Question

I have the following Java code:

final Future future = exeService.submit(
    new Runnable() {
        public void run() {
            myObject.doSomething();
        }
    }
);

future.get();

where exeService is an instance of

java.util.concurrent.ExecutorService

The problem is that myObject.doSomething() never returns, and, hence, future.get() never returns.

However, if I replace the call to submit with a call to execute like this:

exeService.execute(
    new Runnable() {
        public void run() {
            myObject.doSomething();
        }
    }
);

the call to myObject.doSomething() does return. I don’t know if it matters, but doSomething() is a void method.

Why is doSomething() finishing when using execute but not when using submit?

Also, I don’t need to use Future.get(); that just seemed to be the most natural way of doing this. (I also run into the same problem with CountdownLatch.) The point is that I need to wait for doSomething() to finish before proceeding, and, for complicated reasons I won’t go into here, I need to launch it on a separate thread. If there is another way of doing this that works, that would be fine.

Answer 1

As in Executor.execute() Javadoc:

Executes the given command at some
time in the future. The command may
execute in a new thread, in a pooled
thread, or in the calling thread, at
the discretion of the Executor
implementation.

So, the method execute() returns immediately leaving you with no option to query to status of submitted task.

On the other hand ExecutorService.submit():

Submits a Runnable task for execution
and returns a Future representing that
task. The Future’s get method will
return null upon successful
completion.

The Future.get() will return only after successful competion, so never in your case.

This is further noted in Future.get() documentation:

Waits if necessary for the computation
to complete, and then retrieves its
result.