Synchronized method does not work as expected

Question

I have a variable which is shared by two threads. The two threads will do some operations on it. I don\'t know why the result of sharedVar is different every time I execute

Answer 1

A synchronized method protects the resource this that means that your code is equivalent to:

private void addOne()
{
    synchronized(this)
    {
        for (int i = 0; i < times; ++i)
        {
            Main.sharedVar ++;
        }
    }
}

But you have 2 objects for which addOne method is called. That means this for mt1.addOne is not the same than this for mt2.addOne and therefore you don't have a common resource of synchronization.

Try changing yout addOne code to:

private void addOne()
{
    synchronized(MyThread.class)
    {
        for (int i = 0; i < times; ++i)
        {
            Main.sharedVar ++;
        }
    }
}

And you will observe the expected behaviour. As the comments below suggest, it is better to use a different object than MyThread.class for synchronization since class objects are accesible from many points and it is easy that other code may try to synchronize using the same object.