How to stop an executor thread when Tomcat is stopped?

Question

I am using an executor service by creating a fixed number of threads to do a HTTP GET data retrieval.

executorService = ExecutorServiceFactory.getInstance().getExecutorService(Config.getInstance().getNumberOfThreads(), "ThreadExecutor_"+UIDFactory.getInstance().createSessionID());
executorService.execute(new Retrieve(data));

private class Retrieve implements Runnable{
    private Vector<String> data;

    public WADORetrieve(Vector<String> data) {          
            this.data = data;
    }

    @Override
    public void run() {
        for(int i=0;i<data.length;i++){
            fetch(data[i]);
        }
    }
}    

When Tomcat is stopped we get this error:

SEVERE: The web application [/viewer] appears to have started a thread named [ThreadExecutor_5161616156] but has failed to stop it. This is very likely to create a memory leak.

Is this a real issue? What can I do to stop tomcat properly without these service errors.

Answer 1

It's a real problem. Non-daemon threads pooled by your executor don't go away by themselves, if you don't call shutdown on the executor then the threads will stay alive and prevent the JVM from exiting.

Make a ServletContextListener that calls shutdown on your executor on contextDestroyed.

Alternatively you could make the threads returned by your ThreadFactory daemon threads, so that they would exit when the last non-daemon thread terminates. Since you're doing data retrieval it would seem better to avoid this since it could leave database resources unclosed.