synchronized block vs synchronized method?

Question

Can any one tell me the advantage of synchronized method over synchronized block with an example?Thanks.

Answer 1

Synchronized blocks place locks for shorter periods than synchronized methods.

Answer 2

With synchronized blocks, you can have multiple synchronizers, so that multiple simultaneous but non-conflicting things can go on at the same time.

Answer 3

The only real difference is that a synchronized block can choose which object it synchronizes on. A synchronized method can only use 'this' (or the corresponding Class instance for a synchronized class method). For example, these are semantically equivalent:

synchronized void foo() {
  ...
}

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

The latter is more flexible since it can compete for the associated lock of any object, often a member variable. It's also more granular because you could have concurrent code executing before and after the block but still within the method. Of course, you could just as easily use a synchronized method by refactoring the concurrent code into separate non-synchronized methods. Use whichever makes the code more comprehensible.

Answer 4

Note: static synchronized methods and blocks work on the Class object.

public class MyClass {
   // locks MyClass.class
   public static synchronized void foo() {
// do something
   }

   // similar
   public static void foo() {
      synchronized(MyClass.class) {
// do something
      }
   }
}

Answer 5

The main difference is that if you use a synchronized block you may lock on an object other than this which allows to be much more flexible.

Assume you have a message queue and multiple message producers and consumers. We don't want producers to interfere with each other, but the consumers should be able to retrieve messages without having to wait for the producers. So we just create an object

Object writeLock = new Object();

And from now on every time a producers wants to add a new message we just lock on that:

synchronized(writeLock){
  // do something
}

So consumers may still read, and producers will be locked.

HTH Snyke

Answer 6

Can any one tell me the advantage of synchronized method over synchronized block with an example?Thanks.

There is not a clear advantage of using synchronized method over block.

Perhaps the only one ( but I wouldn't call it advantage ) is you don't need to include the object refence "this".

Method:

public synchronized void method() { // blocks "this" from here.... 
    ...
    ...
    ...
} // to here

Block

public void method() { 
    synchronized( this ) { // blocks "this" from here .... 
        ....
        ....
        ....
    }  /// to here...
}

See? no advantage at all.

Blocks do have advantages over methods, most of all flexibility, but, that was not your question.

Answer 7

Most often I use this to synchronize access to a list or map but I don't want to block access to all methods of the object.

In the following code one thread modifying the list will not block waiting for a thread that is modifying the map. If the methods were synchronized on the object then each method would have to wait even though the modifications they are making would not conflict.

private List<Foo> myList = new ArrayList<Foo>();
private Map<String,Bar) myMap = new HashMap<String,Bar>();

public void put( String s, Bar b ) {
  synchronized( myMap ) {
    myMap.put( s,b );
    // then some thing that may take a while like a database access or RPC or notifying listeners
  }
}

public void hasKey( String s, ) {
  synchronized( myMap ) {
    myMap.hasKey( s );
  }
}

public void add( Foo f ) {
  synchronized( myList ) {
    myList.add( f );
// then some thing that may take a while like a database access or RPC or notifying listeners
  }
}

public Thing getMedianFoo() {
  Foo med = null;
  synchronized( myList ) {
    Collections.sort(myList);
    med = myList.get(myList.size()/2); 
  }
  return med;
}

Answer 8

Synchronized Method

Pros:

• Your IDE can indicate the synchronized methods.
• The syntax is more compact.
• Forces to split the synchronized blocks to separate methods.

Cons:

• Synchronizes to this and so makes it possible to outsiders to synchronize to it too.
• It is harder to move code outside the synchronized block.

Synchronized block

Pros:

• Allows using a private variable for the lock and so forcing the lock to stay inside the class.
• Synchronized blocks can be found by searching references to the variable.

Cons:

• The syntax is more complicated and so makes the code harder to read.

---

Personally I prefer using synchronized methods with classes focused only to the thing needing synchronization. Such class should be as small as possible and so it should be easy to review the synchronization. Others shouldn't need to care about synchronization.

Answer 9

This is somewhat of a duplicate of Avoid synchronized(this) in Java? and In Java critical sections, what should I synchronize on?

Answer 10

In general these are mostly the same other than being explicit about the object's monitor that's being used vs the implicit this object. One downside of synchronized methods that I think is sometimes overlooked is that in using the "this" reference to synchronize on you are leaving open the possibility of external objects locking on the same object. That can be a very subtle bug if you run into it. Synchronizing on an internal explicit Object or other existing field can avoid this issue, completely encapsulating the synchronization.

Answer 11

Hey, What about the speed ? I mean which one is better in terms of performance ? I think synchronized methods() are faster than the blocks.

Answer 12

Synchronized method is used for lock all the objects Synchronized block is used to lock specific object