Understanding join() method example

Question

The Java thread join() method confuses me a bit. I have following example

class MyThread extends Thread { private String name; private int sleepTime; private Thread waitsFor; MyThread(String name, int stime, Thread wa) { … } public void run() { System.out.print("["+name+" "); try { Thread.sleep(sleepTime); } catch(InterruptedException ie) { } System.out.print(name+"? "); if (!(waitsFor == null)) try { waitsFor.join(); } catch(InterruptedException ie) { } System.out.print(name+"] ");

And

public class JoinTest2 { public static void main (String [] args) { Thread t1 = new MyThread("1",1000,null); Thread t2 = new MyThread("2",4000,t1); Thread t3 = new MyThread("3",600,t2); Thread t4 = new MyThread("4",500,t3); t1.start(); t2.start(); t3.start(); t4.start(); } }

In which order are the threads terminated?

just reading some ebook and found that example, actually the executing thread waits for the thread where joins is called on to finish, maybe I shoud quit for today :D — DarkLeafyGreen
– DarkLeafyGreen, Commented May 3, 2011 at 16:50
Waits for the thread to die. download.oracle.com/javase/1.4.2/docs/api/java/lang/… — Gabriel Llamas
– Gabriel Llamas, Commented May 3, 2011 at 16:51
@Giacomo, Correct which is why I deleted it before anyone noticed (almost anyway ;) — Peter Lawrey
– Peter Lawrey, Commented May 3, 2011 at 16:52

matt b · Accepted Answer · 2011-05-03 16:50:45Z

What actually confuses you about Thread.join()? You haven't mentioned anything specific.

Given that Thread.join() (as the documentation states), Waits for this thread to die, then t4 will wait for t3 to complete, which will wait for t2 to complete, which will wait for t1 to complete.

Therefore t1 will complete first, followed by t2, t3, and t4.

Lawrence McAlpin · Accepted Answer · 2011-05-03 16:49:20Z

It would terminate in order t1, t2, t3, t4... join causes the currently executing thread to wait until the thread it is called on terminates.

Aniket Thakur · Accepted Answer · 2013-09-06 05:23:09Z

There is a main thread which starts the four customs threads you created t1,t2,t3 and t4. Now when join() method is invoked on a thread and no argument(time) is supplied(which makes it 0 by default meaning maximum wait time is forever) then the calling thread will wait for the thread on which join was invoked to terminate.

When t1.start() is called it's corresponding run() method is invoked by the JVM. Since waitsFor argument is null for t1 it will simply terminate by printing it's name. Since you are adding sleep t2,t3,t4 would be started by the time t1 completes it's sleep. Now t2 will wait for t1 to terminate as we are calling join on t1 from t2. Similarly t3 will wait for t2 and t4 will wait for t3. So youe execution will go t1->t2->t3->t4.

However if t1 terminates before t2 calls join on it, join will simply return because at that point(when t2 calls join on t1) isAlive() will be false.

Manoj Rawat · Accepted Answer · 2015-07-23 12:11:56Z

Surely the order of thread termination would be from t1, t2, and so on here. May be you will get a clear understanding of join() method in java thread from here also http://findnerd.com/list/view/Java-Thread-Join-Example/4465/

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