Synchornized inside run

[M Rama]

Continuing with my Thread vs Runnable doubts, in the following code:

public class SynchInRunnable implements Runnable{
public void run() {
synchronized(this){
System.out.println("this is the object of "+ this);
}
 
}
public static void main(String[] args) {
SynchInRunnable s = new SynchInRunnable();
Thread t = new Thread(s);
t.start();
}
}
 

Here, which object is locked for the synchronized. Is it Thread object or Runnable?

The output prints:

this is the object of thread1.SynchInRunnable@119c082



which doesn't tell me a whole lot

[vinuharan haran]

hi,
i just called the holdsLock method in ur code.

 public class SynchInRunnable implements Runnable
{
public void run() 
{
synchronized(this)
{
System.out.println("this is the object of "+ this);
        System.out.println(Thread.holdsLock((Runnable)this));
} 
}
public static void main(String[] args)
{
SynchInRunnable s = new SynchInRunnable();
Thread t = new Thread(s);
t.start();
}
}  

The output is true.

If u type cast this(current object) to Thread it will give compiler error.
so it holds the runnable object lock.

[Joe Sondow]

Here's another way of knowing that "this" is a Runnable, not a Thread:

The "this" object always refers to an instance of the enclosing class, which is SynchInRunnable in this case. Since SynchInRunnable is a Runnable, not a Thread, "this" must be a Runnable.

To find out what class an object is an instance of, you can do this:

public class SynchInRunnable implements Runnable{
    public void run() {
        synchronized(this){
            System.out.println("this is an instance of " + this.getClass().getName() );
        }
    }
 
    public static void main(String[] args) {
        SynchInRunnable s = new SynchInRunnable();
        Thread t = new Thread(s);
        t.start();
    }
}

The output is:
this is an instance of SynchInRunnable
[ April 14, 2005: Message edited by: Joe Sanowitz ]

[Joe Sondow]

Getting a little off topic, but if you really want to test whether something is a subclass or implementer of something else, you can run reflection code like this:

// obj can be a reference to any type of object
if (obj!=null)
{
    Class thisClass = obj.getClass();
    System.out.println("this is an instance of " + thisClass.getName() );
    Class[] interfaces = thisClass.getInterfaces();
    for (int i=0; i<interfaces.length; i++)
        System.out.println(thisClass.getName() + " implements " + interfaces[i]);
    Class superclass = thisClass.getSuperclass();
    System.out.println(thisClass.getName() + " extends " + superclass);
}

[ April 14, 2005: Message edited by: Joe Sanowitz ]