Is synchronized inherited in Java?

问题:

I have superclass Point and a synchronized method draw(). Will the subclasses of Point inherit synchronized if I override method draw() in them or I have to always write it?

回答1:

No, you will always have to write synchronized. If you call the synchronized method of the super class this will of course be a synchronized call. synchronized is not part of the method signature.

See http://gee.cs.oswego.edu/dl/cpj/mechanics.html for detailed description from Doug Lea, Java threading boss (or so).

回答2:

You can check it yourself by writing this:

public class Shape {

    protected int sum = 0;

    public synchronized void add(int x) {
        sum += x;
    }
}


public class Point extends Shape{

    public void add(int x) {
        sum += x;
    }

    public int getSum() {
        return sum;
    }
}

And test class

public class TestShapes {

    public final static int ITERATIONS = 100000;

    public static void main(String[] args) throws InterruptedException {

        final Point p = new Point();

        Thread t1 = new Thread(){
            @Override
            public void run() {

                for(int i=0; i< ITERATIONS; i++){
                    p.add(1);
                }
            }
        };

        Thread t2 = new Thread(){
            @Override
            public void run() {

                for(int i=0; i< ITERATIONS; i++){
                    p.add(1);
                }
            }
        };

        t1.start();
        t2.start();

        t1.join();
        t2.join();


        System.out.println(p.getSum()); // should equal 200000

    }
}

On my machine it was 137099 instead of 200000.

回答3:

your Overriden method will no longer be synchronized if you Override it and remove the synchronized. Found it here and here