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From da...@cocoon.zones.apache.org
Subject [DAISY] Created: Error Handling
Date Thu, 07 Jun 2007 16:23:57 GMT
A new document has been created.

http://cocoon.zones.apache.org/daisy/documentation/1379.html

Document ID: 1379
Branch: main
Language: default
Name: Error Handling
Document Type: Cocoon Document
Created: 6/7/07 4:23:54 PM
Creator (owner): Reinhard Pötz
State: publish

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<h1>Error Handling</h1>

<p>During the execution of a Cocoon pipeline exceptions may occur within the
involved components like generators, transformers etc. There are two
possibilities to deal with them: The one would be not to handle them explicitly
in the sitemap, which causes them to be logged and a default Cocoon error page
to be displayed in the browser. The second is to define an error handling by
using the sitemap tag &lt;map:handle-errors&gt;. Therein you are able to define
any pipeline, that is executed in case of an exception occurred and displays an
appropriate page.</p>

<h2>ExceptionSelector</h2>

<p>The ExceptionSelector allows to realize conditional error handling within
&lt;map:handle-errors&gt;-tags depending on the type of the occurred exception.
Each exception is configured centrally at the selector in the sitemap by
associating a symbolic name to its class.</p>

<p>Furthermore it is possible to define, what exceptions are to be "unrolled".
This means, that if an exception has been rethrown embedded in another
exception, this original exception can be considered for choosing the correct
error handling.</p>

<p>Example:</p>

<pre>          
&lt;map:selector name="exception" src="org.apache.cocoon.selection.ExceptionSelector"&gt;
  &lt;exception name="processing" class="ProcessingException" unroll="true"/&gt;
  &lt;exception name="sax" class="SAXException"/&gt;
  &lt;exception name="application" class="ApplicationException"/&gt;
&lt;/map:selector&gt;
...
&lt;map:pipeline&gt;
  &lt;map:match pattern="resource"&gt;
    ...
  &lt;/map:match&gt;
  &lt;map:handle-errors&gt;
    &lt;map:select type="exception"&gt;
      &lt;map:when test="processing"&gt;...&lt;/map:when&gt;
      &lt;map:when test="sax"&gt;...&lt;/map:when&gt;
      &lt;map:when test="application"&gt;...&lt;/map:when&gt;
    &lt;/map:select&gt;
  &lt;/map:handle-errors&gt;
&lt;/map:pipeline&gt;
          
        </pre>

<p>Let's consider the following nested exceptions to occur:</p>

<ol>
<li>ProcessingException ( ApplicationException ): The ProcessingException is
unrolled, so the error pipeline for "application" will be executed.</li>
<li>ProcessingException ( ValidationException ): Since ValidationException is
not configured at all and therefore unknown, the ProcessingException is not
unrolled even if unrolling is enabled. Therefore the pipeline for "processing"
will be executed.</li>
<li>SAXException ( ApplicationException ): The unrolling is not enabled for
SAXException, so the pipeline for "sax" will be executed.</li>
</ol>

<p>Please notice that the selector configuration is processed from top to bottom
and stops at the first matching exception. Therefore the most specific classes
must be configured first. This behaviour is the same as with Java catch
statements.</p>

<h2>XPathExceptionSelector</h2>

<p>The XPathExceptionSelector is an extension to the standard selector described
above. It adds the possibility to configure additional conditions for each
exception type by using JXPath expressions, that operate on the exception
object. This configuration is also done centrally at the selector in the
sitemap, where symbolic names are defined for all specific error situations.
</p>

<p>Example:</p>

<pre>          
&lt;map:selector name="exception" src="org.apache.cocoon.selection.XPathExceptionSelector"&gt;
  &lt;exception name="Denied" class="AuthenticationFailure"&gt;
    &lt;xpath name="PasswordWrong" test="authCode=10"/&gt;
    &lt;xpath name="PasswordExpired" test="errorCode=11"/&gt;
    &lt;xpath name="AccessForbidden" test="errorCode&amp;gt;11"/&gt;
  &lt;/exception&gt;
&lt;/map:selector&gt;
...
&lt;map:pipeline&gt;
  &lt;map:match pattern="login"&gt;
    ...
  &lt;/map:match&gt;
  &lt;map:handle-errors&gt;
    &lt;map:select type="exception"&gt;
      &lt;map:when test="PasswordWrong"&gt;...&lt;/map:when&gt;
      &lt;map:when test="PasswordExpired"&gt;...&lt;/map:when&gt;
      &lt;map:when test="AccessForbidden"&gt;...&lt;/map:when&gt;
      &lt;map:when test="Denied"&gt;...&lt;/map:when&gt;
      &lt;map:otherwise&gt;...&lt;/map:otherwise&gt;
    &lt;/map:select&gt;
  &lt;/map:handle-errors&gt;
&lt;/map:pipeline&gt;
          
        </pre>

<p>In this example the exception AuthenticationFailure is configured under name
"Denied". Additionally three further conditions "PasswordWrong",
"PasswordExpired" and "AccessForbidden" are defined by using JXPath expressions.
Therefore instances of AuthenticationFailure are expected to have methods
getAuthCode() and getErrorCode(). Now the error handler defined for resource
"login" has five branches: If situation "PasswordWrong" occurs, which means that
an AuthenticationFailure exception with auth code 10 has been thrown, the first
error pipeline is executed. If the error code equals to 11 the second pipeline
is executed, if it is greater that 11 the third one and all other
AuthenticationFailure errors are handled by the fourth one. In any other error
situation the fifth branch would be chosen.</p>

<p>Please notice that the selector stops when it finds the first JXPath
expression in the configuration that matches:</p>

<p>Example:</p>

<pre>          
  &lt;map:selector name="exception" src="org.apache.cocoon.selection.XPathExceptionSelector"&gt;
    &lt;exception name="application" class="ApplicationException"&gt;
      &lt;xpath name="error3" test="errorCode&amp;gt;3"/&gt;
      &lt;xpath name="error6" test="errorCode&amp;gt;6"/&gt;
    &lt;/exception&gt;
  &lt;/map:selector&gt;
  ...
  &lt;map:pipeline&gt;
    &lt;map:match pattern="processForm"&gt;
      ...
    &lt;/map:match&gt;
    &lt;map:handle-errors&gt;
      &lt;map:select type="exception"&gt;
        &lt;map:when test="error6"&gt;...&lt;/map:when&gt; &lt;!-- handler
1 --&gt;
        &lt;map:when test="error3"&gt;...&lt;/map:when&gt; &lt;!-- handler
2 --&gt;
      &lt;/map:select&gt;
    &lt;/map:handle-errors&gt;
  &lt;/map:pipeline&gt;
          
        </pre>

<p>If an ApplicationException with error code 9 occurs, handler 2 is executed
since error situation "error3" is configured before "error6" at the selector
even if the expression for "error6" also evaluates to "true".</p>

<h2>Error Handler Hierarchy</h2>

<p>The tag &lt;map:handle-errors&gt; may be attached to any &lt;map:pipeline&gt;
or the &lt;map:pipelines&gt; tag of the root sitemap or a subsitemap. Therefore
it is possible to define two kinds of error handlers: A default handler may be
defined within &lt;map:pipelines&gt; for applying to all resources of a sitemap.
Alternatively individual handlers may be configured for sets of resources within
&lt;map:pipeline&gt;.</p>

<p>Example:</p>

<pre>          
&lt;map:pipelines&gt;
  &lt;map:pipeline name="pipe1"&gt;
    &lt;map:match pattern="res1"&gt;
      ...
    &lt;/map:match&gt;
    &lt;map:handle-errors&gt;
      &lt;!-- this is an individual handler for pipe1 --&gt;
    &lt;/map:handle-errors&gt;
  &lt;/map:pipeline&gt;
  &lt;map:pipeline name="pipe2"&gt;
    &lt;map:match pattern="res2"&gt;
      ...
    &lt;/map:match&gt;
  &lt;/map:pipeline&gt;
  &lt;map:pipeline name="pipe3"&gt;
    &lt;map:match pattern="res3"&gt;
      ...
    &lt;/map:match&gt;
  &lt;/map:pipeline&gt;
  &lt;map:handle-errors&gt;
    &lt;!-- this is the default handler for the whole sitemap --&gt;
  &lt;/map:handle-errors&gt;
&lt;/map:pipelines&gt;
          
        </pre>

<p>In conjunction with the ExceptionSelector resp. the XPathExceptionSelector it
is possible to define a hierarchy of error handlers for an application. The
behaviour then is the following: If an error occurs within a pipeline, Cocoon at
first checks if an individual handler for this pipeline is defined. If so and it
is able to handle the error due to its selection the processing terminates.
Otherwise Cocoon looks for a default handler of the current sitemap. If one is
found it is called. Now there is the same behaviour as above: If it can handle
the exception the processing terminates otherwise the searching proceeds within
the pipeline where the subsitemap is mounted. This goes on until the default
handler of the root sitemap has been considered. If an error could not be
handled at all, it is processed by the Cocoon engine in the end.</p>

<p>Please notice that a &lt;map:otherwise&gt; breaks the hierarchy since all
errors will be handled on this level. Therefore all levels above will be called
never.</p>

<p>Example:</p>

<pre>          
Root sitemap:
&lt;map:pipelines&gt;
  &lt;map:pipeline&gt;
    &lt;map:mount uri-prefix="sub" src="sub/"/&gt;
    &lt;map:handle-errors&gt;
      &lt;map:select type="exception"&gt;
        &lt;map:when test="resourceNotFound"&gt;...&lt;/map:when&gt;
      &lt;/map:select&gt;
    &lt;/map:handle-errors&gt;
  &lt;/map:pipeline&gt;
  &lt;map:handle-errors&gt;
    &lt;map:generate src="generalerror.htm"/&gt;
    &lt;map:serialize/&gt;
  &lt;/map:handle-errors&gt;
&lt;/map:pipelines&gt;

Subsitemap:
&lt;map:pipelines&gt;
  &lt;map:pipeline&gt;
    &lt;map:match pattern="processForm"&gt;
      ...
    &lt;/map:match&gt;
    &lt;map:handle-errors&gt;
      &lt;map:select type="exception"&gt;
        &lt;map:when test="validation"&gt;...&lt;/map:when&gt;
      &lt;/map:select&gt;
    &lt;/map:handle-errors&gt;
  &lt;/map:pipeline&gt;
  &lt;map:handle-errors&gt;
    &lt;map:select type="exception"&gt;
      &lt;map:when test="application"&gt;...&lt;/map:when&gt;
    &lt;/map:select&gt;
  &lt;/map:handle-errors&gt;
&lt;/map:pipelines&gt;
          
        </pre>

<p>Let's consider four situations concerning the above example:</p>

<ol>
<li>A ValidationException occurs, because for instance the user entered an
invalid value: The defined pipeline's handler is called. Since it has a matching
&lt;map:when&gt;-section it is able to handle such an exception and therefore
the processing is finished.</li>
<li>An ApplicationException occurs, because for instance a database connection
has failed: The pipeline's handler is not able to handle the exception, so next
the subsitemap's default handler is called. It has a matching
&lt;map:when&gt;-section and is therefore able to handle the exception.</li>
<li>A ResourceNotFoundException occurs, because for instance some file is
missing. Both the pipeline's and the subsitemaps' handlers are not able to
handle it. Now Cocoon proceeds after the mount point of the subsitemap and finds
its pipeline's handler next. It is able to handle a ResourceNotFoundException
and therefore produces the output in this case.</li>
<li>A NullPointerException occurs, because something went completely wrong in
the application: All handlers are not configured for such an exception and so
the root sitemaps default handler will apply to it showing a general error page.
</li>
</ol>

<p>When handling exceptions in error handlers one has to take care about
recursion when working with redirects. Consider the following sitemap:</p>

<p>Example:</p>

<pre>          
&lt;map:pipelines&gt;
  &lt;map:pipeline&gt;
    &lt;map:match pattern="resource"&gt;
      ...
      &lt;map:transformer type="foo"/&gt;
      ...
    &lt;/map:match&gt;
    &lt;map:match pattern="error"&gt;
      ...
      &lt;map:transformer type="foo"/&gt;
      ...
    &lt;/map:match&gt;
    &lt;map:handle-errors&gt;
      &lt;map:select type="exception"&gt;
        &lt;map:when test="connection"&gt;
          &lt;map:act type="redirect" src="cocoon:/error"/&gt;
        &lt;/map:when&gt;
      &lt;/map:select&gt;
    &lt;/map:handle-errors&gt;
  &lt;/map:pipeline&gt;
&lt;/map:pipelines&gt;
          
        </pre>

<p>This configuration may lead to an infinite loop: Imagine to call "resource"
where the FooTransformer throws a ConnectionException, because the connection to
a backend system has broken. The defined error handler will handle it and the
used action internally redirects to resource "error". This resource itself uses
the FooTransformer to get some data from the backend, which of cause also causes
a ConnectionException. This is handled by the error handler, which redirects to
resource "error" and so on. Such an infinite loop may also occur when using
several "nested" redirects, i.e. the error handler redirects to a resource,
which redirects to another resource, which might produce the original exception.
</p>

<p>When defining error handlers for an application such situation must be
avoided. An easy rule would be: An error handling routine must never redirect to
a resource for which the routine itself is responsible and which might produce
the same error as just handled.</p>

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