来自不同类的同步语句

Question

I have two classes B and C that implements the interface A. In both classes exist a method with a statement that needs to be synchronized. I have this:

    class B implements A {
         @Override
         public void method(){
           //some code here
           synchronized (this.getClass()) {
             //some code here
           }    
        }
    }



        class C implements A {
             @Override
             public void method(){
               //some code here
               synchronized (this.getClass()) {
                 //some code here
               }    
            }
        }   

I need to synchronize the statements in both classes but with this.getClass return different classes and it can execute at the same time. How can I synchronized these two statement from different classes in a way that it will execute only once at the same time?

Answer 1

The most likely reason for needing to synchronize them is that they are going to access some common object. If so, synchronize on that object.

If not, and if you control interface A, you can add:

public static final Object LOCK = new Object();

and have them both synchronize on A.LOCK.

Answer 2

Synchronize on some other object that both implementations of method() have access to.

class D {
    public static final Object SYNC_OBJ = new Object();
}

class B implements A {
     @Override
     public void method(){
       //some code here
       synchronized (D.SYNC_OBJ) {
         //some code here
       }    
    }
}

class C implements A {
     @Override
     public void method(){
       //some code here
       synchronized (D.SYNC_OBJ) {
         //some code here
       }    
    }
}

Answer 3

You could declare a shared monitor object in A , for example:

static final Object MONITOR = new Object();

Then use synchronized(MONITOR) in B and C .

Answer 4

You can easily modify your code without introducing a new class/variable to do exactly what you want (assuming you want to synchronize all implementations of A):

synchronized (A.class) {
    // do something
}

This is not a good solution however. The right way to do it is using a lock.

Answer 5

Synchronized block in java is synchronized on some objects. synchronized blocks synchronized on same object reference only allows 1 thread at a time. other will be blocked until it leaves. So use same object reference for both methods. https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html