Implementing enum-based singleton

Question

J. Bloch in his Effective Java suggests we use an enum-based singleton implementation. For instance:

public enum Application {

    INSTANCE;

    //methods, fields

}

This implementation is nice in the case of serialization because enums provide us with the capability of serialization by default (and we don't have to be afraid of getting two different instances while deserializing the object).

My question is how this implementation respects multhreading. How to make it thread-safe? What would we probably get if we try to access it from different threads?

Answer 1

The actual enum behavior of instatiating the instance doesn't have an issue with thread safety. However, you will need to make sure that the instance state itself is thread-safe.

The interactions with the fields and methods of Application are the risk--using either careful synchronization and locking, or purely concurrent data and careful verification that other inconsistencies can't happen, will be your best bet here.

Answer 2

Singleton ensures you only have one instance of a class per class loader.

You only have to take care about concurrency if your singleton has a mutable state. I mean if singleton persist some kind of mutable data.

In this case you should use some kind of synchronization-locking mechanishm to prevent concurrent modification of the state and/or use thread-safe data structures.