線程安全的單例模式

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:內部類延遲加載實例化

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

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

方案三:類初始化時建立靜態成員
public class ThreadSafeSingleton {
　　private static ThreadSafeSingleton sThreadSafeSingleton = new ThreadSafeSingleton();
   public static ThreadSafeSingleton getInstance() {
　　　　return sThreadSafeSingleton;

   }
}

測試代碼:
    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();
        }
    }

兩個的輸出結果是同樣的，根據Java Concurrency in practice的理論，方案一存在unsafe publication風險，
即Thread A在初始化sThreadSafeSingleton時，Thread B在讀取該變量，因爲此處讀取操做無鎖，
若compile reorder後，sThreadSafeSingleton的賦值操做在構造函數以前，可能會致使Thread B讀取了一個不完整的sThreadSafeSingleton對象。
而方案二利用內部類延遲加載(調用getInstance時才初始化內部類，而後實例化對象)，所以無需鎖操做，性能和安全性上較方案一優。

輸出結果爲:

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2、三的差異在實例化的時機上，方案三在初始化類時便會實例化(如調用ThreadSafeSingleton靜態成員、函數時觸發)，

方案二在初始化內部類時纔會觸發實例化(主動式引用方式，即調用getInstance，而引用其餘靜態變量或靜態函數時不會觸發內部類初始化，

從而實現lazy initialization + thread safe instance的效果)。