Re: Singleton Pattern

Eric Sosman <esosman@ieee-dot-org.invalid>
Sat, 13 Aug 2011 20:56:12 -0400
On 8/13/2011 4:56 PM, wrote:

People have been coming up with creative solutions to lazily implement
the singleton pattern in a thread-safe way. We have seen things like
double-checked locking and creating instance via a single-elemnt enum

I have thought of yet another way to implement this in a lazy and
thread-safe way. I haven't seen this proposed anywhere and it seems to
work unless I am missing something. Here it goes:

public class Singleton {

    private static Singleton _instance;
    private Singleton(){}

    private synchronized static void createInstance(){
        _instance = new Singleton();

    public static Singleton getInstance(){
        if (_instance == null){
        return _instance;

The synchronized createInstance() method would eliminate the need to
do double-checked locking and the synchronization would happen only
when multiple threads call getInstance() before _instance has been

Anyone see any issues with this?


    T1: if (_instance == null)
        "Aha! It's null! Let's go make one."

    ** context switch **

    T2: if (_instance == null)
        "Aha! It's null! Let's go make one."

    T2: _instance = createInstance(); // instance #1

    ** context switch **

    T1: _instance = createInstance(); // instance #2

.... and the two threads go merrily on their way with references
to two different Singleton instances. With N threads, you could
get as many as N distinct instances.

Eric Sosman

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