Thread-safe code without using the `synchronized` keyword?

Question

What are the possible ways to make code thread-safe without using the synchronized keyword?

Answer 1

Actually, lots of ways:

1. No need for synchronization at all if you don't have mutable state.
2. No need for synchronization if the mutable state is confined to a single thread. This can be done by using local variables or java.lang.ThreadLocal.
3. You can also use built-in synchronizers. java.util.concurrent.locks.ReentrantLock has the same functionality as the lock you access when using synchronized blocks and methods, and it is even more powerful.

Answer 2

Only have variables/references local to methods. Or ensure that any instance variables are immutable.