Question

How thread-safe is enum in java? I am implementing a Singleton using enum (as per Bloch's Effective Java), should I worry at all about thread safety for my singleton enum? Is there a way to prove or disprove that it is thread safe?

// Enum singleton - the preferred approach
public enum Elvis { 
    INSTANCE;
    public void leaveTheBuilding() { ... }
}

Thanks

Answer 1

As @Mike is saying, creation of enum is guaranteed to be thread safe. However, methods that you add to an enum class do not carry any thread safety guarantee. In particular, the method leaveTheBuilding may be executed, concurrently, by multiple threads. If this method has side effects (changes the state of some variable) then you need to think about protecting it (i.e., make it synchronized) or parts thereof.