Question

Right now i am trying to learn more about java threading, and i have a small question that i cannot find a direct answer to anywhere. Lets say i have two threadsthat both share an object:

public class FooA implements Runnable
{
    Object data;

    public FooA(final Object newData)
    {
        data = newData;
    }

    public void doSomething()
    {
        synchronized(data)
        {
            data = new Integer(1);
        }
    }

    public void run() {
        // Does stuff
    }
}

public class FooB implements Runnable
{
    Object data;

    public FooB(final Object newData)
    {
        data = newData;
    }

    public void doSomething()
    {
        synchronized(data)
        {
            System.out.println(data);
        }
    }
}

Would FooA block FooB when it is in the doSomething section of the code? Or vice versa? My gut feeling says yes, but according to the book i am reading it says no. Hence the need for monitor objects. I made a slightly more complex version of this, and everything worked fine.

I looked around a bit, but couldn’t find a concrete answer.

Answer 1

The problem is that one of the synchronised blocks assigns a new object to data. If that block starts first, and changes data, subsequent runs will be using a different object to lock on. So from then on, both will be able to run simultaneously.