tomcat 6 thread pool for asynchronous processing

java, multithreading, threadpool, tomcat

Solution

Answering your short question:

In JVM thread pools are abstracted behind `java.util.concurrent.ExecutorService` interface. There are different implementations of this interface but in most cases methods of this interface would suffice.

To create specific thread pool, take a look at `java.util.concurrent.Executors` class: http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/Executors.html which contains static factory methods for creating different implementations of `ExecutorService` interface. You may be interested in `newFixedThreadPool(int threadsNumber)` and `newCachedThreadPool` methods.

For more general information on `Executors` in JVM you may want to read following Oracle's tutorial: http://docs.oracle.com/javase/tutorial/essential/concurrency/executors.html

So, to use thread pool (`ExecutorService`) under Tomcat you should do the following:

.1. Create and register in `web.xml` instance of `javax.servlet.ServletContextListener` interface (which would act like an entry point to your webapplication) if it's not done yet.

.2. In `contextInitialized(ServletContextEvent)` method, you create instance of `ExecutorService` (thread pool) and store it in `ServletContext` attribute map, so that it can be accessed from any point in you webapp e.g.:

// following method is invoked one time, when you web application starts (is deployed)
@Override
public void contextInitialized(ServletContextEvent servletContextEvent) {
    // ...
    final int numberOfThreads = ...;
    final ExecutorService threadPool = Executors.newFixedThreadPool(numberOfThreads); // starts thread pool
    final ServletContext servletContext = servletContextEvent.getServletContext();
    servletContext.setAttribute("threadPoolAlias", threadPool);
    // ...
}

// following method is invoked one time when your web application stops (is undeployed)
public void contextDestroyed(ServletContextEvent servletContextEvent) {
    // following code is just to free resources occupied by thread pool when web application is undeployed
    final ExecutorService threadPool = (ExecutorService) servletContextEvent.getServletContext().getAttribute("threadPoolAlias");
    threadPool.shutdown();
}

.3. Somewhere in `Servlet.service` method or anywhere in your webapp (you should be able to obtain reference to `ServletContext` almost anywhere from webapp):

Callable<ResultOfMyTask> callable = new Callable<ResultOfMyTask>() {
    public ResultOfMyTask call() {
        // here goes your task code which is to be invoked by thread pool 
    }
};

final ServletContext servletContext = ...;
final ExecutorService threadPool = (ExecutorService) servletContext.getAttribute("threadPoolAlias");
final Future<ResultOfMyTask> myTask = threadPool.submit(callable);;

You should store reference to myTask and can query it from other threads to find out whether it's finished and what is the result.

Hope this helps...

Problem

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