synchronized block with different class as lock

Question

I want to know if my understanding of synchronized(x.class){..} is correct.

Assumption:

1. I have two variables within a class which will be initialized, if needed. But only once.

2. I want to block multiple access to each variable from different threads.

3. I only want to block variable wise and not to the complete SingletonExample instance.

Example:

public class SingletonExample{

    private volatile SingletonA var;
    private volatile SingletonB tar;

    public void initVarOnDemand1() {
        if (var == null) {
            synchronized (SingletonA.class) {
                if (var == null) {
                    var = SingletonA.getInstance(1);
                }
            }
        }
    }

    public void initVarOnDemand2() {
        if (var == null) {
            synchronized (SingletonA.class) {
                if (var == null) {
                    var = SingletonA.getInstance(2);
                }
            }
        }
    }

    public void initTarOnDemand1() {
        if (tar == null) {
            synchronized (SingletonB.class) {
                if (tar == null) {
                    tar = new SingletonB.getInstance(1);
                }
            }
        }
    }

    public void initTarOnDemand2() {
        if (tar == null) {
            synchronized (SingletonB.class) {
                if (tar == null) {
                    tar = SingletonB.getInstance(2);
                }
            }
        }
    }
}

My final goal is, that no two threads can simultaneously change variable var and no two threads can change simultaneously variable tar .

But I do not want to block two threads, that access different variables.

Therefore I use block synchronization with each class as the lock object.

Is this the correct way for what I want? (I hope I explained it well enough - sorry for my bad english).

Answer 1

A better approach to this would be to use AtomicReference.compareAndSet :

private final AtomicReference<Integer> var = new AtomicReference<>();

public void foo() {
  var.compareAndSet(null, 1);
}

public void bar() {
  var.compareAndSet(null, -1);
}

This only sets the value of var if its current value is equal to the first parameter (in this case null ).