Synchronization variants

Question

What is the difference between the two synchronizations:

public synchronized void set (int i) { 
   this.i = i;
}

and

public void set (int i) {
       synchronized (this) {
               this.i = i;
         }
    }

See also: http://stackoverflow.com/questions/4394976/what-is-the-difference-between-synchronizedthis-and-synchronized-method — assylias, Sep 19 '13 at 08:03

score 2 · Answer 1 · edited Sep 19 '13 at 08:06

2

First one is synchronized method and second one is synchronized block.

Here as you synchronized on this object in block both represent same. In synchronized-method thread acquire the lock of current object.

Note: in synchronized block you can synchronize block of code instead whole method body, as well use different resource for locking(except this).

edited Sep 19 '13 at 08:06

Suresh Atta

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answered Sep 19 '13 at 08:00

Subhrajyoti Majumder

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Prasad Kharkar · Answer 2 · 2013-09-19T08:08:05.013

1

first case : whole method is synchronized
second case: only block of code in synchronized for which synchronized(this) is written

So if you write second method as

public void set (int i) {
       // Code here is not synchronized
       synchronized (this) { // only this block of code is synchronized
               this.i = i;
         }
       // code after this is also not synchronized. 
    }

But in case of second block, you can also synchronize of some other object.

public void set (int i) {
       synchronized (someObject) {
               this.i = i;
         }
    }

edited Sep 19 '13 at 08:08

answered Sep 19 '13 at 07:59

Prasad Kharkar

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But it is the same, isn't it? – ceving Sep 19 '13 at 08:00
1

@ceving, your example would be funcionally the same. But if you want a method that should be be completely synchronized but only a code blcok from it, then you'll have to use second approach – Prasad Kharkar Sep 19 '13 at 08:03
I could write an extra method for this code block. – ceving Sep 19 '13 at 08:10
@ceving, yes you can, but if you write a separate method and synchronize it, you can't synchronize on some other object as mentioned in the answer. – Prasad Kharkar Sep 19 '13 at 08:14

score 1 · Accepted Answer · answered Sep 19 '13 at 08:07

They are different ways of writing the same thing. Java could have just had the second form. Synchronizing the whole body of a method on the method's this object is a particularly common case, so the language provides a quick, simple way of doing it.

The Java Language Specification states the equivalence in 8.4.3.6 synchronized Methods. synchronized void bump() { count++; } has exactly the same effect as

void bump() {
    synchronized (this) {
        count++;
    }
}

score 1 · Answer 4 · answered Sep 19 '13 at 08:09

These two synchronisation methods are equivalent because you gave this in the synchronized block.

The synchronization is based on the Intrinsic Lock or Monitor Lock, an attribute of each Object

As the other answers stated your first proposition is a synchronized method which means that the thread will acquire the lock of the current Object.

From the Oracle Tutorials :

When a thread invokes a synchronized method, it automatically acquires the intrinsic lock for that method's object and releases it when the method returns. The lock release occurs even if the return was caused by an uncaught exception.

Your second proposition is a synchronized block or synchronized statement. In this case the thread acquire the lock of the object put in argument.

Again from the Oracle Tutorials :

Unlike synchronized methods, synchronized statements must specify the object that provides the intrinsic lock:

In your case you put this so it will take the lock of the current object as the synchronized method did.

But you can also give it another object and it will take its lock leaving the current object lock "unchanged"

score 0 · Answer 5 · answered Sep 19 '13 at 08:06

0

There is no difference between the two synchronizations, but the second one is more flexible: you can add unsynchronized code outside the synchronized block in the same method, or synchronize on an object different from this.

answered Sep 19 '13 at 08:06

aUserHimself

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Synchronization variants

5 Answers5