Question

This seems to be a pretty frequent question though I didn’t find one. Suppose I have this piece of code:

public class MyClass {
    private AnotherClass mField;

    public void changeOne(AnotherClass newOne) {
        // <...> lines of code here
        synchronized (mField) {
            mField = newOne;
        }
        // <...> lines of code here

    }

    public void changeTwo(AnotherClass newTwo) {
        // <...> lines of code here
        mField = newTwo;
        // <...> lines of code here
    }
}

Let’s say changeOne() and changeTwo() are called from different threads. Is it enough to have a synchronized block in changeOne() to protect mField from changing by changeTwo()? Or I need to explicitly wrap each place where mField is changed into the synchronized block? (please leave behind synchronized methods and others).

Answer 1

You need to explicitly synchronize all modifications to mField using either synchronized block (or) synchronized method. Otherwise more than one thread can change mField by executing changeTwo at a time.

EDIT: As Tedd Hopp suggested, if variable is not volatile reads also need to synchronized and lock you get should be on same object.