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From "Terence M. Bandoian" <tere...@tmbsw.com>
Subject Re: Initiating httpservletrequest from inside Tomcat / TomEE
Date Mon, 06 May 2019 18:36:37 GMT
On 5/6/2019 10:45 AM, Paul Carter-Brown wrote:
> Yea, but the issue is that only works when calling in the context of a
> current servlet call.
>
> Here is the kind of problem I want to solve:
>
> @WebServlet(name = "MyExample", urlPatterns = {"/example"}, loadOnStartup =
> 1)
> public class Example extends HttpServlet {
>
>      @PersistenceContext
>      private EntityManager em;
>
>      @Override
>      public void init(ServletConfig config) {
>          Thread t = new Thread(() -> {
>              while (true) {
>                  try {
>                      // Do a GET to /example/ and get the response without
> going out on localhost and back in....
>                      // We cant just call doGet as we want the request to
> flow through the servlet filters, do the entitymanager injection etc
>                      Thread.sleep(10000);
>                  } catch (Exception e) {
>                  }
>              }
>          });
>          t.start();
>
>      }
>
>      @Override
>      protected void doGet(HttpServletRequest req, HttpServletResponse resp)
> throws ServletException, IOException {
>          // do stuff like use em
>          resp.setStatus(200);
>          resp.getWriter().write("Hello World");
>      }
>
> }
>


If I understand the problem you're trying to solve, it seems like there 
could be two ways to approach it.

1) Can your service be initialized with the context information it needs 
to use the request dispatcher?
2) Can the servlets you want to use be refactored so that the underlying 
functionality is exposed in a way that doesn't require any context?

Hope that helps.

-Terence


>
>
> On Mon, May 6, 2019 at 5:35 PM John Dale <jcdwrrc@gmail.com> wrote:
>
>> For reference, I did find this after searching "calling a servlet
>> programmatically":
>> https://docs.oracle.com/cd/E19146-01/819-2634/abxbn/index.html
>>
>> On 5/6/19, Paul Carter-Brown <paul.carter-brown@jini.guru> wrote:
>>> I think we are completely missing each other. Forget sockets - that was
>>> just an example. I have code running in a Tomcat App server which is not
>>> managed by Tomcat and is not initiated by anything within Tomcat. That
>> code
>>> now wants to call a servlet hosted in that very same JVM. Any way to do
>>> that without going out and back in on TCP?
>>>
>>>
>>> On Mon, May 6, 2019 at 5:14 PM John Dale <jcdwrrc@gmail.com> wrote:
>>>
>>>> Sockets are an implementation of TCP/UDP inherently.
>>>>
>>>> Perhaps a mountaintop signal fire?
>>>>
>>>> ;)
>>>>
>>>> John
>>>>
>>>>
>>>> On 5/6/19, Paul Carter-Brown <paul.carter-brown@jini.guru> wrote:
>>>>> lol on the Semaphore Telegraph,
>>>>>
>>>>> I can't use a request dispatcher as the request is being initiated
>> from
>>>>> code that has no context. I already have it working with HTTP using
>>>>> asynchttp library, but I want to avoid the overhead. E.g. lets say I
>>>> wrote
>>>>> my own server socket listener on port 10000 running in the Tomcat JVM
>>>>> and
>>>>> got some request in some propriatary protocol called X. Now I want to
>>>> call
>>>>> a Tomcat servlet in the current JVM with some info I got over X
>> without
>>>>> going out on TCP and back in....
>>>>>
>>>>> On Mon, May 6, 2019 at 4:40 PM John Dale <jcdwrrc@gmail.com> wrote:
>>>>>
>>>>>> If you're wanting to forward control to another servlet deployed
in
>>>>>> the same context:
>>>>>> https://www.javatpoint.com/requestdispatcher-in-servlet
>>>>>>
>>>>>> If you are okay going through TCP to facilitate some future or
>> current
>>>>>> distribution of services, Use HTTPURLConnection (not sure what you're
>>>>>> wanting to do with the result of the request, if anything):
>>>>>>
>>>>>>
>> https://stackoverflow.com/questions/2793150/how-to-use-java-net-urlconnection-to-fire-and-handle-http-requests
>>>>>> If you need more sophisticated HTTP interactions, Apache maintains
a
>>>>>> very useful library for that:  http://hc.apache.org/
>>>>>>
>>>>>> If these don't work-out for you, rather than using .NET, PHP, Python,
>>>>>> or some other Java facsimile at best, I recommend using the semaphore
>>>>>> telegraph:
>>>>>> https://en.wikipedia.org/wiki/Semaphore_telegraph
>>>>>>
>>>>>> Sincerely,
>>>>>>
>>>>>> John
>>>>>> DB2DOM
>>>>>>
>>>>>> On 5/6/19, Paul Carter-Brown <paul.carter-brown@jini.guru>
wrote:
>>>>>>> Hi John,
>>>>>>>
>>>>>>> Thanks for your feedback.
>>>>>>>
>>>>>>> The request I'm initiating should not or need not carry any context
>>>>>>> from
>>>>>>> the originating code. There is also no session to worry about
as
>> its
>>>>>>> just
>>>>>>> for rest calls. So basically I have the headers, path and body
and
>>>> need
>>>>>> to
>>>>>>> generate a http servlet request and get an http servlet response
>> (or
>>>>>>> similar) back. I have this working by calling into localhost
but
>>>>>>> ideally
>>>>>>> want to skip the trombone out and back in.
>>>>>>>
>>>>>>> Have you got any basic code examples?
>>>>>>>
>>>>>>> Paul
>>>>>>>
>>>>>>> On Tue, Apr 30, 2019 at 5:27 PM John Dale <jcdwrrc@gmail.com>
>> wrote:
>>>>>>>> Another thought .. you can do some request dispatching, but
>> without
>>>>>>>> knowing more about the tools you're using, I can't say for
sure if
>>>>>>>> this is the direction you'll want to go.
>>>>>>>>
>>>>>>>> On 4/29/19, Paul Carter-Brown <paul.carter-brown@jini.guru>
>> wrote:
>>>>>>>>> Hi
>>>>>>>>>
>>>>>>>>> I'm trying to design a Kafka consumer and producer that
will run
>>>>>> inside
>>>>>>>> the
>>>>>>>>> tomcat jvm and pick up messages off a Kafka topic and
translate
>>>> them
>>>>>>>> into a
>>>>>>>>> servlet request and pass it through tomcat and then when
the
>>>>>>>>> response
>>>>>>>>> is
>>>>>>>>> complete then translate it into a Kafka message and put
it onto
>>>>>> another
>>>>>>>>> topic as a reply. This way I can reuse our existing jax-rs
rest
>>>>>>>>> services
>>>>>>>>> and expose them as an async api over Kafka. The idea
is to make
>>>>>>>>> the
>>>>>>>>> Kafka
>>>>>>>>> messages similar to http in that they would consist of
headers
>>>>>>>>> and
>>>> a
>>>>>>>> body.
>>>>>>>>> The body would be json.
>>>>>>>>>
>>>>>>>>> Now I know this could be done by calling localhost with
an http
>>>> call
>>>>>> to
>>>>>>>>> trombone the requests back into tomcat but I'd like to
avoid the
>>>>>>>> associated
>>>>>>>>> latency and overhead. Is it possible to call tomcat directly
>>>>>>>>> in-process.
>>>>>>>>> This does not need to be portable to other containers
so can be
>>>>>>>>> proprietary.
>>>>>>>>>
>>>>>>>>> I'm using tomcat 8. In fact its tomee 8 but guessed this
is more
>>>>>>>>> a
>>>>>>>>> tomcat
>>>>>>>>> question than tomee but have sent to both groups just
in case.
>>>>>>>>>
>>>>>>>>> Thanks for any insights.
>>>>>>>>>
>>>>>>>>> Paul
>>>>>>>>>
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>>>>>>
>>>>>>
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>>>>
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