Question

public void execute(Runnable command)

This command object contains the submitted object, but it seems to have been wrapped.

How can I access the submitted object from within a custom thread pool executor? Or is it not such a good idea to try and access the submitted object from inside a ThreadPoolExecutor’s before/after/execute methods?

Answer 1

Don’t use execute, use submit, which returns a Future, which is a handle to the task. Here’s some example code:

ExecutorService service = Executors.newCachedThreadPool();

Callable<String> task = new Callable<String>() {
    public String call() throws Exception {
        return "hello world";
    }
};

Future<String> future = service.submit(task);

Although you can’t access the task directly, you can still interact with it:

future.cancel(); // Won't start task if not already started
String result = future.get(); // blocks until thread has finished calling task.call() and returns result
future.isDone(); // true if complete

You can also interact with the service:

service.shutdown(); //etc    

EDITED TO INCORPORATE COMMENTS:

If you want to do some logging, use an anonymous class to override the afterExecute() method, like this:

    ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 1, 1, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(1)) {
        @Override
        protected void afterExecute(Runnable r, Throwable t)
        {
            // Do some logging here
            super.afterExecute(r, t);
        }
    };

Override other methods as required.

Quick plug: IMHO, the bible for this subject is Java Concurrency in Practice – I recommend you buy it and read it.