Java Thread Join Example

There are three variant of join method:

public final void join() throws InterruptedException :

Thread on which join method is getting called to die.

public final void join(long millis) throws InterruptedException:
This method when called on the thread, it waits for either of following:

• Thread on which join method is getting called, to die.
• Specified milliseconds

public final void join(long millis,int nanos) throws InterruptedException:
This method when called on the thread, it waits for either of following:

• Thread on which join method is getting called, to die.
• Specified milliseconds + nano seconds

Example:

Lets take simple example:

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package org.arpit.java2blog.thread;   public class MyRunnable implements Runnable{   public void run() {   try {    System.out.println(Thread.currentThread().getName()+" Start");    // thread sleeps for 4 secs    Thread.sleep(4000);    System.out.println(Thread.currentThread().getName()+" end");   } catch (InterruptedException e) {    // TODO Auto-generated catch block    e.printStackTrace();   }   } }

Create ThreadExampleMain.java

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package org.arpit.java2blog.thread;   public class ThreadExampleMain { public static void main(String args[]) {                  System.out.println("Main thread execution starts");   MyRunnable mr=new MyRunnable();      Thread t1=new Thread(mr,"Thread 1");   Thread t2=new Thread(mr,"Thread 2");   Thread t3=new Thread(mr,"Thread 3");      t1.start();   // lets waits for  t1 to die   try {    t1.join();   } catch (InterruptedException e) {       e.printStackTrace();   }   t2.start();   try { // lets waits for 1 sec or t2 to die which ever occurs first   t2.join(1000);      } catch (InterruptedException e1) {       e1.printStackTrace();   }   t3.start() ;      // complete all threads before completing main thread   try {    t2.join();    t3.join();      } catch (InterruptedException e1) {       e1.printStackTrace();   }   System.out.println("Main thread execution ends");   } }

When you run above program, you will get following output.

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Main thread execution starts Thread 1 Start Thread 1 end Thread 2 Start Thread 3 Start Thread 2 end Thread 3 end Main thread execution ends

Lets analysis output now.

1. Main thread execution starts
2. Thread 1 starts(Thread 1 start) and as we have put t1.join() , it will wait for t1 to die(Thread 1 end).
3. Thread 2 starts(Thread 2 start) and waits for either 1 seconds or die but as we have put sleep for 4 seconds in run method, it will not die in 1 second. so main thread resumes and Thread 3 starts(Thread 3 start)
4. As we have put t2.join() and t3.join(). These 2 threads will get completed before exiting main thread.So Thread 2 will end(Thread 2 end ) and then thread 3 will end(Thread 3 end).
5. Main thread execution ends.