Java Synchronized Block for .class

The snippet synchronized(X.class) uses the class instance as a monitor. As there is only one class instance (the object representing the class metadata at runtime) one thread can be in this block.

With synchronized(this) the block is guarded by the instance. For every instance only one thread may enter the block.

synchronized(X.class) is used to make sure that there is exactly one Thread in the block. synchronized(this) ensures that there is exactly one thread per instance. If this makes the actual code in the block thread-safe depends on the implementation. If mutate only state of the instance synchronized(this) is enough.

To add to the other answers:

static void myMethod() {
  synchronized(MyClass.class) {
    //code
  }
}

is equivalent to

static synchronized void myMethod() {
  //code
}

and

void myMethod() {
  synchronized(this) {
    //code
  }
}

is equivalent to

synchronized void myMethod() {
  //code
}