What will happen in the following mulththreaded scenario?

Question

Suppose I have a class where in the have methods synchronized in the way as follows.

public class Test{
    public static synchronized void method1(){
    }
    public static synchronized void method2(){
    }
    public synchronized void method3(){
    }
    public synchronized void method4(){
    }
}

So there is a scenario when two threads are calling method1 and method2 simultaneously.I feel that only one of the methods be allowed to call.What will be the case if they call method1 and method3.Will there be a same scenario here too?what will be the case with method3 and method4 from same object?

http://stackoverflow.com/questions/30208524/why-is-this-class-not-thread-safe — Liviu Stirb, Dec 28 '15 at 16:36
You could try it.. If you call a synchronized method from two different places at the same time, both calls will complete, one after the other (afaik). — Jonas Czech, Dec 28 '15 at 16:38

Patricia Shanahan · Accepted Answer · 2015-12-28T16:48:16.460

If you have two objects of class Test, referenced by x and y, there will be three monitors, one for Test, one for x, and one for y.

method1 and method2 both use the Test monitor, and so exclude each other. method3 and method4 each use the monitor for their target object, so they exclude each other if called for the same object, but not if called for different objects. They don't involve the Test monitor, so they don't exclude the static methods.

score 0 · Answer 2 · answered Dec 28 '15 at 16:41

So there is a scenario when two threads are calling method1 and method2 simultaneously.I feel that only one of the methods be allowed to call

This does not make any sense to me.

A single thread can call only one method at a time.On the other hand a single method can be called by multiple threads.

Liviu Stirb · Answer 3 · 2015-12-28T16:56:46.213

By using synchronized on a static method you will synchronize on the Class object. So basically only one synchronized method can be run from different threads. The one that can't acquire the lock will wait for the first method to finish

Normal method do synchronize on the instance. So a static method and a non static method can be run at once by different threads. 2 different non static synchronized methods won't run at once if one method is called from different threads. One method will wait for the first to finish

So:

method1 and method2 won't run at the same time if they are called from different threads

method3 and method4 won't run at the same time if they are called from different threads

method1 and method3 will run at the same time if they are called from different threads (same for method2 and method 4)

score 0 · Answer 4 · answered Dec 28 '15 at 16:42

threads trying to call synchronized static and non-static methods will never block each other.

a thread calling method1 and method2 will be locked on Test.class, while formethod3 and method4 it will be locked on this.

so threads calling method1 and method3 won't block each other , while threads calling method3 and method4 will block each other if they use the same instance of Test.

What will happen in the following mulththreaded scenario?

4 Answers4