Joining threads

Sometimes one thread needs to know when another thread is ending. In java, isAlive() and join() are two different methods to check whether a thread has finished its execution.

The isAlive() method returns true if the thread upon which it is called is still running otherwise it returns false.

final boolean isAlive()

But, join() method is used more commonly than isAlive(). This method waits until the thread on which it is called terminates.

final void join() throws InterruptedException

Using join() method, we tell our thread to wait until the specified thread completes its execution. There are overloaded versions of join() method, which allows us to specify time for which you want to wait for the specified thread to terminate.

final void join(long milliseconds) throws InterruptedException

Example of isAlive method

public class MyThread extends Thread
{
	public void run()
	{
		System.out.println("r1 ");
		try {
        		Thread.sleep(500);
    		}
    		catch(InterruptedException ie) { }
       		System.out.println("r2 ");
  	}
	public static void main(String[] args)
	{
		MyThread t1=new MyThread();
		MyThread t2=new MyThread();
		t1.start(); 
		t2.start();
		System.out.println(t1.isAlive());
		System.out.println(t2.isAlive());
	}
}

Output :

r1 
true
true
r1 
r2 
r2 

Example of thread without join() method

public class MyThread extends Thread
{
	public void run()
   	{
       		System.out.println("r1 ");
       		try {
        		Thread.sleep(500);
    		}
    		catch(InterruptedException ie){ }
       		System.out.println("r2 ");
  	}
	public static void main(String[] args)
	{
		MyThread t1=new MyThread();
		MyThread t2=new MyThread();
		t1.start(); 
		t2.start();
	}
}

Output :

r1
r1
r2
r2

In this above program two thread t1 and t2 are created. t1 starts first and after printing "r1" on console thread t1 goes to sleep for 500 ms. At the same time Thread t2 will start its process and print "r1" on console and then go into sleep for 500 ms. Thread t1 will wake up from sleep and print "r2" on console similarly thread t2 will wake up from sleep and print "r2" on console. So you will get output like r1 r1 r2 r2


Example of thread with join() method

public class MyThread extends Thread
{
	public void run()
   	{
       		System.out.println("r1 ");
       		try {
        		Thread.sleep(500);
    		}catch(InterruptedException ie){ } 
       		System.out.println("r2 ");
  	}
	public static void main(String[] args)
	{
		MyThread t1=new MyThread();
		MyThread t2=new MyThread();
		t1.start(); 
		
		try{
  			t1.join();	//Waiting for t1 to finish
		}catch(InterruptedException ie){}

		t2.start();
	}
}

Output :

r1 
r2 
r1 
r2

In this above program join() method on thread t1 ensures that t1 finishes it process before thread t2 starts.


Specifying time with join()

If in the above program, we specify time while using join() with t1, then t1 will execute for that time, and then t2 will join it.

t1.join(1500);
Doing so, initially t1 will execute for 1.5 seconds, after which t2 will join it.

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