「Android」單例的五種寫法

單例

發現博客園可以很好的設置自己的博客文章的展示，很開心，然后特此發一篇

其實這幾種寫法大家應該都會的，就權當拿來記錄一下吧，以后復習鞏固也比較方便。 
這篇文章中的代碼，來自一篇視頻（我想找視頻貼上，但我沒找到，以后有機會看到再貼吧，實不好意思），上面有講到，我覺得很好，擇錄了一些下來。

1.

//懶人寫法|原始
//缺點：類加載的時候就初始化實例，會拖慢應用啟動速度
public class PlainOldSingleton {
    private static PlainOldSingleton INSTANCE = new PlainOldSingleton();
    private PlainOldSingleton(){}
    public static PlainOldSingleton getInstance(){
        return INSTANCE;
    }
}

2.

//懶加載
//缺點：不是線程安全的，有可能線程A和線程B來同時訪問INSTANCE的話，會new出兩個實例，如果有多個線程的話，就會new出多個實例
public class LazyNotThreadSafe{
    private static LazyNotThreadSafe INSTANCE;
    private LazyNotThreadSafe(){}
    public static LazyNotThreadSafe getInstance(){
        if(INSTANCE == null){
            INSTANCE = new LazyNotThreadSafe();
        }
        return INSTANCE;
    }
}

3.

//同步鎖
//缺點：每次獲取都會加鎖，會有一定的性能開銷
//優點：線程安全
public class LazyThreadSafeSynchronized{
    private static LazyThreadSafeSynchronized INSTANCE;
    private LazyThreadSafeSynchronized(){}
    public static synchronized LazyThreadSafeSynchronized getInstance(){
        if(INSTANCE == null){
            INSTANCE = new LazyThreadSafeSynchronized();
        }
        return INSTANCE;
    }
}

4.

//雙重驗證
//面試多考，volatile是java1.5之后的一個內容，要理解這個需要懂Java虛擬機
public class LazyThreadSafeDoubleCheck{
    private static voltatile LazyThreadSafeDoubleCheck INSTANCE;
    private LazyThreadSafeDoubleCheck(){}
    public static LazyThreadSafeDoubleCheck getInstance(){
        if(INSTANCE == null){
            synchronized (LazyThreadSafeDoubleCheck.class){
                if(INSTANCE == null){
                    INSTANCE = new LazyThreadSafeDoubleCheck();
                }
            }
        }
        return INSTANCE;
    }
}

5.

//靜態內部類
//優雅，實現了懶加載和線程安全，線程安全利用了虛擬機的機制
public class LazyThreadSafeStaticInnerClass{
    private static class Holder{
        private static LazyThreadSafeStaticInnerClass INSTANCE == new LazyThreadSafeStaticInnerClass();
    }
    private LazyThreadSafeStaticInnerClass(){}
    public static LazyThreadSafeStaticInnerClass getInstance(){
        return Holder.INSTANCE;
    }
}