Java同步超类的私有方法

Question

I have class A and B which inherit from Base.

Base has a private method. It is possible that A and B execute this method at the same time.

So I want to synchronize that method. If I just put "synchronize" keyword in the signature, will that make that A and B execution is synchronized (share the same lock) ? Or do I need to generate a static lock object...?

Sorry, a bit confused...

Answer 1

If your code looks a bit like the example below, then it is not thread safe and you need to include some form of synchronization to make sure the call to the private method increment is atomic and ensure visibility. In that simple example, making getNewCounter synchronized would be enough. Using a lock object would work too.

public class Base {
    private int counter;

    private void increment() {
        counter++;
    }
    protected int getNewCounter() {
        increment();
        return counter;
    }
}

public class A extends Base {
    public int aMethod() {
        return getNewCounter();
    }
}

public class B extends Base {
    public int anotherMethod() {
        return getNewCounter();
    }
}

Answer 2

Base has a private method. It is possible that A and B execute this method at the same time.

Private methods or fields are not inherited by derived class, so answer to your question is no.

Edit As per comment:

If you have two objects A and B and they are trying to manipulate Singleton object C, in that case it is C which should have synchronization and not A and B.

General rule followed in multi-threaded application is, Every class which needs protection from concurrent access in order to maintain a stable state is responsible for using synchronization correctly, you shouldn't and cant expect caller of the method to access the object state in a synchronized way.

Answer 3

What I can understand from your post is that you have a base class with a private method. You are creating different objects of this base class or the classes derived from the base class.

Now if you make the private method syncronized It will not make the execution of that method syncronized, as you have multiple objects. Instance level lock is only per object.

For eg You have ObjectA and ObjectB of class Test, both these objects can execute private method say myMethod() defined in class Test simultaneously even if its synchronized because the lock is on the object.

Hope this helps.

Answer 4

What i know is that, private members are NOT inherited. So class A and class B cant access this private method with either using inheritance or creating an object of Base class to call this method, until there is a public method in Base class which has the access to this private method.