Asynchronous Servlets in Java: An excessive number of threads running simultaneously in a Java application may consume a lot of resources. This is no exception for a web based Java application. Incoming requests are handled by dedicated HTTP worker threads which will then process those incoming requests until a response is finally assembled and sent back to the client. Considering a web application scenario where the number of simultaneous users is expected to be very high, problems regarding resource consumption is what matters to active threads may become a real problem. HTTP worker threads belong to a dedicated pool, which may become exhausted. Additionally, the threads themselves have to consume their own resources, so incrementing the HTTP thread pool size to a (very) large number will also lead to system resources exhaustion: it simply does not scale. This is where Asynchronous Servlets may be helpful. In short, an asynchronous servlet enables an application to process incoming requests in an asynchronous fashion: A given HTTP worker thread handles an incoming request and then passes the request to another background thread which in turn will be responsible for processing the request and send the response back to the client. The initial HTTP worker thread will return to the HTTP thread pool as soon as it passes the request to the background thread, so it becomes available to process another request. This approach by itself may solve the problem of HTTP thread pool exhaustion, but will not solve the problem of system resources consumption. After all, another background thread was created for processing the request, so the number of simultaneous active threads will not decrease and the system resource consumption will not be improved.

1. Web Component Development
with Servlet & JSP Technologies
(EE 6)
Module-11: Asynchronous Servlets and Clients

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Objectives
Upon completion of this module, you should be able
to:
● Use the Asynchronous Servlet mechanism
● Use JavaScript to send an HTTP request from a client
● Process an HTTP response entirely in JavaScript
● Combine these techniques to create the effect of
server-push

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Relevance
Discussion – The following questions are relevant to
understanding what technologies are available for
developing web applications and the limitations of
those technologies:
● What happens if the response to an HTTP request
cannot be made until a message is rteceived from a
third party? Must the servlet thread be blocked
indefinitely?
● Suppose dozens of clients are cooperating, for
example, in a chat room. Must a request from every
client be blocked waiting until one client triggers an
update?

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Asynchronous Servlets
Java EE 6 adds the option to perform request processing
asynchronously. If this is done, then the servlet
execution thread can be freed up to service requests
from other clients, and the generation of the response
may be performed in another thread, for example the
thread that creates the trigger condition that allows the
response to be prepared.

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Asynchronous Servlets
Java EE 6 adds the option to perform request processing
asynchronously. If this is done, then the servlet
execution thread can be freed up to service requests
from other clients, and the generation of the response
may be performed in another thread, for example the
thread that creates the trigger condition that allows the
response to be prepared.

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Separating Request Receipt from Response
Generation
● To allow for separation of request from response generation, the
servlet API provides a class called AsyncContext. A servlet that
wishes to hand off response generation to another thread can
obtain an AsyncContext object and pass this to another thread,
perhaps using a queue.
● The AsyncContext object provides access to the original
HttpServletRequest and HttpServletResponse objects.
● RequestDispatcher, is achieved using one of serveral dispatch
methods in the AsyncContext itself.

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Asynchronous Servlet Example
protected void processRequest(HttpServletRequest request,
HttpServletResponse response) throws ServletException,
IOException {
AsyncContext ac = request.startAsync();
handler.addJob(ac);
}

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Forwarding and Filtering
Asynchronous handlers are permitted to dispatch or forward their
processing to other servlets or JSPs, including those that are not
asynchronous (that is, they do not carry the asyncSupported=true
element in the annotation). This allows a standard response page to
be used as a view for both synchronous and asynchronous responses.
Filters can be invoked on asynchronous invocations too. If the
AsyncContext.dispatch call is used on a matching URL, the dispatcher
type will by ASYNC

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Asynchronous Listeners
The AsyncListener interface must be implemented by the listener
that will be notified when these situations arise. The interface
defines four listener methods:
● onComplete(AsyncEvent)
● onError(AsyncEvent)
● onTimeout(AsyncEvent)
● onStartAsync(AsyncEvent)

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Asynchronous JavaScript Clients(AJAX)
AJAX is a technique for creating fast and dynamic web
pages.
AJAX allows web pages to be updated asynchronously by
exchanging small amounts of data with the server behind
the scenes. This means that it is possible to update parts
of a web page, without reloading the whole page.

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Combining Asynchronous Servlets With Asynchronous JavaScript
● Consider a page containing JavaScript code that makes an
asynchronous request for update from the server, and a server-side
implementation that uses the asynchronous servlet techniques.
● Now, if the server does not choose to respond for several minutes, the
user is not inconvenienced, as the body of the page is operating
normally.
● Similarly, because of the use of asynchronous servlet execution, the
resource load on the server is minimized. Consequently, the response
may be sent at a time convenient to the server, and the effect of
server-push is achieved in an architecturally manageable way.

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