How to correctly make a thread safe Singleton Factory in Java? [duplicate]

Answer 1

Thread safe implementations(examples) on Wiki - Singleton Pattern on Wikipedia

As in the link above, a single-element enum type is the best way to implement a Singleton for any Java that supports enums.

One of the best yet simple ones:

public class ClientFactory{
    private ClientFactory() {}

    private static ClientFactory INSTANCE=null;

    public static ClientFactory getInstance() {
        if(INSTANCE==null){
            synchronize(ClientFactory.class){
                if(INSTANCE==null) // check again within synchronized block to guard for race condition
                    INSTANCE=new ClientFactory();
            }
        }
        return INSTANCE;
    }
}

Source: Wikipedia