synchronized methods

[Ismael Upright]

Hi,

I have a question regarding jqplusv6.

There is a following code:

public class Test extends Thread
{
 static int x, y;
 public synchronized void run(){  for(; :wink: { x++; y++; System.out.println(x+" "+y);} }
 public static void main(String[] args)
 {
   new Test().start();
   new Test().start();
 }
}

The answer is that nothing can be surely said about this code (like it will keep on printing same values for x and y or will always increment these values by 1 etc.)

What I don't understand is the explanation:

"You may be tempted by the synchronized keyword on the run method. But note that, there are two diferent thread objects and both the threads are accquiring locks for their own thread object. So, in this case synchronized doesn't make any sense."


I started to wonder, if that's true, then when there is a sense to make the method synchronized?

[Wirianto Djunaidi]

To make it easier to discuss, let assume the following changes:

 
public static void main(String[] args)
{
   Test testA = new Test();
   Test testB = new Test();
   testA.start();
   testB.start();
}

What the explanation was saying is "the synchronized method when run will check the lock against the instance of the object itself". In this case you have instance testAand testB.

Usually synchronized method is perfectly fine because you put it on to prevent concurrent access to data that belong to that object instance.

In your case you are manipulating static x and y, which belong to the Test.class itself .. not instance of Test. So your synchronized method is guarding the wrong door. That is why it does not make sense.

If you want to guard concurrent access to static variables, then you need to move the synchronized inside and use Test.class itself or another static variable object as the key.
[ April 18, 2008: Message edited by: Wirianto Djunaidi ]

[Ismael Upright]

So, if x any y would not be static and both threads testA and testB would try to change these variables in, let's say threadA, then the synchronized methods would have a sense?

[vinal sen]

hai! Djunaidi!

could you please modify the above code and give concurrent protection to static variables.

Please explain.Thank you.

[Ismael Upright]

Hi,

I guess that the static variables will receive the protection if the fragment of the code that operates on them will be synchronized i.e. on the Test class object:

public class Test extends Thread
{
 static int x, y;
 
 public void run()
{
    synchronized(Test.class) 
    { 
        for( ; ; )  { x++; y++; System.out.println(x+" "+y);}
    } 
}
 
 public static void main(String[] args)
 {
   new Test().start();
   new Test().start();
 }
}

This way we synchronize this code on the one object, not on the several objects.
[ April 20, 2008: Message edited by: Ismael Upright ]

[Henry Wong]

This way we synchronize this code on the one object, not on the several objects.

And don't forget to remove the previous synchronization (of the run() method).

Henry

[Ismael Upright]

ops, true
fixed

[Ismael Upright]

I cannot find a way to edit my post..
So here's the code:

...

Okay, found it
[ April 20, 2008: Message edited by: Ismael Upright ]