Async Servlet Example

Async servlet was introduced in Servlet 3. It’s a great way to deal with thread starvation problem with long running threads.

Async Servlet

Before we jump into understanding what Async Servlet is, let’s try to understand why do we need it.

Let’s say we have a Servlet that takes a lot of time to process, something like below.

package com.journaldev.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/LongRunningServlet")
public class LongRunningServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;

	protected void doGet(HttpServletRequest request,
			HttpServletResponse response) throws ServletException, IOException {
		long startTime = System.currentTimeMillis();
		System.out.println("LongRunningServlet Start::Name="
				+ Thread.currentThread().getName() + "::ID="
				+ Thread.currentThread().getId());

		String time = request.getParameter("time");
		int secs = Integer.valueOf(time);
		// max 10 seconds
		if (secs > 10000)
			secs = 10000;

		longProcessing(secs);

		PrintWriter out = response.getWriter();
		long endTime = System.currentTimeMillis();
		out.write("Processing done for " + secs + " milliseconds!!");
		System.out.println("LongRunningServlet Start::Name="
				+ Thread.currentThread().getName() + "::ID="
				+ Thread.currentThread().getId() + "::Time Taken="
				+ (endTime - startTime) + " ms.");
	}

	private void longProcessing(int secs) {
		// wait for given time before finishing
		try {
			Thread.sleep(secs);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

}

If we hit above servlet through browser with URL as http://localhost:8080/AsyncServletExample/LongRunningServlet?time=8000, we get response as “Processing done for 8000 milliseconds!!” after 8 seconds. Now if you will look into server logs, you will get following log:

LongRunningServlet Start::Name=http-bio-8080-exec-34::ID=103
LongRunningServlet Start::Name=http-bio-8080-exec-34::ID=103::Time Taken=8002 ms.

So our Servlet Thread was running for ~8+ seconds, although most of the processing has nothing to do with the servlet request or response.

This can lead to Thread Starvation – since our servlet thread is blocked until all the processing is done. If server gets a lot of requests to process, it will hit the maximum servlet thread limit and further requests will get Connection Refused errors.

Prior to Servlet 3.0, there were container specific solution for these long running threads where we can spawn a separate worker thread to do the heavy task and then return the response to client. The servlet thread returns to the servlet pool after starting the worker thread. Tomcat’s Comet, WebLogic’s FutureResponseServlet and WebSphere’s Asynchronous Request Dispatcher are some of the example of implementation of asynchronous processing.

The problem with container specific solution is that we can’t move to other servlet container without changing our application code, that’s why Async Servlet support was added in Servlet 3.0 to provide standard way for asynchronous processing in servlets.

Asynchronous Servlet Implementation

Let’s see steps to implement async servlet and then we will provide async supported servlet for above example.

First of all the servlet where we want to provide async support should have @WebServlet annotation with asyncSupported value as true.
Since the actual work is to be delegated to another thread, we should have a thread pool implementation. We can create thread pool using Executors framework and use servlet context listener to initiate the thread pool.
We need to get instance of AsyncContext through ServletRequest.startAsync() method. AsyncContext provides methods to get the ServletRequest and ServletResponse object references. It also provides method to forward the request to another resource using dispatch() method.
We should have a Runnable implementation where we will do the heavy processing and then use AsyncContext object to either dispatch the request to another resource or write response using ServletResponse object. Once the processing is finished, we should call AsyncContext.complete() method to let container know that async processing is finished.
We can add AsyncListener implementation to the AsyncContext object to implement callback methods – we can use this to provide error response to client incase of error or timeout while async thread processing. We can also do some cleanup activity here.

Once we will complete our project for Async servlet Example, it will look like below image.

Initializing Worker Thread Pool in Servlet Context Listener

package com.journaldev.servlet.async;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class AppContextListener implements ServletContextListener {

	public void contextInitialized(ServletContextEvent servletContextEvent) {

		// create the thread pool
		ThreadPoolExecutor executor = new ThreadPoolExecutor(100, 200, 50000L,
				TimeUnit.MILLISECONDS, new ArrayBlockingQueue<Runnable>(100));
		servletContextEvent.getServletContext().setAttribute("executor",
				executor);

	}

	public void contextDestroyed(ServletContextEvent servletContextEvent) {
		ThreadPoolExecutor executor = (ThreadPoolExecutor) servletContextEvent
				.getServletContext().getAttribute("executor");
		executor.shutdown();
	}

}

The implementation is pretty straight forward, if you are not familiar with Executors framework, please read Thread Pool Executor.

For more details about listeners, please go through Servlet Listener Example.

Worker Thread Implementation

package com.journaldev.servlet.async;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.AsyncContext;

public class AsyncRequestProcessor implements Runnable {

	private AsyncContext asyncContext;
	private int secs;

	public AsyncRequestProcessor() {
	}

	public AsyncRequestProcessor(AsyncContext asyncCtx, int secs) {
		this.asyncContext = asyncCtx;
		this.secs = secs;
	}

	@Override
	public void run() {
		System.out.println("Async Supported? "
				+ asyncContext.getRequest().isAsyncSupported());
		longProcessing(secs);
		try {
			PrintWriter out = asyncContext.getResponse().getWriter();
			out.write("Processing done for " + secs + " milliseconds!!");
		} catch (IOException e) {
			e.printStackTrace();
		}
		//complete the processing
		asyncContext.complete();
	}

	private void longProcessing(int secs) {
		// wait for given time before finishing
		try {
			Thread.sleep(secs);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

Notice the use of AsyncContext and it’s usage in getting request and response objects and then completing the async processing with complete() method call.

AsyncListener Implementation

package com.journaldev.servlet.async;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebListener;

@WebListener
public class AppAsyncListener implements AsyncListener {

	@Override
	public void onComplete(AsyncEvent asyncEvent) throws IOException {
		System.out.println("AppAsyncListener onComplete");
		// we can do resource cleanup activity here
	}

	@Override
	public void onError(AsyncEvent asyncEvent) throws IOException {
		System.out.println("AppAsyncListener onError");
		//we can return error response to client
	}

	@Override
	public void onStartAsync(AsyncEvent asyncEvent) throws IOException {
		System.out.println("AppAsyncListener onStartAsync");
		//we can log the event here
	}

	@Override
	public void onTimeout(AsyncEvent asyncEvent) throws IOException {
		System.out.println("AppAsyncListener onTimeout");
		//we can send appropriate response to client
		ServletResponse response = asyncEvent.getAsyncContext().getResponse();
		PrintWriter out = response.getWriter();
		out.write("TimeOut Error in Processing");
	}

}

Notice the implementation of onTimeout() method where we are sending timeout response to client.