Java - Thread Joining

[Updated: Feb 7, 2017, Created: Mar 23, 2016]

Thread#join() halts the calling thread execution until the thread represented by this instance terminates.

That means aThreadInstance.join() will block the thread in which this method is called until 'aThreadInstance' fishes executing.

Following simple example demonstrates the concept.



public class ThreadJoinExample {
    public static void main (String[] args) throws InterruptedException {
        Task task1 = new Task();
        Thread thread1 = new Thread(task1);
        thread1.start();
        thread1.join();//here the main thread will wait until thread1 fishes.
        System.out.println("after join");

        Task task2 = new Task();
        new Thread(task2).start();
    }

    private static class Task implements Runnable {

        @Override
        public void run () {
            int c = 0;
            String threadName = Thread.currentThread()
                                      .getName();

            System.out.println(threadName + " started.");
            for (int i = 0; i < 1000; i++) {
                c+=i;
            }

            System.out.println(threadName + " ended.");
        }
    }
}

Output:

Thread-0 started.
Thread-0 ended.
after join
Thread-1 started.
Thread-1 ended.

Note Thread-1 starts only after Thread-0 terminates.


Calculating Factorial of multiple integers in parallel

This is more meaningful example of using join method.

We spawn a single thread A from the main thread which in turn creates n number of threads to calculate factorial of n integers. The threads A waits for all calculator threads to finish before printing the final results.

public class ThreadJoinExample2 {
    public static void main (String[] args) {
        final List<Integer> integers = Arrays.asList(10, 12, 13, 14, 15, 20);

        new Thread(new Runnable() {//thread A
            @Override
            public void run () {
                List<FactorialCalculator> threads = new ArrayList<>();
                for (Integer integer : integers) {
                    FactorialCalculator calc = new FactorialCalculator(integer);
                    threads.add(calc);
                    calc.start();
                }
                for (FactorialCalculator calc : threads) {
                    try {
                        calc.join();
                        System.out.println(calc.getNumber() + "! = "
                                                        + calc.getFactorial());

                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }

    private static class FactorialCalculator extends Thread {

        private final int number;
        private BigDecimal factorial;

        FactorialCalculator (int number) {
            this.number = number;
        }

        @Override
        public void run () {
            factorial = calculateFactorial(number);
        }

        private static BigDecimal calculateFactorial (int number) {
            BigDecimal factorial = BigDecimal.ONE;
            for (int i = 1; i <= number; i++) {
                factorial = factorial.multiply(new BigDecimal(i));
            }
            return factorial;
        }

        public BigDecimal getFactorial () {
            return factorial;
        }

        public int getNumber () {
            return number;
        }
    }
}

Output:

10! = 3628800
12! = 479001600
13! = 6227020800
14! = 87178291200
15! = 1307674368000
20! = 2432902008176640000

Other overloaded join methods

  • void join(long millis): Waits at most millis milliseconds for this thread to finish. A timeout of 0 means to wait forever.
  • void join(long millis, int nanos) : Waits at most millis milliseconds plus nanos nanoseconds for this thread to finish.

Example Project

Dependencies and Technologies Used :

  • JDK 1.8
  • Maven 3.0.4

Java Thread Join Example Select All Download
  • java-thread-join
    • src
      • main
        • java
          • com
            • logicbig
              • example

See Also