Sunday, July 15, 2007

Lazy Initialization Using an On Demand Holder

I was reminded of the "On Demand Holder" idiom the other day. If you're not familiar with it, the idiom is a thread safe replacement for lazy initialization using double checked locking. As we all know double checked locking is broken. Here is an example implementation -
public class Something {
private Something() { }

private static class Holder {
private static final Something instance = new Something();

public static Something getInstance() {
return Holder.instance;

This works because of the way classes are loaded. The inner class Holder is not loaded and initialized until a thread references it, so the static instance of Something is not created until the first time that the getInstance() method is called. Here is more information and references for "On Demand Holder".


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