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

Yes, that is one difference. The other is that you can acquire a lock on other objects than this.

Answer 2

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 method, the lock is held only during the synchronized block (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.

Answer 3

A synchronized method is shorthand. This:

class Something {
    public synchronized void doSomething() {
        ...
    }

    public static synchronized void doSomethingStatic() {
        ...
    }
}

is, for all intents and purposes, equivalent to this:

class Something {
    public void doSomething() {
        synchronized(this) {
            ...
        }
    }

    public static void doSomethingStatic() {
        synchronized(Something.class) {
            ...
        }
    }
}

(Where Something.class is the class object for the class Something.)

So indeed, with a synchronized block, you can be more specific about your lock, and more fine-grained about when you want to use it, but other than that there's no difference.

Answer 4

A synchronized method locks on the object instance the method is contained in.

Where as a synchronized block can lock on ANY object - typically a mutex obect defined as an instance variable. This allows more control over what locks are in operation.

Answer 5

The key difference is this: if you declare a method to be synchronized, then the entire body of the method becomes synchronized; if you use the synchronized block, however, then you can surround just the "critical section" of the method in the synchronized block, while leaving the rest of the method out of the block.

If the entire method is part of the critical section, then there effectively is no difference. If that is not the case, then you should use a synchronized block around just the critical section. The more statements you have in a synchronized block, the less overall parallelism you get, so you want to keep those to the minimum.