Question

How do threads that rely on one another communicate in Java?

For example, I am building a web crawler with threads that need data that comes from other threads.

Answer 1

That depends on the nature of the communication.

• Is it duplex (ie A talks to B and B talks to A)?
• Is it communication of data or communication of completion?

and so on.

The simplest and most advisable form of inter-thread communication is simply to wait for the completion of other threads. That’s most easily done by using Future:

ExecutorService exec = Executors.newFixedThreadPool(50);
final Future f = exec.submit(task1);
exec.submit(new Runnable() {
    @Override
    public void run() {
        f.get();
        // do stuff
    }
});

The second task won’t execute until the first completes.

Java 5+ has many concurrent utilities for dealing with this kind of thing. This could mean using LinkedBlockingQueues, CountDownLatch or many, many others.

For an in-depth examination of concurrency Java Concurrency in Practice is a must-read.