synchronized keyword usage

[Devadoss Poornachari]

Can anyone explain the difference between synchronized(this) and synchronized(MyClass.class) usage.

[Peter den Haan]

Originally posted by Devadoss Poornachari:
Can anyone explain the difference between synchronized(this) and synchronized(MyClass.class) usage.

The first synchronizes on an object, i.e. a single instance of the class; as you no doubt know, there can be any number of instances of a class. The second synchronizes on the class object itself; for a given class, there is just one class object. As a consequence, the last style of synchronization allows only one thread into the block at a time, whereas the first style allows one thread per instance.
Incidentally,

public synchronized foobar() {
   // blah
}

Is equivalent to

public foobar() {
   synchronized (this) {
      // blah
   }
}

As you know, static methods are associated with a class rather than any specific instance of a class, so it shouldn't surprise you that

public static synchronized foobar() {
   // blah
}

Is equivalent to

public static foobar() {
   synchronized (MyClass.class) {
      // blah
   }
}

Does that help?
- Peter