How a static synchronized function works?

Question

When a Java member needs to be thread-safe, we do like the following:

 public synchronized void func() {
     ...
 }

This syntax equivalent to:

 public void func() {
      synchronized(this) {
           ....
      }
 }

That is, it actually uses this for a lock.

My question is, if I use synchronized with a static method, as follows:

class AA {
    private AA() {}

    public static synchronized AA getInstance() {
        static AA obj = new AA();
        return obj;
    }
}

In this case, on what is the lock made for the synchronized method?

The class AA will be locked `synchronized(AA.class)` , but no instances — , Jul 16 '13 at 05:49
Isn't the static declaration of the local variable `obj` a syntax error? — Thilo, Jul 16 '13 at 05:53

sanbhat · Accepted Answer · 2013-07-16T06:05:53.273

13

In case of static synchronized method, the class object of your class AA will be implicit lock

its equivalent to

class AA {
    private AA() {}

    public static AA getInstance() {
        synchronized(AA.class) {
           AA obj = new AA();
           return obj;
        }
    }
}

edited Jul 16 '13 at 06:05

answered Jul 16 '13 at 05:47

sanbhat

17,522
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3

+1. equivalent to `synchronized (AA.class) { ` – Thilo Jul 16 '13 at 05:48
but what about `static AA obj = new AA();` - wouldn't it give compile time error ? – exexzian Jul 16 '13 at 06:00
thanks @Bingo i have corrected it now.. I had not paid attention to that code – sanbhat Jul 16 '13 at 06:10
1

@sanbhat well it happens, but u surely deserve upvote +1 – exexzian Jul 16 '13 at 06:11
@sanbhat So when we synchronze on `this`, another object cannot execute the synchronized thread, but when synchronized on class? What happens exactly? True that it acquired the lock of the class; does that mean another class cannot execute the synchronized piece of code? – Prasad Kharkar Jul 16 '13 at 06:16
@PrasadKharkar since the lock object is the `Class` object associated with the class, and there will be only **one** `Class` object, only one thread can execute `static synchronized` methods of a class **at any instance of time**. To make multiple methods of a same class, execute in a mutually exclusive manner, all the method can be declared as `static synchronized` – sanbhat Jul 16 '13 at 06:32
@sanbhat, got it :) thanks but what is the use of it? I mean is there any practical use for it? sorry if its a dumb question. But I would like to know – Prasad Kharkar Jul 16 '13 at 07:10
An old blog written by me i hope should help.. http://bhat86.blogspot.in/2011/06/mutual-exclusion-among-2-different.html – sanbhat Jul 16 '13 at 09:01

score 7 · Answer 2 · edited Jun 20 '20 at 09:12

From section 8.4.3.6 of the JLS:

A synchronized method acquires a monitor (§17.1) before it executes.

For a class (static) method, the monitor associated with the Class object for the method's class is used.

So your code acquires the monitor for AA.class. As sanbhat says, it's like

synchronized(AA.class) {
    ...
}

... just as with an instance method it would be

synchronized(this) {
    ...
}

score 0 · Answer 3 · answered Jul 16 '13 at 05:51

0

It worked on AA.class lock.

public static AA getInstance() {
        synchronized(AA.class){
            static AA obj = new AA();
            return obj;
        }

}

answered Jul 16 '13 at 05:51

lichengwu

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How a static synchronized function works?

3 Answers3