Question

Question regarding deadlock in Java

Asked by: wrynn

http://java.sun.com/docs/books/tutorial/essential/concurrency/deadlock.html

can someone explain to me why each method is able to print out the first statement, but cannot ever call the returnBow()?

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Asked On: 2009-11-04 at 19:32:57ID24873401
Topics: New to Java Programming
,; Open Source Programming
,; Java Server Pages (JSP)
Participating Experts: 4
Points: 500
Comments: 20

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Answers

by: a_bPosted on 2009-11-04 at 20:34:59ID: 25746800

It's because they are in a deadlock, waiting for each other to finish. Try executing this -

public class Deadlock {
    static class Friend {
        private final String name;
        public Friend(String name) {
            this.name = name;
        }
        public String getName() {
            return this.name;
        }
        public synchronized void bow(Friend bower) {
            System.out.format("%s: %s has bowed to me!%n", 
                    this.name, bower.getName());
            bower.bowBack(this);
        }
        public void bowBack(Friend bower) {
            System.out.format("%s: %s has bowed back to me!%n",
                    this.name, bower.getName());
        }
    } 
    public static void main(String[] args) {
        final Friend alphonse = new Friend("Alphonse");
        final Friend gaston = new Friend("Gaston");
        new Thread(new Runnable() {
            public void run() { alphonse.bow(gaston); }
        }).start();
        new Thread(new Runnable() {
            public void run() { gaston.bow(alphonse); }
        }).start();
    }
}

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by: Ravi_KallaPosted on 2009-11-04 at 20:37:59ID: 25746810

Let us imagine each operation is done in a time frame of one second.

Second1 : "Friend" object for "alphonse" is created

Second2 : "Friend" object for "gaston" is created

Second3 : "bow()" method is called by "alphonse" and this will run in an individual thread

Second4 : "bow()" method is called by "gaston" and this will run in an individual thread

Second5 : "bowBack()" method is called by both "alphonse" and "gaston" objects at the same time. Since, "bowBack()" is a synchronized method, only one instance of that method can run at a time. So, if two objects call a synchronized method approximately at same time, each thread will wait for another thread to finish its job with the synchronized method "bowBack()". This waiting will last for ever and is called as DeadLock.

by: a_bPosted on 2009-11-04 at 20:39:38ID: 25746816

@wrynn: That was a very good explanation by Ravi_Kalla

by: afibarraPosted on 2009-11-04 at 21:10:48ID: 25746930

public synchronized void bow(Friend bower) {
System.out.format("%s: %s has bowed to me!%n",
this.name, bower.getName());
bower.bowBack(this); //Here each Thread is waiting for the other to complete
}

by: afibarraPosted on 2009-11-04 at 21:12:35ID: 25746942

I´m so slow, Ravi_Kalla´s explanation is very good.

by: rrz@871311Posted on 2009-11-04 at 21:20:29ID: 25746966

Ravi_Kalla's explanation is good.
But I think the example code is confusing. In the code a Friend does not bow on his own initiative. A bower bows only when a Friend's bow method is called. To see how it really works without the deadlock, add a delay between the two thread starts. Try the code below here. The output is
Alphonse: Gaston has bowed to me!
Gaston: Alphonse has bowed back to me!
Gaston: Alphonse has bowed to me!
Alphonse: Gaston has bowed back to me!

public class Deadlock {
    static class Friend {
        private final String name;
        public Friend(String name) {
            this.name = name;
        }
        public String getName() {
            return this.name;
        }
        public synchronized void bow(Friend bower) {
            System.out.format("%s: %s has bowed to me!%n", 
                    this.name, bower.getName());
            bower.bowBack(this);
        }
        public synchronized void bowBack(Friend bower) {
            System.out.format("%s: %s has bowed back to me!%n",
                    this.name, bower.getName());
        }
    } 
    public static void main(String[] args) {
        final Friend alphonse = new Friend("Alphonse");
        final Friend gaston = new Friend("Gaston");
        new Thread(new Runnable() {
            public void run() { alphonse.bow(gaston); }
        }).start();
        try{
             Thread.sleep(1000);
        }catch(InterruptedException ie){
                                        System.out.print("sleep Interrupted");
          }
        new Thread(new Runnable() {
            public void run() { gaston.bow(alphonse); }
        }).start();
    }
}

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by: rrz@871311Posted on 2009-11-04 at 22:42:26ID: 25747216

On second thought, maybe a better explanation of the code's behavior is that if
one synchronized method in an object calls another synchronized method in the same object, then a deadlock will occur.

by: rrz@871311Posted on 2009-11-05 at 08:15:45ID: 25750993

Please ignor my last comment. I was tired and confused last night.

by: wrynnPosted on 2009-11-05 at 18:52:22ID: 25756263

im confused because the output i'm getting is

Alphonse: Gaston has bowed to me!
Gaston: Alphonse has bowed to me!

Suggesting that the 2nd thread was able to begin executing the synchronized method "bow" before the 1st thread was finished executing. isnt that supposed to be impossible? i thought that synchronized means that no other thread can call the method until the other thread is finished?

by: wrynnPosted on 2009-11-05 at 18:53:28ID: 25756267

what is also confusing is how sometimes i get this output:

Gaston: Alphonse has bowed to me!
Alphonse: Gaston has bowed back to me!
Alphonse: Gaston has bowed to me!
Gaston: Alphonse has bowed back to me!

so there seems to be some random nature to deadlock. can someone explain why in this case there is random output?

by: afibarraPosted on 2009-11-05 at 19:02:56ID: 25756299

>im confused because the output i'm getting is

>Alphonse: Gaston has bowed to me!
>Gaston: Alphonse has bowed to me!

>Suggesting that the 2nd thread was able to begin executing the synchronized method "bow" before the >1st thread was finished executing. isnt that supposed to be impossible? i thought that synchronized >means that no other thread can call the method until the other thread is finished?

That only happens if two threads are about to execute a synchronized method in a class, and BOTH threads are using the same instance of the class to invoke the method. In this case each thread is using its own instance.

by: Ravi_KallaPosted on 2009-11-05 at 19:04:45ID: 25756304

If the synchronized method "bowBack()" is called by two different threads at approximately same time, it will go into deadlock. Sometimes, the time difference at which the two threads call "bowBack()" will be more. In such cases, the programs run without deadlock.

>>so there seems to be some random nature to deadlock.

It behaves randomly because, threads run independently and there is no hard and fast rule which thread has to be given more preference... and so the the time difference at which the "bowBack()" is called. If time difference is more, it wont go to dead lock.

by: wrynnPosted on 2009-11-05 at 19:17:15ID: 25756345

i just found this link here: http://forums.sun.com/thread.jspa?forumID=54&threadID=5341448

and it says the same thing that i just saw afibarra write regarding it being two DIFFERENT instances and thus DIFFERENT locks being used. and that the deadlock is because each method tries to cross over and grab the lock from the other object which is already locking itself.

for whatever reason i did not notice that they were different instances. and i was unaware of the rule regarding unique locks for each instance.

A confusing topic for even "veteran" java programmers. thanks for all of your help.

by: wrynnPosted on 2009-11-05 at 19:20:44ID: 31650348

wow im impressed. im so glad i use experts exchange because i can rely on the very smart people on here who almost always have great suggestions and explanations. great work guys, keep it up you are helping others SUCCEED!!

by: rrz@871311Posted on 2009-11-05 at 19:30:22ID: 25756395

> i thought that synchronized means that no other thread can call the method until the other thread is finished?
As we see on this page
http://java.sun.com/docs/books/tutorial/essential/concurrency/syncmeth.html
we have
"When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object."
In this example, we have two Friend objects. When each object is in its own bow method, it has it has acquired the intrinsic lock for itself.
As we see on this page
http://java.sun.com/docs/books/tutorial/essential/concurrency/syncmeth.html
we have
" When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
So, this example each Friend is waiting for the other Friend. They are both in their own bow method and are calling each others bowback method. That is the deadlock.

>what is also confusing is how sometimes i get this output:
On the page that you linked to in your question it states
"When Deadlock runs, it's extremely likely that both threads will block when they attempt to invoke bowBack."
It all depends on whether one Friend starts and finishs before the other starts. Did you run the code I posted yesterday? I insert a time delay. That way the deadlock is always avoided.

by: afibarraPosted on 2009-11-05 at 19:34:54ID: 25756416

yw =)

by: Ravi_KallaPosted on 2009-11-05 at 19:37:55ID: 25756427

I feel that 100 points is very less for ID: 25746810 ... there are good appreciations but no good points :)

by: wrynnPosted on 2009-11-06 at 12:32:08ID: 25762777

i wanted to reward afibarra for pointing out the most confusing part which is that each instance of an object has its own separate lock for all its synchronized methods.

but i guess i gave him too many points. next time i will try to spread the points around more evenly.

by: wrynnPosted on 2009-11-06 at 12:47:44ID: 25762901

afibarra was able to explain *why* the bowback method was unable to be called. because the instances had locked themselves during bow(), and that were able to have access to run each of their synchronized bow() methods because they were dealing with their own instance lock. but once they try to call a synchronized method on the OTHER object instance, it is unable to do so because it is locked, so it must wait. and if both threads by chance call .bowback() at the same time they will both begin to wait indefinitely for the locks to be released but they never will.

by: rrz@871311Posted on 2009-11-06 at 13:39:25ID: 25763304

I think you got it now.

20120131-EE-VQP-002

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Question regarding deadlock in Java

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