Thread - Join method

[solomon_13000]

When we run a java program, JVM will create the main thread in which the main function will be called. Inside the main thread we can have child thread. Now for the main thread to terminate the child thread must first be dead. To know if the child thread is alive or not we can do the following.

t1.join();
t1.isAlive() // returns a boolean value

Im assuming t1 as a thread object. Now how does the join() method play a role in finding out the status of the child thread. Also how does it tell the main thread that the child thread is all dead?. Besides that without the syntax t1.join(), the t1.isAlive() statement will result in an error. So what does the join() method do to the t1 thread object?.

[solomon_13000]

It causes the main thread to wait for the child thread to complete processing before the main thread terminates.

[Razee Marikar]

Calling join function of a thread causes the caller thread (Main thread in this case) to wait for the called thread (t1) to complete. In other words, t1.join() will pause Main thread till t1 completes.

isAlive() is a method in thread to check whether t1 (called thread) is running or not. So, from Main, if you call it after t1.join(), it should always return false (because join wait till t1 dies). Calling t1.isAlive should NOT throw an error unless t1 is uninitialized. Try the code below:

public class ThreadTest {

	public static void main(String[] args) throws Exception
	{
		boolean state;
		MyThread t1 = new MyThread();
		state = t1.isAlive(); // No error, state=false, because thread not started yet
		System.out.println("Before start() T1::isAlive = " + state);
		t1.start();
		state = t1.isAlive(); // No error, state=true here, but maybe false under some circumstances depending on wether t1 is still has terminated or not
		System.out.println("Just after start() T1::isAlive = " + state);
		t1.join();
		state = t1.isAlive(); // Always false
		System.out.println("After join() T1::isAlive = " + state);
	}
	
	static class MyThread extends Thread
	{
		public void run()
		{
			try{
				Thread.sleep(1000);
			}
			catch(Exception ex)
			{
				throw new RuntimeException(ex);
			}
		}
	}
	
}