Java Concurrency - Thread.join()

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Thread.join() is the most basic mechanism of inter-thread synchronization in Java.

Thread.join() Method

The calling thread goes into wating state. It waits for the referenced thread to terminate.

Thread.join() throws InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

Example

CountDownRunnable Class

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public class CountDownRunnable implements Runnable {
    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName() + " - start execution");
        for(int i = 3; i >= 0; i--) {
            try {
                Thread.sleep(1000);
                System.out.println(Thread.currentThread().getName() + " - " + i);
            } catch (InterruptedException e) {
                return; // when interrupted, stop execution and return
            }
        }
        System.out.println(Thread.currentThread().getName() + " - finish execution");
    }
}

main method

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public static void main(String[] args) {
    System.out.println(Thread.currentThread().getName() + " - start execution");

    Thread countdownThread = new Thread(new CountDownRunnable(), "countdown");

    countdownThread.start();
    try {
        countdownThread.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    System.out.println(Thread.currentThread().getName() + " - finish execution");
}

output

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main - start execution
countdown - start execution
countdown - 3
countdown - 2
countdown - 1
countdown - 0
countdown - finish execution
main - finish execution

if there is no thread join, main thread will finish before countdown thread. see output

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main - start execution
main - finish execution
countdown - start execution
countdown - 3
countdown - 2
countdown - 1
countdown - 0
countdown - finish execution

Thread.join(long millis)

The calling thread goes into wating state. It Waits at most millis milliseconds for the referenced thread to die. A timeout of 0 means to wait forever.

This method is very useful to prevent the calling thread from waiting too long.

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countdownThread.start();
countdownThread.join(1500);

Reference