Java synchronized method lock on object, or method?

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If you declare the method as synchronized (as you’re doing by typing public synchronized void addA()) you synchronize on the whole object, so two thread accessing a different variable from this same object would block each other anyway.

If you want to synchronize only on one variable at a time, so two threads won’t block each other while accessing different variables, you have synchronize on them separately in synchronized () blocks. If a and b were object references you would use:

public void addA() {
    synchronized( a ) {
        a++;
    }
}

public void addB() {
    synchronized( b ) {
        b++;
    }
}

But since they’re primitives you can’t do this.

I would suggest you to use AtomicInteger instead:

import java.util.concurrent.atomic.AtomicInteger;

class X {

    AtomicInteger a;
    AtomicInteger b;

    public void addA(){
        a.incrementAndGet();
    }

    public void addB(){ 
        b.incrementAndGet();
    }
}

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