线程安全的单例模式

方案一:Double Check Lock

public class ThreadSafeSingleton {
    private static ThreadSafeSingleton sThreadSafeSingleton;

    public static ThreadSafeSingleton getInstance() {
        if (sThreadSafeSingleton == null) {
            createSingleton();
        }
        return sThreadSafeSingleton;
    }

    private synchronized static void createSingleton() {
        if (sThreadSafeSingleton == null) {
            sThreadSafeSingleton = new ThreadSafeSingleton();
        }
    }

方案二:内部类延迟加载实例化

public class ThreadSafeSingleton {
    public static ThreadSafeSingleton getInstance() {
        return ThreadSafeSingletonHolder.sThreadSafeSingleton;
    }

    private static class ThreadSafeSingletonHolder {
        private static ThreadSafeSingleton sThreadSafeSingleton = new ThreadSafeSingleton();
    }
}

方案三:类初始化时创建静态成员
public class ThreadSafeSingleton {
　　private static ThreadSafeSingleton sThreadSafeSingleton = new ThreadSafeSingleton();
   public static ThreadSafeSingleton getInstance() {
　　　　return sThreadSafeSingleton;

   }
}

测试代码:
    public static void main(String[] args) {
        for (int i = 0; i<3; ++i) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    TestUtils.print(String.format("thread=%s, singleton=%s",
                            Thread.currentThread().getName(), getInstance()));
                }
            }).start();
        }
    }

两个的输出结果是一样的，根据Java Concurrency in practice的理论，方案一存在unsafe publication风险，
即Thread A在初始化sThreadSafeSingleton时，Thread B在读取该变量，由于此处读取操作无锁，
若compile reorder后，sThreadSafeSingleton的赋值操作在构造函数之前，可能会导致Thread B读取了一个不完整的sThreadSafeSingleton对象。
而方案二利用内部类延迟加载(调用getInstance时才初始化内部类，然后实例化对象)，因此无需锁操作，性能和安全性上较方案一优。

输出结果为:

thread=Thread-1, singleton=com.tony.ThreadSafeSingleton@142b7711
thread=Thread-2, singleton=com.tony.ThreadSafeSingleton@142b7711
thread=Thread-0, singleton=com.tony.ThreadSafeSingleton@142b7711

方案二、三的差别在实例化的时机上，方案三在初始化类时便会实例化(如调用ThreadSafeSingleton静态成员、函数时触发)，

方案二在初始化内部类时才会触发实例化(主动式引用方式，即调用getInstance，而引用其他静态变量或静态函数时不会触发内部类初始化，

从而实现lazy initialization + thread safe instance的效果)。