昨天朋友面試栽在了單例模式的線程安全問題

面試常見的幾種單例模式

不支持多線程的單例模式：

懶漢式線程不安全：

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

支持多線程

synchronized意味着不高效

• 懶漢式：
  效率很低

public class Singleton {  
    private static Singleton instance;  
    private Singleton (){}  
    public static synchronized Singleton getInstance() {  
    if (instance == null) {  
        instance = new Singleton();  
    }  
    return instance;  
    }  
}

• 餓漢式
  基於ClassLoader實現加載時就被創建出對象，推薦使用，但是同時也會浪費內存

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

• 雙重校驗鎖

  在同步的情況下仍然能高效

public class Singleton {  
    private volatile static Singleton singleton;  
    private Singleton (){}  
    public static Singleton getSingleton() {  
    if (singleton == null) {  
        synchronized (Singleton.class) {  
        if (singleton == null) {  
            singleton = new Singleton();  
        }  
        }  
    }  
    return singleton;  
    }  
}