synchronized method

[Harikrishna Gorrepati]

Hi, this is from K&B. How can we say synchronized (args) here at Line 3? I thought it should be either synchronized (this) or synchronized (P763_Question05.class). Please advice

public class P763_Question05 {
    public static void main(String[] args) {
	synchronized (args) {
	    System.out.print("2 ");
	    try {
		args.wait();
	    } catch (InterruptedException ie) {

	    }
	}
	System.out.print("3 ");
    }
}    

[Mohamed Sanaulla]

Harikrishna Gorrepati wrote:Hi, this is from K&B. How can we say synchronized (args) here at Line 3? I thought it should be either synchronized (this) or synchronized (P763_Question05.class). Please advice

Is it from the perspective of avoiding Deadlock? if not, then as args is also an object, it can be used right?

[Harikrishna Gorrepati]

If we can get lock on any object, Please advice what is the difference between synchronized(args) and synchronized(this)

[Henry Wong]

Harikrishna Gorrepati wrote:If we can get lock on any object, Please advice what is the difference between synchronized(args) and synchronized(this)

Different objects?

And in your example, the second case will not work, as static methods don't have access to a "this" object.

Henry

[Mohamed Sanaulla]

I was trying some code example to show the difference. Came up with one-

public class MyjavaRanch {
    public static void main(String[] args) {
        MyThread mt = new MyThread();
        Thread myT = new Thread(mt);
        Thread myT2 = new Thread(mt);
        myT.start();
        myT2.start();
    }
}

class MyThread implements Runnable
{
    int i=0;
    public void run()
    {
        myMethod1(new String[]{"sana","mohamed"});
        myMethod2();
    }
    public void myMethod1(String[] args)
    {
        synchronized(args) //Try executing this by replacing with this. 
        {
            i=i+1;
            System.out.println("In myMethod1 "+i);
            i=i+1;
        }

    }
    public void myMethod2()
    {
        synchronized(this)
        {
            i=i+1;
            System.out.println("In myMethod2 "+i);
            i=i+1;
        }
    }
}

You can see the difference in the output- the way myMethod1 increments and prints in both the cases.

[Harikrishna Gorrepati]

You are right. I meant to ask something like this. Let me change my question. Assuming that there is no "static" context, Please advice what is the difference between synchronized(args) and synchronized(this). Here is the code.

public class P763_Question05 {
    public void callMe(String[] args)
	synchronized (args) {
	    System.out.print("2 ");
	    try {
		args.wait();
	    } catch (InterruptedException ie) {

	    }
	}
	System.out.print("3 ");
    }
}       

[Mohamed Sanaulla]

Harikrishna Gorrepati wrote:You are right. I meant to ask something like this. Let me change my question. Assuming that there is no "static" context, Please advice what is the difference between synchronized(args) and synchronized(this).

If you try to execute the above code which I tried-
with synchronized(args)
One of the output:
In myMethod1 1
In myMethod2 4
In myMethod1 2
In myMethod2 7

with synchronized(this)
one of the output:
In myMethod1 1
In myMethod2 3
In myMethod1 5
In myMethod2 7

You can see that- the second set of output is clearly what we were expecting.

or better, you can make the following changes-

class MyThread implements Runnable
{
    int i=0;
    int j=0; //new variable which wil be used by myMethod2
    public void run()
    {
        myMethod1(new String[]{"sana","mohamed"});
        myMethod2();
    }
    public void myMethod1(String[] args)
    {
        synchronized(args)
        {
            i=i+1;
            System.out.println("In myMethod1 "+i);
            i=i+1;
        }

    }
    public void myMethod2()
    {
        synchronized(this)
        {
            j=j+1;
            System.out.println("In myMethod2 "+j);
            j=j+1;
        }
    }
}

[Abimaran Kugathasan]

Harikrishna Gorrepati wrote:You are right. I meant to ask something like this. Let me change my question. Assuming that there is no "static" context, Please advice what is the difference between synchronized(args) and synchronized(this). Here is the code.

public class P763_Question05 {
    public void callMe(String[] args)
	synchronized (args) {
	    System.out.print("2 ");
	    try {
		args.wait();
	    } catch (InterruptedException ie) {

	    }
	}
	System.out.print("3 ");
    }
}       

As, Henry said, Both the code snaps are synchronized on different objects. synchronized(this) means, you are synchronizing the below code snaps on the currently executing object. And, you can specify any other object instead of this currently executing object. In the second case, the below code snaps are synchronized on the args object.

[Mohamed Sanaulla]

I just wrote a blog post related to this. Might be helpful.