Re: Singleton Pattern

Lew <>
Sat, 13 Aug 2011 21:12:01 -0700 (PDT)
Eric Sosman wrote: 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

Which, as you know, is broken. As it happens, it's broken in pretty much e=
xactly the same way as your proposal.

This is one of the most well-discussed idioms in the literature. The flaws=
 and solutions are pretty much out there for anyone to google.

Bear in mind that lazy initialization should be done judiciously, if at all=
.. See Joshua Bloch's /Effective Java/, Item 71, for example.

The Singleton (anti)pattern is also heavily abused.

and creating instance via a single-elemnt enum type.

I have thought of yet another [sic] 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.

See Doug Lea's and Brian Goetz's articles and books on Java threading, as w=
ell as the Java Language Specification. IBM Developerworks (Java) is a goo=
d online resource for articles. I recommend in particular /Java Concurrenc=
y in Practice/ by Goetz, et al., and many people urge /Concurrent Programmi=
ng in Java/ by Mr. Lea. The aforementioned /Effective Java/ has several ef=
fective tips on the subject as well.

Concurrent programming is subtle and quick to anger. It pays to study the =
literature thoroughly.


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