How do you hang a thread in Java in one line?

Question

By one line I mean at most 100 chars per line.

(I basically need this to keep the program alive. The main thread registers callback listeners that are run in separate threads. I just need the main one to hang forever and let the other threads do their work)

Answer 1

synchronized(this) {
    while (true) {
        this.wait();
    }
}

(thanks to Carlos Heuberger. Exception handling omitted in the above code)

This will make the current thread wait on the monitor of the current class until someone calls notify(), or forever.

Answer 2

There are a few things you could do that would be better than hanging the initial thread forever:

1. Use otherThread.join(). This will cause the current thread you are running in to sleep until the other thread has finished executing.
2. As @nanda suggests, use ExcecutorService.shutdown() to wait until a pool of threads has finished.
3. Use otherThread.setDaemon(false) and simply let your initial thread exit. This will set your new threads as user threads. Java will not shut down until the only threads running are daemon threads.

Answer 3

Thread.sleep(Long.MAX_VALUE);

Ok, so it isn't forever, but talk about a really long time :)

Answer 4

Use executor. By using method shutdown() you'll force the executor to wait until all threads are finished.

Answer 5

With a CountDownLatch you can wait untill the count down reached 0, if you make sure it never counts down, maybe only when it needs to end. (This also result in 0% cpu, the opposite of loops that will run forever, and with join() your app will still finish when all other threads finished, The option of the executor is better, but will also end when all executed task have finished)

Answer 6

You can use thread.join to wait for all of the threads.

Answer 7

Here is a solution that is a one-liner, in that you only have to add one extra line. (You do have to add synchronized and throws InterruptedException to your main declaration though.) Also, it does not need access to, or even knowledge of the threads in the library you are using.

public static synchronized void main(String[] args) throws InterruptedException{
  ...
  YourMainClass.class.wait();  // wait forever
}

It assumes you will never call notify on your main class and that you want to exit if you get an InterruptedException. (You can add a while (true) { ... } around the wait line if you really want to guard against that.)

Answer 8

while(true) { Thread.sleep(1000); }

Answer 9

public static void main(String[] args) {
    Thread t = new Thread() {
        @Override
        public void run() {
            try {
                while (true) {
                    Thread.sleep(1000);
                }
            } catch (InterruptedException e) {
            }
        }
    };
    t.setDaemon(false);
    t.start();
}

Answer 10

for(;;);

But it's very unlikely that hanging the thread is what you want. Instead, you should consider options like joining on the other threads.