Join() method in Java

[Matloob Hussain]

Hi,

Here is the code example, the output of two methods named method1() and method2() is different. The code difference in second method is thread T2 is started after join() method which is invoked on thread T1 but in first method, both threads T1 and T2 are started first and then join() method is invoked.

Could anyone please explain it..... thanks

public class Test{

	public static void main(String[] arg){

		Test test = new Test();
		test.method1();
		test.method2();
	}	//	end main method


	public void method1(){
		System.out.println("Method1 is started");
		ClientDB db1 = new ClientDB();
		Thread t1 = new Thread(db1);
		t1.setName("T1");
		Thread t2 = new Thread(db1);
		t2.setName("T2");
		t1.start();
		t2.start();
		try{
			t1.join();
		}catch(InterruptedException ie){}
	}	// end method1


		public void method2(){
			System.out.println("Method2 is started");
			ClientDB db1 = new ClientDB();
			Thread t1 = new Thread(db1);
			t1.setName("T1");
			Thread t2 = new Thread(db1);
			t2.setName("T2");
			t1.start();
			try{
				t1.join();
			}catch(InterruptedException ie){}
			t2.start();
		}	// end method2

}	// end class Test

class ClientDB implements Runnable{

	public void run(){

		for(int x=1; x<5; x++){

			System.out.println("Thread: " + Thread.currentThread().getName());
			try{
				Thread.sleep(1000);
			}catch(InterruptedException ie){}

		}	// end for loop
 	}	// end run method

}	// end class ClientDB

[Piyush Joshi]

I am not very good in threads but as far as I understand it: (if I am wrong please somebody correct me)

t1.join() call here in method1 and method2 will block the main thread from becoming runnable till the thread t1 completes.

In method1 we start thread t2 before we call t1.join() so t1 and t2 both will be in runnable state and

In method2 we call t1.join() before we start thread t2, so as soon as we call t1.join() the main thread will be blocked.
Therefore t2 will now be started only after the t1 completes execution.

[Seetharaman Venkatasamy]

Suppose there are two threads T1 and T2 are running . and T1 starts T2 and now if you say T2.join() [in the T1 thread flow], then T1 blocks until T2 complete the job.

[Suhrid Karthik]

In method1(), there is a possibility of both T1 and T2 running at the same time. In method2(), the main thread waits for T1 to complete by join()'ing itself to the end of T1. When T1 completes, the main thread resumes and starts T2.

[Piyush Joshi]

Seetharaman Venkatasamy wrote:Suppose there are two threads T1 and T2 are running . and T1 starts T2 and now if you say T2.join() [in the T1 thread flow], then T1 blocks until T2 complete the job.

Yes you are right.

T2.join() will block the current executing thread. that is T1 if you call T2.join() from T1