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

Answer 1

To simply put it, you use synchronized when you have mutliple threads accessing the same method of the same instance which will change the state of the object/or application.

It is meant as a simple way to prevent race conditions between threads, and really you should only use it when you are planning on having concurrent threads accessing the same instance, such as a global object.

Now when you are reading the state of an instance of a object with concurrent threads, you may want to look into the the java.util.concurrent.locks.ReentrantReadWriteLock -- which in theory allows many threads to read at a time, but only one thread is allowed to write. So in the getter and setting method example that everyone seems to be giving, you could do the following:

public class MyClass{
    private ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    private int myValue = 0;

    public void setValue(){
        rwl.writeLock().lock();
        myValue++;
       rwl.writeLock().unlock();
    }

    public int getValue(){
       rwl.readLock.lock();
       int result = myValue;
       rwl.readLock.unlock();
       return result;
    }
}