How to Join Multiple Threads in Java - Thread Join Example

Join method from Thread class is an important method and used to impose order on execution of multiple Threads. Concept of joining multiple threads is very popular on  mutithreading interview question. Here is one of such question, “You have three threads T1, T2 and T3, How do you ensure that they finish in order T1, T2, T3 ?. This question illustrate power of join method on multithreaded programming. Unlike classical thread questions like difference between wait and sleep method or solving producer consumer problem in Java, This one is bit tricky. You can do this by using join method, by calling T1.join() from T2 and T2.join() from T3. In this case thread T1 will finish first, followed by T2 and T3. In this Java multithreading tutorial we will have a closer look on join method with a simple example. Idea is to illustrate how join method works in simple words. By the way from Java 5 onwards you can also use CountDownLatch and CyclicBarrier classes to implement scenarios like one thread is waiting for other threads to finish there task.

When to use join method in Java

As I said join is an important and useful method from Thread class but many times overlooked. Similar to wait method, by using join method, we can make one Thread to wait for another. Primary use of Thread.join() is to wait for another thread and start execution once that Thread has completed execution or died. Join is also a blocking method, which blocks until thread on which join has called die or specified waiting time is over. By the way understanding, how join method works, is not straight forward. Many developers get confused on things like, which thread will wait,  which thread will join etc. These points will be more clear when we go through an example of joining multiple Thread in Java using join() method.

Thread Join Example  in Java

Here is a simple example of joining two threads using Thread.join() method. By the way unlike Thread.sleep() method, join() is not a static method, it needs to be call on a java.lang.Thread object. Current thread, which calls join method will wait until thread on which join has called die or wait at most specified millisecond for this thread to die.

/**

 * Sample Java class to illustrate How to join two threads in Java.

 * join() method allows you to serialize processing of two threads.

 */

public class Join {
 
    public static void main(String args[]) throws InterruptedException{
     
        System.out.println(Thread.currentThread().getName() + " is Started");
     
        Thread exampleThread = new Thread(){
            public void run(){
                try {
                    System.out.println(Thread.currentThread().getName() + " is Started");
                    Thread.sleep(2000);
                    System.out.println(Thread.currentThread().getName() + " is Completed");
                } catch (InterruptedException ex) {
                    Logger.getLogger(Join.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        };
     
        exampleThread.start();
        exampleThread.join();
     
        System.out.println(Thread.currentThread().getName() + " is Completed");
    }
 
}

Output:
main is Started
Thread-0 is Started
Thread-0 is Completed
main is Completed

If you look at above example, first main thread is started and than it creates another thread, whose name is "Thread-0" and started it. Since Thread-0 sleep for 2 seconds, it require at least 2 seconds to complete and in between main thread called join method on Thread-0 object. Because of join method, now main thread will wait until Thread-0 completes its operation or You can say main thread will join Thread-0. If you look on output, it confirms this theory.

Important point on Thread.join method

Now we know How to use join method in Java, it’s time to see some important points about Thread.join() method.

1. Join is a final method in java.lang.Thread class and you cannot override it.

2) Join method throw IntrupptedException if another thread interrupted waiting thread as a result of join() call.

3) Join is also an overloaded method in Java, three version of join() available, check javadoc for details.

That’s all on How to join two Threads in Java with example. You can Join multiple threads by using Thread.join() method. Join is particularly useful to make one thread wait for other, or serializing two function e.g. first load your cache and than start processing request.

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