Thread Synchronization

[Balaji Bang]

Synchronization won work if it is on different objects. Here how it is working? Please explain.....

public class AnoThreads{

     static volatile java.io.PrintStream s=System.out;

     public static void main(String args[]){

          Runnable r1=new Runnable(){
               public void run(){
                    synchronized(s){
                         System.out.print("SCJP-1 ");
                         System.out.print("SCJP-2 ");
                         System.out.print("SCJP-3 ");
                    }
               }
          };

          Runnable r2=new Runnable(){
               public void run(){
                    synchronized(s){
                         System.out.print("SCEA-1 ");
                         System.out.print("SCEA-2 ");
                         System.out.print("SCEA-3 ");
                    }
               }
          };
//r1.run();
//r2.run();
Thread t1=new Thread(r1);
Thread t2=new Thread(r2);
          //Thread t1=(Thread)r1;
          //Thread t2=(Thread)r2;
          t1.start();
          t2.start();

     }

}
OUTPUT : SCJP-1 SCJP-2 SCJP-3 SCEA-1 SCEA-2 SCEA-3 or SCEA-1 SCEA-2 SCEA-3 SCJP-1 SCJP-2 SCJP-3

[Jelle Klap]

If I told you that System.out is a static final reference to a PrintStream instance, and that a PrintStream's print() method uses a synchronized block that synchronizes on its intrinsic lock (using the this reference), could you then explain this behavior?

[Ankit Garg]

Just to be more specific than jelle, just look at the synchronization statement

synchronized(s)

[Balaji Bang]

System.out is a static and if it is locking on this then I think Synchronization works. But there are 2 different anonymous Inner classes. In different classes does the synchronization work???

[Ankit Garg]

Yes you have different anonymous inner classes, but both will share the same instance s i.e. System.out. And since you synchronize on s, so both the run methods in the anonymous inner classes will synchronize on the same object. If I modify your code, then the synchronization will not work

public class AnoThreads{

     static volatile java.io.PrintStream s=System.out;

     public static void main(String args[]){

          Runnable r1=new Runnable(){
               public void run(){
                    synchronized(this){
                         System.out.print("SCJP-1 ");
                         System.out.print("SCJP-2 ");
                         System.out.print("SCJP-3 ");
                    }
               }
          };

          Runnable r2=new Runnable(){
               public void run(){
                    synchronized(this){
                         System.out.print("SCEA-1 ");
                         System.out.print("SCEA-2 ");
                         System.out.print("SCEA-3 ");
                    }
               }
          };
          Thread t1=new Thread(r1);
          Thread t2=new Thread(r2);
          t1.start();
          t2.start();

     }
}

Now both methods will synchronize on different objects...

[Ruben Soto]

Jelle Klap wrote:If I told you that System.out is a static final reference to a PrintStream instance, and that a PrintStream's print() method uses a synchronized block that synchronizes on its intrinsic lock (using the this reference), could you then explain this behavior?

I understand what you are saying, but the statement marked as bold is not relevant for this example, correct?

[Ankit Garg]

You are right Ruben. The matter of concern for the question is that the synchronized blocks synchronize on the System.out object. It's not related to the print method...

[Jelle Klap]

Ruben Soto wrote:

Jelle Klap wrote:If I told you that System.out is a static final reference to a PrintStream instance, and that a PrintStream's print() method uses a synchronized block that synchronizes on its intrinsic lock (using the this reference), could you then explain this behavior?

I understand what you are saying, but the statement marked as bold is not relevant for this example, correct?

Well, yes and no.
It's important to understand that the output of both run() methods will never intertwine, because both run() methods synchronize on the same lock i.e. System.out's intrinsic lock.
It's also important to understand that, although the print() method also synchronizes on this lock, this is not a dead-lock scenario, because intrinsic locks are reentrant.

[Ruben Soto]

Jelle Klap wrote:

Ruben Soto wrote:

Jelle Klap wrote:If I told you that System.out is a static final reference to a PrintStream instance, and that a PrintStream's print() method uses a synchronized block that synchronizes on its intrinsic lock (using the this reference), could you then explain this behavior?

I understand what you are saying, but the statement marked as bold is not relevant for this example, correct?

Well, yes and no.
It's important to understand that the output of both run() methods will never intertwine, because both run() methods synchronize on the same lock i.e. System.out's intrinsic lock.
It's also important to understand that, although the print() method also synchronizes on this lock, this is not a dead-lock scenario, because intrinsic locks are reentrant.

I see what you mean. I wasn't aware of this, but unless you use some specialized API lock related class, aren't locks in Java always reentrant?

[Ruben Soto]

Ankit Garg wrote:You are right Ruben. The matter of concern for the question is that the synchronized blocks synchronize on the System.out object. It's not related to the print method...

Thanks Ankit, that's what I meant. But I think Jelle was trying to make a point about locks being reentrant. It's just that it wasn't clear from the wording used.