单例模式的最佳实现

public class Singleton {
    // Private constructor prevents instantiation from other classes
    private Singleton() { }

    /**
    * 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;
    }
}

99%的Java程序员实现不了一个完美的单例模式，看看Wiki上给出的最佳实现：http://t.cn/zOvZMy3 通过单例模式全面了解Double-checked locking以及并发时候的锁的问题：http://t.cn/aKbmIb 

Eager initialization

If the program will always need an instance, or if the cost of creating the instance is not too large in terms of time/resources, the programmer can switch to eager initialization, which always creates an instance:

public class Singleton {
    private static final Singleton instance = new Singleton(); 
    public static Singleton getInstance() {
        return instance;
    }
}

This method has a number of advantages:
The instance is not constructed until the class is used.
There is no need to synchronize the getInstance() method, meaning all threads will see the same instance and no (expensive) locking is required.
The final keyword means that the instance cannot be redefined, ensuring that one (and only one) instance ever exists.