Any Solution of Deadlock?

[Abdul Mohsin]

What's the best way to detect deadlock and remove deadlock?

Example:

public class RunDeadLock
{
public static void main(String[] args)
{
Thread1 t1= new Thread1();
Thread2 t2= new Thread2();

t1.start();
t2.start();

}
}


public class Thread1 extends Thread
{
public void run(){
ThreadResource1.go1();
}
}

public class Thread2 extends Thread
{
public void run(){
ThreadResource2.go2();
}
}


public class ThreadResource2
{
public static synchronized void go2(){
System.out.println("Entered in resource 2");
System.out.println(Thread.currentThread().getName());
try{
Thread.sleep(5000);
}catch(Exception e){}

ThreadResource1.go1();
System.out.println("Exiting from resource");


}
}


public class ThreadResource1
{
public static synchronized void go1(){
System.out.println("Entered in resource1");
System.out.println(Thread.currentThread().getName());
try{
Thread.sleep(5000);
}catch(Exception e){}
ThreadResource2.go2();
System.out.println("Exiting from resource");

}
}

How to remove deadlock?

[Barry Gaunt]

This question seems to be a general thread programming issue, rather than SCJP specific. Moving to our Threads forum. Interested SCJP'ers can follow there.

[Burkhard Hassel]

Howdy, cowboys!

To recognize a lock look for threads calling other threads that may be blocked who call the calling thread by themselves.

In this example each synchronized static go-Method calls the go Method of the other class and vice versa.
And (important) it is calling it from synchronized code.
Perhaps somebody can provide a nice graphic for this...


Easiest solution:
get the call of go1/2 out of the sync background!

Extra tipp: only sleep 500 ms instead of 5000. Makes it much faster.

By the way: the code is still an endless loop (or better two endless loops). It will never end.


Yours,
Bu.

[Abdul Mohsin]

Hi Hassel,

I know that code is in endless loop but before that it stucked in deadlock
I just want to remove this deadlock condition.
Please explain me what are the different ways to detect and resolve deadlock situation.

Regards,

Abdul Mohsin

[Nitesh Kant]

Originally posted by Abdul Mohsin:
Hi Hassel,

I know that code is in endless loop but before that it stucked in deadlock
I just want to remove this deadlock condition.
Please explain me what are the different ways to detect and resolve deadlock situation.

Regards,

Abdul Mohsin

Hi Abdul,
Addressing the two parts of your question individually:

Detecting deadlock:
In order to detect a deadlock, you have to take a thread dump of the JVM.
In order to analyze these thread dumps there are a few tools available. this article gives you some insight into one such tool. Googling will give you some more.

Removing deadlocks:
Well, there is no solution to bad programming but to change the program itself
So, you have to change the code to remove the deadlock. There is no way you can resolve a deadlock solution without changing the code.
Once you have correctly analyzed the thread dump, you can easily rectify such a situation.

[Abdul Mohsin]

Hi,

to make my above post work what changes you will do to remove deadlock situation.

Regards,

Abdul Mohsin

[Nitesh Kant]

to make my above post work what changes you will do to remove deadlock situation.

The above code, which i assume is just a demo code, is designed to cause a deadlock. The only way out is to make the methods un-synchronized OR to remove nested calls to synchronize methods in different threads.
Probably, if you have a business specific use case, then this exercise will make some sense.

[Ernest Friedman-Hill]

Originally posted by Nitesh Kant:

The only way out is to make the methods un-synchronized OR to remove nested calls to synchronize methods in different threads.

Well, not exactly.

The way to avoid deadlock when locking multiple resources is to always lock them in the same order in every case. This program wouldn't deadlock if go1() and go2() were written as shown below. Note that both methods lock ThreadResource1.class before ThreadResource2.class; therefore only one method can proceed at a time, and the other must wait. Once a method locks ThreadResource1.class, it will always proceed to completion.

 
public static void go2(){
  synchronized (ThreadResource1.class) {
    synchronized (ThreadResource2.class) {
        System.out.println("Entered in resource 2");
        System.out.println(Thread.currentThread().getName());
        try{
            Thread.sleep(5000);
        }catch(Exception e){}
        ThreadResource1.go1();
        System.out.println("Exiting from resource");
    }
  }
}
 
...
 
public static synchronized void go1(){
    System.out.println("Entered in resource1");
    System.out.println(Thread.currentThread().getName());
    try{
        Thread.sleep(5000);
    }catch(Exception e){}
    ThreadResource2.go2();
    System.out.println("Exiting from resource");
}

[Nitesh Kant]

Thanks for pointing that out Ernest. Missed the obvious in my last post.

[Abdul Mohsin]

Hi Ernest ,

Excellent explaination !!

Thanks,

Abdul Mohsin