why is static inner class singleton thread safe?
http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom
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public class Singleton { // Private constructor prevents instantiation from other classes private Singleton() { System.out.println("constructor"); } /** * SingletonHolder is loaded on the first execution of * Singleton.getInstance() or the first access to SingletonHolder.INSTANCE, * not before. */ private static class SingletonHolder { public static final Singleton INSTANCE = new Singleton(); } public static Singleton getInstance() { return SingletonHolder.INSTANCE; } public static void main(String[] args) { System.out.println("main"); getInstance(); }}?
When to use it
Use this pattern if the initialization of the class is expensive and it cannot be done safely at class-loading time and the initialization is highly concurrent. The crux of the pattern is the safe removal of the synchronization overhead associated with accessing a singleton instance.
其中有一种使用该模式的情况就是在class-loading阶段如果初始化不安全
Since the class initialization phase is guaranteed by the JLS to be serial, i.e., non-concurrent, no further synchronization is required in the static getInstance method during loading and initialization.
类初始化阶段是连续的，也就是非并发的
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不过构造时也会引起double-checked locking问题 就是会存在某一个瞬间引用不为空但构造未完成。
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from: http://topic.csdn.net/u/20110222/13/87f22039-9ef0-48d0-8270-c91516e5cee5.html