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I'm trying to explore static synchronized method I got the theoretical concept that it's acquire lock on class, not on instance. But I'm failed to create an example where I Can test it.

Here I have created the Code to test, But both instance are accessing static synchronized method at the same time.

class Demo{ public static synchronized void a(){ System.out.println("A Method " + Thread.currentThread().getName()); } } public class StaticSyn{ public static void main(String[] args){ Demo obj = new Demo(); Demo obj2 = new Demo(); Thread one = new Thread(){ @Override public void run(){ int i=0; while(i<5){ obj.a(); try{ Thread.sleep(100); }catch(InterruptedException e){ } i++; } } }; Thread two = new Thread(new Runnable(){ @Override public void run(){ int i=0; while(i<5){ obj2.a(); try{ Thread.sleep(100); }catch(InterruptedException e){ } i++; } } }); one.start(); two.start(); } }

With static synchronized I'm getting this output.

A Method Thread-0 A Method Thread-1 A Method Thread-1 A Method Thread-0 A Method Thread-1 A Method Thread-0 A Method Thread-0 A Method Thread-1 A Method Thread-1 A Method Thread-0

Without static keyword I'm getting this output.

A Method Thread-0 A Method Thread-1 A Method Thread-1 A Method Thread-0 A Method Thread-0 A Method Thread-1 A Method Thread-0 A Method Thread-1 A Method Thread-1 A Method Thread-0

So, where is the problem? and How I can test that Just one object is accessing static synchronized method.

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  • A static synchronized method in Demo is equivalent to wrapping the method body in synchronized (Demo.class) { ... }. Commented May 5, 2016 at 13:16
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    There is no problem, neither of your threads is inside a() while the other is. Commented May 5, 2016 at 13:17
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    What are you actually expecting to happen? Commented May 5, 2016 at 13:17
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    And because System.out is internally synchronized already, there will be no difference whether Demo.a() is synchronized or not. Commented May 5, 2016 at 13:18
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    I'd add that calling a static method on object instances is a code smell (at least a compiler warning in Java), your grasp on "what you want to achieve" does not seem good enough just yet. Commented May 5, 2016 at 13:20

2 Answers 2

Reset to default
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You can add a sleep in your method a() as next:

public static synchronized void a(){ System.out.println("Before sleep: A Method " + Thread.currentThread().getName()); try{ Thread.sleep(100); } catch(InterruptedException e){ Thread.currentThread().interrupt(); } System.out.println("After sleep: A Method " + Thread.currentThread().getName()); }

You will then see that you won't have 2 threads executing this block at the same time like in the next output, indeed you never have twice consecutively Before sleep:

Before sleep: A Method Thread-0 After sleep: A Method Thread-0 Before sleep: A Method Thread-1 After sleep: A Method Thread-1 Before sleep: A Method Thread-0 After sleep: A Method Thread-0 Before sleep: A Method Thread-1 After sleep: A Method Thread-1 Before sleep: A Method Thread-0 After sleep: A Method Thread-0 Before sleep: A Method Thread-1 After sleep: A Method Thread-1 Before sleep: A Method Thread-0 After sleep: A Method Thread-0 Before sleep: A Method Thread-1 After sleep: A Method Thread-1 Before sleep: A Method Thread-0 After sleep: A Method Thread-0 Before sleep: A Method Thread-1 After sleep: A Method Thread-1

What is the difference between static synchronized method and synchronized method?

The main difference is the object used to synchronize the access.

Static

For example doing this:

class Demo { public static synchronized void a() { // Rest of the method } }

Is equivalent to this:

class Demo { public static void a() { synchronized (Demo.class) { // Rest of the method } } }

In other words in case of a static method it uses the object representing the class itself to synchronize the access.

Non Static

For example doing this:

class Demo { public synchronized void a() { // Rest of the method } }

Is equivalent to this:

class Demo { public void a() { synchronized (this) { // Rest of the method } } }

In other words in case of a non static method it uses the current instance of the class to synchronize the access.

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2 Comments

Really thanks it clearing, but little confused. can you write an example of static synchronized method and synchronized method in code that could describe difference between them.
sure, hold on a little bit
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You need to make the a() method run long to make sure the method calls aren't overlapping. This is one way of doing it:

package com.example; public class Demo { public static synchronized void a() { for (int i = 0; i < 5; i++) { System.out.println("A Method " + Thread.currentThread().getName()); try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } } } public static void main(String[] args) { final Demo obj = new Demo(); final Demo obj2 = new Demo(); Thread one = new Thread() { @Override public void run() { obj.a(); // same as Demo.run() } }; Thread two = new Thread() { @Override public void run() { obj2.a(); // same as Demo.run() } }; one.start(); two.start(); } }
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