why is static inner class singleton thread safe？

http://en.wikipedia.org/wiki/Initialization_うむdemand_ホールディングスアイドル
 

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 a at class-loading time andthe initialization is highly concurrent.The crux of the patteinininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininininincにあります。lass-loading段階で不安全Since the class initialization phase is garanted by the JLS to be serialを初期化すれば、i.e.non-concurrent、no further synchronization is required in the static get Instance method during loring段階は連続的に初期化されます。
 
しかし、構造時にもdoub-checked locking問題が発生します。ある瞬間に引用が空ではなく、構造が未完成です。
 
from:http://topic.csdn.net/u/20110222/13/87f22039-9ef0-48d0-8270-c91516e5cee5.html