deadlock?

[Vali GB]

Hi all,

try to get a deadlock accessing via 2 threads 2 resources in synchronized blocks like:

synchronized(A){
synchronized(B){
}
}
and
synchronized(B){
synchronized(A){
}
}
i try to put the threads on sleep, use system.outs just to make sure my threads are accessing these objects in a convenient way, i know this because i got this output(copied/pasted from one run):
1. u tryin to acquire lock on A
2. u got lock on A, tryin to get lock on B
3. v tryin to acquire lock on B
4. v got lock on B, tryin to get lock on A
5. v got lock on A
6. v thread, freed all locks
7. u got lock on B
... and so on..

so, my question, how's it possible to read the 5th line if the u thread has already the lock on A?

Thanks,
vali

[Dirk Schreckmann]

Moving this to the Threads and Synchronization forum...

[Siyaa Hoffman]

Hi Vali,

Try this code. This creates a deadlock between the two threads.

public class TestThreadSync implements Runnable
{
String A = new String("Siyaa1");
String B = new String("Siyaa2");

public static void main(String[] args)
{
TestThreadSync mainClass = new TestThreadSync();
Thread t1 = new Thread(mainClass);
Thread t2 = new Thread(mainClass);
t1.setName("T1");
t2.setName("T2");
t1.start();
t2.start();
}


public void run()
{
try{
if(Thread.currentThread().getName().equals("T1"))
{
System.out.println("Thread T1 launched");
synchronized(A)
{
System.out.println("Thread T1 got lock on A");
Thread.sleep(10000);

synchronized(B){
System.out.println("T1 managed both");
}
}
}

if(Thread.currentThread().getName().equals("T2"))
{
System.out.println("Thread T2 launched");
synchronized(B)
{
System.out.println("Thread T2 got lock on B");
Thread.sleep(10000);

synchronized(A){
System.out.println("T2 managed both");
}
}
}

}
catch(Exception e)
{
System.out.println("Exception - "+e.getMessage());
}
}




}

The output that I see is -

C:\Test>java TestThreadSync
Thread T1 launched
Thread T1 got lock on A
Thread T2 launched
Thread T2 got lock on B


And the program just hangs there. This is what I expected it to do when entered in a deadlock situation.

[Vali GB]

Thanx a lot Siyaa!!
your example worked just fine into a deadlock..of course

actually my test program ressembled a lot yours with a few differences, and I still can't understand something...

For example, if I modify your program just a bit :

  
//public class TestThreadSync implements Runnable {
public class TestThreadSync extends Thread {
    String A = new String("Siyaa1");
    String B = new String("Siyaa2");
 
    public static void main(String[] args) {
//        TestThreadSync mainClass = new TestThreadSync();
//        Thread t1 = new Thread(mainClass);
//        Thread t2 = new Thread(mainClass);
        TestThreadSync t1 = new TestThreadSync();
        TestThreadSync t2 = new TestThreadSync();
        t1.setName("T1");
        t2.setName("T2");
        t1.start();
        t2.start();
    }
 

    public void run() {
        try {
            if (Thread.currentThread().getName().equals("T1")) {
                System.out.println("Thread T1 launched");
                synchronized (A) {
                    System.out.println("Thread T1 got lock on A");
                    Thread.sleep(10000);
 
                    synchronized (B) {
                        System.out.println("T1 managed both");
                    }
                }
            }
 
            if (Thread.currentThread().getName().equals("T2")) {
                System.out.println("Thread T2 launched");
                synchronized (B) {
                    System.out.println("Thread T2 got lock on B");
                    Thread.sleep(10000);
 
                    synchronized (A) {
                        System.out.println("T2 managed both");
                    }
                }
            }
 
        } catch (Exception e) {
            System.out.println("Exception - " + e.getMessage());
        }
    }
 

}

the're will be no more deadlock:

Thread T1 launched
Thread T1 got lock on A
Thread T2 launched
Thread T2 got lock on B
T1 managed both
T2 managed both
Process terminated with exit code 0

Why?

[Vali GB]

I somehow managed to solve the prev problem.
It was about the share on the 2 objects I wanted to deadlock...