What is the difference between a synchronized method and synchronized block in Java?

Question

What is the difference between a synchronized method and synchronized block in Java ?

I have been searching the answer on the Net, people seem to be so unsure about this one :-(

My take would be there is no difference between the two, except that the synch block might be more localized in scope and hence the lock will be of lesser time ??

And in case of Lock on a static method, on what is the Lock taken ? What is the meaning of a Lock on Class ?

Answer 1

A synchronized method uses the method receiver as a lock (i.e. this for non static methods, and the enclosing class for static methods). Synchronized blocks uses the expression as a lock.

So the following two methods are equivalent from locking prospective:

synchronized void mymethod() { ... }

void mymethod() {
  synchronized (this) { ... }
}

For static methods, the class will be locked:

class MyClass {
  synchronized static mystatic() { ... }

  static mystaticeq() {
    syncrhonized (MyClass.class) { ... }
  }
}

For synchronized blocks, you can use any non-null object as a lock:

synchronized (mymap) {
  mymap.put(..., ...);
}

Lock scope

For synchronized methods, the lock will be held throughout the method scope, while in the synchronized block, the lock is held only during that block scope (otherwise known as critical section). In practice, the JVM is permitted to optimize by removing some operations out of the synchronized block execution if it can prove that it can be done safely.