What Cases Require Synchronized Method Access in Java?

Question

In what cases is it necessary to synchronize access to instance members? I understand that access to static members of a class always needs to be synchronized- because they are shared across all object instances of the class.

My question is when would I be incorrect if I do not synchronize instance members?

for example if my class is

public class MyClass {
    private int instanceVar = 0;

    public setInstanceVar()
    {
        instanceVar++;
    }

    public getInstanceVar()
    {
        return instanceVar;
    }
}

in what cases (of usage of the class MyClass) would I need to have methods: public synchronized setInstanceVar() and public synchronized getInstanceVar() ?

Thanks in advance for your answers.

Answer 1

It depends on whether you want your class to be thread-safe. Most classes shouldn't be thread-safe (for simplicity) in which case you don't need synchronization. If you need it to be thread-safe, you should synchronize access or make the variable volatile. (It avoids other threads getting "stale" data.)