually referred to as a connection operator. OperatedClientConnection interface extends HttpClientConnection interface and define additional methods to manage connection socket.  The ClientConnectionOperator interface represents a strategy for creating OperatedClientConnection instances and updating the underlying Socket of those objects. Implementations will most likely make use SocketFactorys to create Socket instances. The ClientConnectionOperator interface enables the users of HttpClient to provide a custom strategy for connection operators as well as an ability to provide alternative implementation of the OperatedClientConnection interface.
- 
- === Managed connections and connection managers ===
- 
-     HTTP connections are complex, stateful, thread-unsafe objects which need to be properly managed to function correctly. HTTP connections can only be used by one execution thread at a time. HttpClient employs a special entity to manage access to HTTP connections called HTTP connection manager and represented by the ClientConnectionManager interface. The purpose of an HTTP connection manager is to serve as a factory for new HTTP connections, manage persistent connections and synchronize access to persistent connections making sure that only one thread can have access to a connection at a time.
- 
-     Internally HTTP connection managers work with instances of OperatedClientConnection, but they hands out instances of ManagedClientConnection to the service consumers. ManagedClientConnection acts as a wrapper for a OperatedClientConnection instance that manages its state and controls all I/O operations on that connection. It also abstracts away socket operations and provides convenience methods for opening and updating sockets in order to establish a route. ManagedClientConnection instances are aware of their link to the connection manager that spawned them and of the fact that they must be returned back to the manager when no longer in use. ManagedClientConnection classes also implement ConnectionReleaseTrigger interface that can be used to trigger the release of the connection back to the manager. Once the connection release has been triggered the wrapped connection gets detached from the ManagedClientConnection wrapper and the OperatedClientConnection instance is ret
 urned back to the manager. Even though the service consumer still holds a reference to the ManagedClientConnection instance, it is no longer able to execute any I/O operation or change the state of the OperatedClientConnection either intentionally or unintentionally.
- 
-     This is an example of acquiring a connection from a connection manager:  
-     
- {{{
- HttpParams params = new BasicHttpParams();
- Scheme http = new Scheme("http", PlainSocketFactory.getSocketFactory(), 80);
- SchemeRegistry sr = new SchemeRegistry();
- sr.register(http);
- 
- ClientConnectionManager connMrg = new SingleClientConnManager(params, sr);
- 
- // Request new connection. This can be a long process
- ClientConnectionRequest connRequest = connMrg.requestConnection(
-         new HttpRoute(new HttpHost("localhost", 80)), null);
- 
- // Wait for connection up to 10 sec
- ManagedClientConnection conn = connRequest.getConnection(10, TimeUnit.SECONDS);
- try {
-     // Do useful things with the connection.
-     // Release it when done.
-     conn.releaseConnection();
- } catch (IOException ex) {
-     // Abort connection upon an I/O error.
-     conn.abortConnection();
-     throw ex;
- }
- }}}
-     
-     The connection request can be terminated prematurely by calling ClientConnectionRequest#abortRequest() if necessary. This will unblock the thread blocked in the ClientConnectionRequest#getConnection() method.
- 
-     BasicManagedEntity wrapper class can be used to ensure automatic release of the underlying connection once the response content has been fully consumed. HttpClient uses this mechanism internally to achieve transparent connection release for all responses obtained from HttpClient#execute methods:
-     
- {{{
- ClientConnectionRequest connRequest = connMrg.requestConnection(
-         new HttpRoute(new HttpHost("localhost", 80)), null);
- ManagedClientConnection conn = connRequest.getConnection(10, TimeUnit.SECONDS);
- try {
-     BasicHttpRequest request = new BasicHttpRequest("GET", "/");
-     conn.sendRequestHeader(request);
-     HttpResponse response = conn.receiveResponseHeader();
-     conn.receiveResponseEntity(response);
-     HttpEntity entity = response.getEntity();
-     if (entity != null) {
-         BasicManagedEntity managedEntity = new BasicManagedEntity(entity, conn, true);
-         // Replace entity
-         response.setEntity(managedEntity);
-     }
-     // Do something useful with the response
-     // The connection will be released automatically 
-     // as soon as the response content has been consumed
- } catch (IOException ex) {
-     // Abort connection upon an I/O error.
-     conn.abortConnection();
-     throw ex;
- }
- }}}
-    
- === Connection manager parameters ===
- 
-     These are parameters that be used to customize standard HTTP connection manager implementations:
- 
-     * '''http.conn-manager.timeout''': Defines the timeout in milliseconds used when retrieving an instance of ManagedClientConnection from the ClientConnectionManager This parameter expects a value of type java.lang.Long. If this parameter is not set connection requests will not time out (infinite timeout).
- 
-     * '''http.conn-manager.max-per-route''': Defines the maximum number of connections per route. This limit is interpreted by client connection managers and applies to individual manager instances. This parameter expects a value of type ConnPerRoute.
- 
-     * '''http.conn-manager.max-total''': Defines the maximum number of connections in total. This limit is interpreted by client connection managers and applies to individual manager instances. This parameter expects a value of type java.lang.Integer.
-    
- === Simple connection manager ===
- 
-     SingleClientConnManager is a simple connection manager that maintains only one connection at a time. Even though this class is thread-safe it ought to be used by one execution thread only. SingleClientConnManager will make an effort to reuse the connection for subsequent requests with the same route. It will, however, close the existing connection and open it for the given route, if the route of the persistent connection does not match that of the connection request. If the connection has been already been allocated IllegalStateException is thrown. 
-     
-     SingleClientConnManager is used by HttpClient per default.
-     
- === Pooling connection manager ===
- 
-     ThreadSafeClientConnManager is a more complex implementation that manages a pool of client connections and is able to service connection requests from multiple execution threads. Connections are pooled on a per route basis. A request for a route which already the manager has persistent connections for available in the pool will be services by leasing a connection from the pool rather than creating a brand new connection.
- 
-     ThreadSafeClientConnManager maintains a maximum limit of connection on a per route basis and in total. Per default this implementation will create no more than than 2 concurrent connections per given route and no more 20 connections in total. For many real-world applications these limits may prove too constraining, especially if they use HTTP as a transport protocol for their services. Connection limits, however, can be adjusted using HTTP parameters.
- 
-     This example shows how the connection pool parameters can be adjusted:
-     
- {{{
- HttpParams params = new BasicHttpParams();
- // Increase max total connection to 200
- ConnManagerParams.setMaxTotalConnections(params, 200);
- // Increase default max connection per route to 20
- ConnPerRouteBean connPerRoute = new ConnPerRouteBean(20);
- // Increase max connections for localhost:80 to 50
- HttpHost localhost = new HttpHost("locahost", 80);
- connPerRoute.setMaxForRoute(new HttpRoute(localhost), 50);
- ConnManagerParams.setMaxConnectionsPerRoute(params, connPerRoute);
- 
- SchemeRegistry schemeRegistry = new SchemeRegistry();
- schemeRegistry.register(
-         new Scheme("http", PlainSocketFactory.getSocketFactory(), 80));
- schemeRegistry.register(
-         new Scheme("https", SSLSocketFactory.getSocketFactory(), 443));
- 
- ClientConnectionManager cm = new ThreadSafeClientConnManager(params, schemeRegistry);
- HttpClient httpClient = new DefaultHttpClient(cm, params);
- }}}
- 
- === Connection manager shutdown ===
- 
-     When an HttpClient instance is no longer needed and is about to go out of scope it is important to shut down its connection manager to ensure that all connections kept alive by the manager get closed and system resources allocated by those connections are released.
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- HttpGet httpget = new HttpGet("http://www.google.com/");
- HttpResponse response = httpclient.execute(httpget);
- HttpEntity entity = response.getEntity();
- System.out.println(response.getStatusLine());
- if (entity != null) {
-     entity.consumeContent();
- }
- httpclient.getConnectionManager().shutdown();
- }}}
- 
- === Multithreaded request execution ===
- 
-     When equipped with a pooling connection manager such as ThreadSafeClientConnManager HttpClient can be used to execute multiple requests simultaneously using multiple threads of execution.
- 
-     ThreadSafeClientConnManager will allocate connections based on its configuration. If all connections for a given route have already been leased, a request for connection will block until a connection is released back to the pool. One can ensure the connection manager does not block indefinitely in the connection request operation by setting 'http.conn-manager.timeout' to a positive value. If the connection request cannot be serviced within the given time period ConnectionPoolTimeoutException will be thrown.
- 
- {{{
- HttpParams params = new BasicHttpParams();
- SchemeRegistry schemeRegistry = new SchemeRegistry();
- schemeRegistry.register(
-         new Scheme("http", PlainSocketFactory.getSocketFactory(), 80));
- 
- ClientConnectionManager cm = new ThreadSafeClientConnManager(params, schemeRegistry);
- HttpClient httpClient = new DefaultHttpClient(cm, params);
- 
- // URIs to perform GETs on
- String[] urisToGet = {
-     "http://www.domain1.com/",
-     "http://www.domain2.com/",
-     "http://www.domain3.com/",
-     "http://www.domain4.com/"
- };
- 
- // create a thread for each URI
- GetThread[] threads = new GetThread[urisToGet.length];
- for (int i = 0; i < threads.length; i++) {
-     HttpGet httpget = new HttpGet(urisToGet[i]);
-     threads[i] = new GetThread(httpClient, httpget);
- }
- 
- // start the threads
- for (int j = 0; j < threads.length; j++) {
-     threads[j].start();
- }
- 
- // join the threads
- for (int j = 0; j < threads.length; j++) {
-     threads[j].join();
- }
- 
- }}}
- 
- {{{
- static class GetThread extends Thread {
-     
-     private final HttpClient httpClient;
-     private final HttpContext context;
-     private final HttpGet httpget;
-     
-     public GetThread(HttpClient httpClient, HttpGet httpget) {
-         this.httpClient = httpClient;
-         this.context = new BasicHttpContext();
-         this.httpget = httpget;
-     }
-     
-     @Override
-     public void run() {
-         try {
-             HttpResponse response = this.httpClient.execute(this.httpget, this.context);
-             HttpEntity entity = response.getEntity();
-             if (entity != null) {
-                 // do something useful with the entity
-                 // ...
-                 // ensure the connection gets released to the manager
-                 entity.consumeContent();
-             }
-         } catch (Exception ex) {
-             this.httpget.abort();
-         }
-     }
-    
- }
- }}}
- 
- === Connection eviction policy ===
- 
-     One of the major shortcoming of the classic blocking I/O model is that the network socket can react to I/O events only when blocked in an I/O operation. When a connection is released back to the manager, it can be kept alive however it is unable to monitor the status of the socket and react to any I/O events. If the connection gets closed on the server side, the client side connection is unable to detect the change in the connection state and react appropriately by closing the socket on its end. 
- 
-     HttpClient tries to mitigate the problem by testing whether the connection is 'stale', that is no longer valid because it was closed on the server side, prior to using the connection for executing an HTTP request. The stale connection check is not 100% reliable and adds 10 to 30 ms overhead to each request execution. The only feasible solution that does not involve a one thread per socket model for idle connections is a dedicated monitor thread used to evict connections that are considered expired due to a long period of inactivity. The monitor thread can periodically call ClientConnectionManager#closeExpiredConnections() method to close all expired connections and evict closed connections from the pool. It can also optionally call ClientConnectionManager#closeIdleConnections() method to close all connections that have been idle over a given period of time. 
-     
- {{{
- public static class IdleConnectionMonitorThread extends Thread {
-     
-     private final ClientConnectionManager connMgr;
-     private volatile boolean shutdown;
-     
-     public IdleConnectionMonitorThread(ClientConnectionManager connMgr) {
-         super();
-         this.connMgr = connMgr;
-     }
- 
-     @Override
-     public void run() {
-         try {
-             while (!shutdown) {
-                 synchronized (this) {
-                     wait(5000);
-                     // Close expired connections
-                     connMgr.closeExpiredConnections();
-                     // Optionally, close connections
-                     // that have been idle longer than 30 sec
-                     connMgr.closeIdleConnections(30, TimeUnit.SECONDS);
-                 }
-             }
-         } catch (InterruptedException ex) {
-             // terminate
-         }
-     }
-     
-     public void shutdown() {
-         shutdown = true;
-         synchronized (this) {
-             notifyAll();
-         }
-     }
-     
- }
- }}}
-     
- === Connection keep alive strategy ===
- 
-     The HTTP specification does not specify how long a persistent connection may be and should be kept alive. Some HTTP servers use non-standard {{{Keep-Alive}}} header to communicate to the client the period of time in seconds they intend to keep the connection alive on the server side. HttpClient makes use of this information if available. If the {{{Keep-Alive}}} header is not present in the response, HttpClient assumes the connection can be kept alive indefinitely. However, many HTTP servers out there are configured to drop persistent connections after a certain period of inactivity in order to conserve system resources, quite often without informing the client. In case the default strategy turns out to be too optimistic, one may want to provide a custom keep-alive strategy.
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- httpclient.setKeepAliveStrategy(new ConnectionKeepAliveStrategy() {
- 
-     public long getKeepAliveDuration(HttpResponse response, HttpContext context) {
-         // Honor 'keep-alive' header
-         HeaderElementIterator it = new BasicHeaderElementIterator(
-                 response.headerIterator(HTTP.CONN_KEEP_ALIVE));
-         while (it.hasNext()) {
-             HeaderElement he = it.nextElement();
-             String param = he.getName(); 
-             String value = he.getValue();
-             if (value != null && param.equalsIgnoreCase("timeout")) {
-                 try {
-                     return Long.parseLong(value) * 1000;
-                 } catch(NumberFormatException ignore) {
-                 }
-             }
-         }
-         HttpHost target = (HttpHost) context.getAttribute(
-                 ExecutionContext.HTTP_TARGET_HOST);
-         if ("www.naughty-server.com".equalsIgnoreCase(target.getHostName())) {
-             // Keep alive for 5 seconds only
-             return 5 * 1000;
-         } else {
-             // otherwise keep alive for 30 seconds
-             return 30 * 1000;
-         }
-     }
-     
- });
- }}}
- 
- = HTTP state management =
- 
-     Originally HTTP was designed as a stateless, request / response oriented protocol that made no special provisions for stateful sessions spanning across several logically related request / response exchanges. As HTTP protocol grew in popularity and adoption more and more systems began to use it for applications it was never intended for, for instance as a transport for e-commerce applications. Thus, the support for state management became a necessity. 
-     
-     Netscape Communications, at that time a leading developer of web client and server software, implemented support for HTTP state management in their products based on a proprietary specification. Later, Netscape tried to standardise the mechanism by publishing a specification draft. Those efforts contributed to the formal specification defined through the  RFC standard track. However, state management in a significant number of applications is still largely based on the Netscape draft and is incompatible with the official specification. All major developers of web browsers felt compelled to retain compatibility with those applications greatly contributing to the fragmentation of standards compliance.
- 
- == HTTP cookies ==
- 
-     Cookie is a token or short packet of state information that the HTTP agent and the target server can exchange to maintain a session. Netscape engineers used to refer to it as as a "magic cookie" and the name stuck. 
-     
-     HttpClient uses Cookie interface to represent an abstract cookie token. In its simples form an HTTP cookie is merely a name / value pair. Usually an HTTP cookie also contains a number of attributes such as version, a domain for which is valid, a path that specifies the subset of URLs on the origin server to which this cookie applies, and maximum period of time the cookie is valid for.
-     
-     SetCookie interface represents a {{{Set-Cookie}}} response header sent by the origin server to the HTTP agent in order to maintain a conversational state. SetCookie2 interface extends SetCookie with {{{Set-Cookie2}}} specific methods.
-     
-     ClientCookie interface extends Cookie interface with additional client specific functionality such ability to retrieve original cookie attributes exactly as they were specified by the origin server. This is important for generating the {{{Cookie}}} header because some cookie specifications require that the {{{Cookie}}} header should include certain attributes only if they were specified in the {{{Set-Cookie}}} or {{{Set-Cookie2}}} header.
-   
- === Cookie versions ===
- 
-     Cookies compatible with Netscape draft specification but non-compliant with the official specification are considered to be of version 0. Standard compliant cookies are expected to have version 1. HttpClient may handle cookies differently depending on the version.
- 
-     Here is an example of re-creating a Netscape cookie:
-         
- {{{
- BasicClientCookie netscapeCookie = new BasicClientCookie("name", "value");
- netscapeCookie.setVersion(0);
- netscapeCookie.setDomain(".mycompany.com");
- netscapeCookie.setPath("/");
- }}}
- 
-     Here is an example of re-creating a standard cookie. Please note that standard compliant cookie must retain all attributes as sent by the origin server:
- 
- {{{
- BasicClientCookie stdCookie = new BasicClientCookie("name", "value");
- stdCookie.setVersion(1);
- stdCookie.setDomain(".mycompany.com");
- stdCookie.setPath("/");
- stdCookie.setSecure(true);
- // Set attributes EXACTLY as sent by the server 
- stdCookie.setAttribute(ClientCookie.VERSION_ATTR, "1");
- stdCookie.setAttribute(ClientCookie.DOMAIN_ATTR, ".mycompany.com");
- }}}
- 
-     Here is an example of re-creating a Set-Cookie2 compliant cookie. Please note that standard compliant cookie must retain all attributes as sent by the origin server:
- 
- {{{
- BasicClientCookie2 stdCookie = new BasicClientCookie2("name", "value");
- stdCookie.setVersion(1);
- stdCookie.setDomain(".mycompany.com");
- stdCookie.setPorts(new int[] {80,8080});
- stdCookie.setPath("/");
- stdCookie.setSecure(true);
- // Set attributes EXACTLY as sent by the server 
- stdCookie.setAttribute(ClientCookie.VERSION_ATTR, "1");
- stdCookie.setAttribute(ClientCookie.DOMAIN_ATTR, ".mycompany.com");
- stdCookie.setAttribute(ClientCookie.PORT_ATTR, "80,8080");
- }}}
-   
- == Cookie specifications ==
- 
-     CookieSpec interface represents a cookie management specification. Cookie management specification is expected to enforce:
-     
-     * rules of parsing {{{Set-Cookie}}} and optionally {{{Set-Cookie2}}} headers.
-     
-     * rules of validation of parsed cookies.
-     
-     * formatting of {{{Cookie}}} header for a given host, port and path of origin.
- 
-     HttpClient ships with several CookieSpec implementations:
-     
-     * '''Netscape draft''': This specification conforms to the original draft specification published by Netscape Communications. It should be avoided unless absolutely necessary for compatibility with legacy code.
- 
-     * '''RFC 2109''': Older version of the official HTTP state management specification superseded by RFC 2965. 
- 
-     * '''RFC 2965''': The official HTTP state management specification.
- 
-     * '''Browser compatibility''': this implementations strives to closely mimic (mis)behavior of common web browser applications such as Microsoft Internet Explorer and Mozilla FireFox.
- 
-     * '''Best match''':  'Meta' cookie specification that picks up a cookie policy based on the format of cookies sent with the HTTP response. It basically aggregates all above implementations into one class.
-     
-     It is strongly recommended to use the Best Match policy and let HttpClient pick up an appropriate compliance level at runtime based on the execution context.
-   
- == HTTP cookie and state management parameters ==
- 
-     These are parameters that be used to customize HTTP state management and behaviour of individual cookie specifications:
-     
-     * '''http.protocol.cookie-datepatterns''': defines valid date patterns to be used for parsing non-standard {{{expires}}} attribute. Only required for compatibility with non-compliant servers that still use {{{expires}}} defined in the Netscape draft instead of the standard  {{{max-age}}} attribute. This parameter expects a value of type java.util.Collection. The collection elements must be of type java.lang.String compatible with the syntax of java.text.SimpleDateFormat. If this parameter is not set the choice of a default value is CookieSpec implementation specific. Please note this parameter applies 
-     
-     * '''http.protocol.single-cookie-header''': defines whether cookies should be forced into a single {{{Cookie}}} request header. Otherwise, each cookie is formatted as a separate {{{Cookie}}} header. This parameter expects a value of type java.lang.Boolean. If this parameter is not set the choice of a default value is CookieSpec implementation specific. Please note this parameter applies to strict cookie specifications (RFC 2109 and RFC 2965) only. Browser compatibility and netscape draft policies will always put all cookies into one request header. 
- 
-     * '''http.protocol.cookie-policy''': defines the name of a cookie specification to be used for HTTP state management. This parameter expects a value of type java.lang.String. If this parameter is not set valid date patterns are CookieSpec implementation specific.
- 
- == Cookie specification registry ==
- 
-     HttpClient maintains a registry of available cookie specifications using CookieSpecRegistry class. The following specifications are registered per default:
-     
-     * '''compatibility''': Browser compatibility (lenient policy).
- 
-     * '''netscape''': Netscape draft.
- 
-     * '''rfc2109''': RFC 2109 (outdated strict policy).
- 
-     * '''rfc2965''': RFC 2965 (standard conformant strict policy).
- 
-     * '''best-match''': Best match meta-policy.
-     
- == Choosing cookie policy ==
- 
-     Cookie policy can be set at the HTTP client and overridden on the HTTP request level if required.
- 
- {{{
- HttpClient httpclient = new DefaultHttpClient();
- // force strict cookie policy per default
- httpclient.getParams().setParameter(
- 		ClientPNames.COOKIE_POLICY, CookiePolicy.RFC_2965);
- 
- HttpGet httpget = new HttpGet("http://www.broken-server.com/");
- // Override the default policy for this request
- httpget.getParams().setParameter(
- 		ClientPNames.COOKIE_POLICY, CookiePolicy.BROWSER_COMPATIBILITY);
- }}}
- 
- == Custom cookie policy ==
- 
-     In order to implement a custom cookie policy one should create a custom implementation of CookieSpec interface, create a CookieSpecFactory implementation to create and initialize instances of the custom specification and register the factory with HttpClient. Once the custom specification has been registered, it can be activated the same way as the standard cookie specifications.
- 
- {{{
- CookieSpecFactory csf = new CookieSpecFactory() {
-     public CookieSpec newInstance(HttpParams params) {
-         return new BrowserCompatSpec() {   
-             @Override
-             public void validate(Cookie cookie, CookieOrigin origin)
- 	        throws MalformedCookieException {
-                 // Oh, I am easy
-             }		
-         };
-     }	
- };
- 
- DefaultHttpClient httpclient = new DefaultHttpClient();
- httpclient.getCookieSpecs().register("easy", csf);
- httpclient.getParams().setParameter(
-      ClientPNames.COOKIE_POLICY, "easy");
- }}}
- 
- == Cookie persistence ==
- 
-     HttpClient can work with any physical representation of a persistent cookie store that implements the CookieStore interface. The default CookieStore implementation called BasicClientCookie is a simple implementation backed by a java.util.List. Cookies stored in an BasicClientCookie object are lost when the container object get garbage collected. Users can provide more complex implementations if necessary. 
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- // Create a local instance of cookie store
- CookieStore cookieStore = new MyCookieStore();
- // Populate cookies if needed
- BasicClientCookie cookie = new BasicClientCookie("name", "value");
- cookie.setVersion(0);
- cookie.setDomain(".mycompany.com");
- cookie.setPath("/");
- cookieStore.addCookie(cookie);
- // Set the store 
- httpclient.setCookieStore(cookieStore);
- }}}
- 
- == HTTP state management and execution context ==
- 
-     In the course of HTTP request execution HttpClient adds the following state management related objects to the execution context: 
- 
-     * '''http.cookiespec-registry''' - CookieSpecRegistry instance representing the actual cookie specification registry. The value of this attribute set in the local context takes precedence over the default one.
- 
-     * '''http.cookie-spec''' - CookieSpec instance representing the actual cookie specification.
-     
-     * '''http.cookie-origin''' - CookieOrigin instance representing the actual details of the origin server.
-     
-     * '''http.cookie-store''' - CookieStore instance represents the actual cookie store. The value of this attribute set in the local context takes precedence over the default one.
- 
-     The local HttpContext object can be used to customize the HTTP state management context prior to request execution or examine its state after the request has been executed:
-     
- {{{
- HttpClient httpclient = new DefaultHttpClient();
- HttpContext localContext = new BasicHttpContext();
- HttpGet httpget = new HttpGet("http://localhost:8080/"); 
- HttpResponse response = httpclient.execute(httpget, localContext);
- 
- CookieOrigin cookieOrigin = (CookieOrigin) localContext.getAttribute(
- 		ClientContext.COOKIE_ORIGIN);
- System.out.println("Cookie origin: " + cookieOrigin);
- CookieSpec cookieSpec = (CookieSpec) localContext.getAttribute(
- 		ClientContext.COOKIE_SPEC);
- System.out.println("Cookie spec used: " + cookieSpec);
- }}}    
-     
- === Per user / thread state management ===
-     
-     One can use an individual local execution context in order to implement per user (or per thread) state management. Cookie specification registry and cookie store defined in the local context will take precedence over the default ones set at the HTTP client level.
- 
- {{{
- HttpClient httpclient = new DefaultHttpClient();
- // Create a local instance of cookie store
- CookieStore cookieStore = new BasicCookieStore();
- // Create local HTTP context
- HttpContext localContext = new BasicHttpContext();
- // Bind custom cookie store to the local context
- localContext.setAttribute(ClientContext.COOKIE_STORE, cookieStore);
- HttpGet httpget = new HttpGet("http://www.google.com/"); 
- // Pass local context as a parameter
- HttpResponse response = httpclient.execute(httpget, localContext);
- }}}
-     
- = HTTP authentication =
- 
-     HttpClient provides full support for authentication schemes defined by the HTTP standard specification. HttpClient's authentication framework can also be extended to support non-standard authentication schemes such as NTLM and SPNEGO.
- 
- == User credentials ==
-   
-   Any process of user authentication requires a set of credentials that can be used to establish user identity. In the simplest form user crednetials can be just a user name / password pair. UsernamePasswordCredentials represents a set of credentials consisting of a security principal and a password in clear text. This implementation is sufficient for standard authentication schemes defined by the HTTP standard specification. 
-   
- {{{
- UsernamePasswordCredentials creds = new UsernamePasswordCredentials("user", "pwd");
- System.out.println(creds.getUserPrincipal().getName());
- System.out.println(creds.getPassword());
- }}}
- 
- stdout>
- {{{
- user
- pwd
- }}}
- 
-     NTCredentials is a Microsoft Windows specific implementation that includes in addition to the user name / password pair a set of additional Windows specific attributes such as a name of the user domain, as in Microsoft Windows network the same user can belong to multiple domains with a different set of authorizations.
- 
- {{{
- NTCredentials creds = new NTCredentials("user", "pwd", "workstation", "domain");
- System.out.println(creds.getUserPrincipal().getName());
- System.out.println(creds.getPassword());
- }}}
- 
- stdout>
- {{{
- DOMAIN/user
- pwd
- }}}
-     
- == Authentication schemes ==
- 
-     The AuthScheme interface represents an abstract challenge-response oriented authentication scheme. An authentication scheme is expected to support the following functions:
- 
-     * Parse and process the challenge sent by the target server in response to request for a protected resource.
-  
-     * Provide properties of the processed challenge: the authentication scheme type and its parameters, such the realm this authentication scheme is applicable to, if available
-     
-     * Generate authorization string for the given set of credentials and the HTTP request in response to the actual authorization challenge.
- 
-     Please note authentication schemes may be stateful involving a series of challenge-response exchanges.
- 
-     HttpClient ships with several AuthScheme implementations:
-     
-     * '''Basic''': Basic authentication scheme as defined in RFC 2617. This authentication scheme is insecure, as the credentials are transmitted in clear text. Despite its insecurity Basic authentication scheme is perfectly adequate if used in combination with the TLS/SSL encryption.
-   
-     * '''Digest''': Digest authentication scheme as defined in RFC 2617. Digest authentication scheme is significantly more secure than Basic and can be a good choice for those applications that do not want the overhead of full transport security through TLS/SSL encryption.
-   
-     * '''NTLM authentication''': NTLM is a proprietary authentication scheme developed by Microsoft and optimized for Windows platforms. NTLM is believed to be more secure than Digest. This scheme is supported only partially and requires an external NTLM engine. For details please refer to the NTLM_SUPPORT.txt document included with HttpClient distributions.
- 
- == HTTP authentication parameters ==
-     
-     These are parameters that be used to customize HTTP authentication process and behaviour of individual authentication schemes:
-     
-     * '''http.protocol.handle-authentication''': defines whether authentication should be handled automatically. This parameter expects a value of type java.lang.Boolean. If this parameter is not set HttpClient will handle authentication automatically.
- 
-     * '''http.auth.credential-charset''': defines the charset to be used when encoding user credentials. This parameter expects a value of type java.lang.String. If this parameter is not set US-ASCII will be used.
-     
- == Authentication scheme registry ==
- 
-     HttpClient maintains a registry of available authentication scheme using AuthSchemeRegistry class. The following schemes are registered per default:
-     
-     * '''Basic''': Basic authentication scheme.
- 
-     * '''Digest''': Digest authentication scheme.
-         
-     NTLM scheme is not registered per default. For details on how to enable NTLM support please refer to the NTLM_SUPPORT.txt document included with HttpClient distributions.
- 
- == Credentials provider ==
-     
-     Credentials providers are intended to maintain a set of user credentials and to be able to produce user credentials for a particular authentication scope. Authentication scope consists of a host name, a port number, a realm name and an authentication scheme name. When registering credentials with the credentials provider one can provide a wild card (any host, any port, any realm, any scheme) instead of a concrete attribute value. The credentials provider is then expected to be able to find the closest match for a particular scope if the direct match cannot be found. 
-     
-     HttpClient can work with any physical representation of a credentials provider that implements the CredentialsProvider interface. The default CredentialsProvider implementation called BasicCredentialsProvider is a simple implementation backed by a java.util.HashMap.
- 
- {{{
- CredentialsProvider credsProvider = new BasicCredentialsProvider();
- credsProvider.setCredentials(
- 		new AuthScope("somehost", AuthScope.ANY_PORT), 
- 		new UsernamePasswordCredentials("u1", "p1"));
- credsProvider.setCredentials(
- 		new AuthScope("somehost", 8080), 
- 		new UsernamePasswordCredentials("u2", "p2"));
- credsProvider.setCredentials(
- 		new AuthScope("otherhost", 8080, AuthScope.ANY_REALM, "ntlm"), 
- 		new UsernamePasswordCredentials("u3", "p3"));
- 
- System.out.println(credsProvider.getCredentials(
- 		new AuthScope("somehost", 80, "realm", "basic")));
- System.out.println(credsProvider.getCredentials(
- 		new AuthScope("somehost", 8080, "realm", "basic")));
- System.out.println(credsProvider.getCredentials(
- 		new AuthScope("otherhost", 8080, "realm", "basic")));
- System.out.println(credsProvider.getCredentials(
- 		new AuthScope("otherhost", 8080, null, "ntlm")));
- }}}
- 
- stdout>
- {{{
- [principal: u1]
- [principal: u2]
- null
- [principal: u3]
- }}}
- 
- == HTTP authentication and execution context ==
- 
-     HttpClient relies on the AuthState class to keep track of detailed information about the state of the authentication process. HttpClient creates two instances of AuthState in the course of HTTP request execution: one for target host authentication and another one for proxy authentication. In case the target server or the proxy require user authentication the respective AuthScope instance will be populated with the AuthScope, AuthScheme and Crednetials used during the authentication process. The AuthState can be examined in order to find out what kind of authentication was requested, whether a matching AuthScheme implementation was found and whether the credentials provider managed to find user credentials for the given authentication scope.
- 
-     In the course of HTTP request execution HttpClient adds the following authentication related objects to the execution context: 
- 
-     * '''http.authscheme-registry''' - AuthSchemeRegistry instance representing the actual authentication scheme registry. The value of this attribute set in the local context takes precedence over the default one.
- 
-     * '''http.auth.credentials-provider''' - CookieSpec instance representing the actual credentials provider. The value of this attribute set in the local context takes precedence over the default one.
-     
-     * '''http.auth.target-scope''' - AuthState instance representing the actual target authentication state. The value of this attribute set in the local context takes precedence over the default one.
-     
-     * '''http.auth.proxy-scope''' - AuthState instance representing the actual proxy authentication state. The value of this attribute set in the local context takes precedence over the default one.
-     
-     The local HttpContext object can be used to customize the HTTP authentication context prior to request execution or examine its state after the request has been executed:
-     
- {{{
- HttpClient httpclient = new DefaultHttpClient();
- HttpContext localContext = new BasicHttpContext();
- HttpGet httpget = new HttpGet("http://localhost:8080/"); 
- HttpResponse response = httpclient.execute(httpget, localContext);
- 
- AuthState proxyAuthState = (AuthState) localContext.getAttribute(
- 		ClientContext.PROXY_AUTH_STATE);
- System.out.println("Proxy auth scope: " + proxyAuthState.getAuthScope());
- System.out.println("Proxy auth scheme: " + proxyAuthState.getAuthScheme());
- System.out.println("Proxy auth credentials: " + proxyAuthState.getCredentials());
- AuthState targetAuthState = (AuthState) localContext.getAttribute(
- 		ClientContext.TARGET_AUTH_STATE);
- System.out.println("Target auth scope: " + targetAuthState.getAuthScope());
- System.out.println("Target auth scheme: " + targetAuthState.getAuthScheme());
- System.out.println("Target auth credentials: " + targetAuthState.getCredentials());
- }}}    
-     
- === Preemptive authentication ===
- 
-     HttpClient does not support preemptive authentication out of the box, because if misused or used incorrectly the preemptive authentication can lead to significant security issues, such as sending user credentials in clear text to an unauthorized third party. Therefore, users are expected to evaluate potential benefits of preemptive authentication versus security risks in the context of their specific application environment and are required to add support for preemptive authentication using standard HttpClient extension mechanisms such as protocol interceptors.
-     
-     This is an example of a simple protocol interceptor that preemptively introduces an instance of BasicScheme to the execution context, if no authentication has been attempted yet. Please note that this interceptor must be added to the protocol processing chain before the standard authentication interceptors.
-     
- {{{
- HttpRequestInterceptor preemptiveAuth = new HttpRequestInterceptor() {
- 	
-     public void process(
-             final HttpRequest request, 
-             final HttpContext context) throws HttpException, IOException {
-         
-         AuthState authState = (AuthState) context.getAttribute(
-                 ClientContext.TARGET_AUTH_STATE);
-         CredentialsProvider credsProvider = (CredentialsProvider) context.getAttribute(
-                 ClientContext.CREDS_PROVIDER);
-         HttpHost targetHost = (HttpHost) context.getAttribute(
-                 ExecutionContext.HTTP_TARGET_HOST);
-         
-         // If not auth scheme has been initialized yet
-         if (authState.getAuthScheme() == null) {
-             AuthScope authScope = new AuthScope(
-             		targetHost.getHostName(), 
-             		targetHost.getPort());
-             // Obtain credentials matching the target host
-             Credentials creds = credsProvider.getCredentials(authScope);
-             // If found, generate BasicScheme preemptively
-             if (creds != null) {
-                 authState.setAuthScheme(new BasicScheme());
-                 authState.setCredentials(creds);
-             }
-         }
-     }
- 	
- };
- 
- DefaultHttpClient httpclient = new DefaultHttpClient();
- // Add as the very first interceptor in the protocol chain
- httpclient.addRequestInterceptor(preemptiveAuth, 0);
- }}}
-     
- = HTTP client =
- 
-     HttpClient interface represents the most essential contract for HTTP request execution. It imposes no restrictions or particular details on the request execution process and leaves the specifics of connection management, state management, authentication and redirect handling up to individual implementations. This should make it easier to decorate the interface with additional functionality such as response content caching.
- 
-     DefaultHttpClient is the default implementation of the HttpClient interface. This class acts as a facade to a number of special purpose handler or strategy interface implementations responsible for handling of a particular aspect of the HTTP protocol such as redirect or authentication handling or making decision about connection persistence and keep alive duration. This enables the users to selectively replace default implementation of those aspects with custom, application specific ones.
-     
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- 
- httpclient.setKeepAliveStrategy(new DefaultConnectionKeepAliveStrategy() {
- 
- 	@Override
- 	public long getKeepAliveDuration(
- 			HttpResponse response,
- 			HttpContext context) {
- 		long keepAlive = super.getKeepAliveDuration(response, context);
- 		if (keepAlive == -1) {
- 			// Keep connections alive 5 seconds if a keep-alive value 
- 			// has not be explicitly set by the server 
- 			keepAlive = 5000;
- 		}
- 		return keepAlive;
- 	}
- 	
- });
- }}}    
-     
-     DefaultHttpClient also maintains a list of protocol interceptors intended for processing outgoing requests and incoming responses and provides methods for managing those interceptors. New protocol interceptors can be introduced to the protocol processor chain or removed from it if needed. Internally protocol interceptors are stored in a simple java.util.ArrayList. They are executed in the same natural order as they are added to the list.
-     
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- httpclient.removeRequestInterceptorByClass(RequestUserAgent.class);
- httpclient.addRequestInterceptor(new HttpRequestInterceptor() {
- 
- 	public void process(
- 			HttpRequest request, HttpContext context)
- 			throws HttpException, IOException {
- 		request.setHeader(HTTP.USER_AGENT, "My-own-client");
- 	}
- 	
- });
- }}}    
-     
-     DefaultHttpClient is thread safe. It is recommended that the same instance of this class is reused for multiple request executions. When an instance of DefaultHttpClient is no longer needed and is about to go out of scope the connection manager associated with it must be shut down by calling the ClientConnectionManager#shutdown() method.
-     
- {{{
- HttpClient httpclient = new DefaultHttpClient();
- // Do something useful
- httpclient.getConnectionManager().shutdown();
- }}}
-     
- == HTTP client parameters ==
- 
-     These are parameters that be used to customize the behaviour of the default HttpClient implementation:
- 
-     * '''http.protocol.handle-redirects''': defines whether redirects should be handled automatically. This parameter expects a value of type java.lang.Boolean. If this parameter is not HttpClient will handle redirects automatically.
- 
-     * '''http.protocol.reject-relative-redirect''': defines whether relative redirects should be rejected. HTTP specification requires the location value be an absolute URI. This parameter expects a value of type java.lang.Boolean. If this parameter is not set relative redirects will be allowed. 
- 
-     * '''http.protocol.max-redirects''': defines the maximum number of redirects to be followed. The limit on number of redirects is intended to prevent infinite loops caused by broken server side scripts. This parameter expects a value of type java.lang.Integer. If this parameter is not set no more than 100 redirects will be allowed.
- 
-     * '''http.protocol.allow-circular-redirects''': defines whether circular redirects (redirects to the same location) should be allowed. The HTTP spec is not sufficiently clear whether circular redirects are permitted, therefore optionally they can be enabled. This parameter expects a value of type java.lang.Boolean. If this parameter is not set circular redirects will be disallowed.
- 
-     * '''http.connection-manager.factory-class-name''': defines the class name of the default ClientConnectionManager implementation. This parameter expects a value of type java.lang.String. If this parameter is not set SingleClientConnManager will be used per default.
-     
-     * '''http.virtual-host''': defines the virtual host name to be used in the {{{Host}}} header instead of the physical host name. This parameter expects a value of type HttpHost. If this parameter is not set name or IP address of the target host will be used.
-     
-     * '''http.default-headers''': defines the request headers to be sent per default with each request. This parameter expects a value of type ava.util.Collection containing Header objects. 
- 
-     * '''http.default-host''': defines the default host. The default value will be used if the target host is not explicitly specified in the request URI (relative URIs). This parameter expects a value of type HttpHost.
-   
- == Automcatic redirect handling ==
- 
-   HttpClient handles all types of redirects automatically, except those explicitly prohibited by the HTTP specification as requiring user intervention. Redirects on POST and PUT requests are converted to GET requests as required by the HTTP specification.
-   
- == HTTP client and execution context ==
- 
-     The DefaultHttpClient treats HTTP requests as immutable objects that are never supposed to change in the course of request execution. Instead, it creates a private mutable copy of the original request object, whose properties can be updated depending on the execution context. Therefore the final request properties such as the target host and request URI can be determined by examining the content of the local HTTP context after the request has been executed.
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- 
- HttpContext localContext = new BasicHttpContext();
- HttpGet httpget = new HttpGet("http://localhost:8080/"); 
- HttpResponse response = httpclient.execute(httpget, localContext);
- HttpHost target = (HttpHost) localContext.getAttribute(
- 		ExecutionContext.HTTP_TARGET_HOST);
- HttpUriRequest req = (HttpUriRequest) localContext.getAttribute(
- 		ExecutionContext.HTTP_REQUEST);
- 
- System.out.println("Target host: " + target);
- System.out.println("Final request URI: " + req.getURI());
- System.out.println("Final request method: " + req.getMethod());
- }}}
- 
- = Advanced topics =
- 
- == Custom client connections ==
- 
-     In certain situations it may be necessary to customize the way HTTP messages get transmitted across the wire beyond what is possible possible using HTTP parameters in order to be able to deal non-standard, non-compliant behaviours. For instance, for web crawlers it may be necessary to force HttpClient into accepting malformed response heads in order to salvage the content of the messages. 
-     
-     Usually the process of plugging in a custom message parser or a custom connection implementation involves several steps:
- 
-     * Provide a custom LineParser / LineFormatter interface implementation. Implement message parsing / formatting logic as required.
- 
- {{{
- class MyLineParser extends BasicLineParser {
- 
- 	@Override
- 	public Header parseHeader(
- 			final CharArrayBuffer buffer) throws ParseException {
- 		try {
- 			return super.parseHeader(buffer);
- 		} catch (ParseException ex) {
-             // Suppress ParseException exception
- 			return new BasicHeader("invalid", buffer.toString());
- 		}
- 	}
- 	
- }
- }}}
- 
-     * Provide a custom OperatedClientConnection implementation. Replace default request / response parsers, request / response formatters with custom ones as required. Implement different message writing / reading code if necessary.
- 
- {{{    
- class MyClientConnection extends DefaultClientConnection {
- 
- 	@Override
- 	protected HttpMessageParser createResponseParser(
- 			final SessionInputBuffer buffer,
- 			final HttpResponseFactory responseFactory, 
- 			final HttpParams params) {
-         return new DefaultResponseParser(
-         		buffer, 
-         		new MyLineParser(), 
-         		responseFactory, 
-         		params);
- 	}
- 	
- }
- }}}
- 
-     * Provide a custom ClientConnectionOperator interface implementation in order to create connections of new class. Implement different socket initialization code if necessary.
- 
- {{{
- class MyClientConnectionOperator extends DefaultClientConnectionOperator {
- 	
- 	public MyClientConnectionOperator(final SchemeRegistry sr) {
- 		super(sr);
- 	}
- 
- 	@Override
- 	public OperatedClientConnection createConnection() {
- 		return new MyClientConnection();
- 	}
- 	
- }
- }}}
- 
-     * Provide a custom ClientConnectionManager interface implementation in order to create connection operator of new class.
- 
- {{{
- class MyClientConnManager extends SingleClientConnManager {
- 	
-     public MyClientConnManager(
-     		final HttpParams params,
-             final SchemeRegistry sr) {
-     	super(params, sr);
-     }
- 
- 	@Override
- 	protected ClientConnectionOperator createConnectionOperator(
- 			final SchemeRegistry sr) {
- 		return new MyClientConnectionOperator(sr);
- 	}
- 	
- }
- }}}
- 
- == Stateful HTTP connections ==
- 
-     While HTTP specification assumes that session state information is always embedded in HTTP messages in the form of HTTP cookies and therefore HTTP connections are always stateless, this assumption does not always hold true in real life. There are cases when HTTP connections are created with a particular user identity or within a particular security context and therefore cannot be shared with other users and can be reused by the same user only. Examples of such stateful HTTP connections are NTLM authenticated connections and SSL connections with client certificate authentication.
- 
- === User token handler ===
-     
-     HttpClient relies on UserTokenHandler interface to determine if the given execution context is user specific or not. The token object returned by this handler is expected to uniquely identify the current user if the context is user specific or to be null if the context does not contain any resources or details specific to the current user. The user token will be used to ensure that user specific resources will not be shared with or reused by other users.
- 
-     The default implementation of the UserTokenHandler interface uses an instance of Principal class to represent a state object for HTTP connections, if it can be obtained from the given execution context. DefaultUserTokenHandler will use the user principle of connection based authentication schemes such as NTLM or that of the SSL session with client authentication turned on. If both are unavailable, null token will be returned.   
-  
-     Users can provide a custom implementation if the default one does not satisfy their needs:
-     
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- httpclient.setUserTokenHandler(new UserTokenHandler() {
- 
- 	public Object getUserToken(HttpContext context) {
- 		return context.getAttribute("my-token");
- 	}
- 	
- });
- }}} 
- 
- === User token and execution context ===
- 
-     In the course of HTTP request execution HttpClient adds the following user identity related objects to the execution context: 
- 
-     * '''http.user-token''' - Object instance representing the actual user identity, usually expected to be an instance of Principle interface.
-     
-     One can find out whether or not the connection used to execute the request was stateful by examining the content of the local HTTP context after the request has been executed.
- 
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- HttpContext localContext = new BasicHttpContext();
- HttpGet httpget = new HttpGet("http://localhost:8080/"); 
- HttpResponse response = httpclient.execute(httpget, localContext);
- HttpEntity entity = response.getEntity();
- if (entity != null) {
-     entity.consumeContent();
- }
- Object userToken = localContext.getAttribute(ClientContext.USER_TOKEN);
- System.out.println(userToken);
- }}}    
- 
- === Persistent stateful connections ===
- 
-     Please note that persistent connection that carry a state object can be reused only if the same state object is bound to the execution context when requests are executed. So, it is really important to ensure the either same context is reused for execution of subsequent HTTP requests by the same user or the user token is bound to the context prior to request execution.
-     
- {{{
- DefaultHttpClient httpclient = new DefaultHttpClient();
- HttpContext localContext1 = new BasicHttpContext();
- HttpGet httpget1 = new HttpGet("http://localhost:8080/"); 
- HttpResponse response1 = httpclient.execute(httpget1, localContext1);
- HttpEntity entity1 = response1.getEntity();
- if (entity1 != null) {
-     entity1.consumeContent();
- }
- Principal principal = (Principal) localContext1.getAttribute(
- 		ClientContext.USER_TOKEN);
- 
- HttpContext localContext2 = new BasicHttpContext();
- localContext2.setAttribute(ClientContext.USER_TOKEN, principal);
- HttpGet httpget2 = new HttpGet("http://localhost:8080/"); 
- HttpResponse response2 = httpclient.execute(httpget2, localContext2);
- HttpEntity entity2 = response2.getEntity();
- if (entity2 != null) {
-     entity2.consumeContent();
- }
- }}}
- 

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