昨天朋友面试栽在了单例模式的线程安全问题

面试常见的几种单例模式

不支持多线程的单例模式：

懒汉式线程不安全：

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

支持多线程

synchronized意味着不高效

• 懒汉式：
  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;  
    }  
}

• 饿汉式
  基于ClassLoader实现加载时就被创建出对象，推荐使用，但是同时也会浪费内存

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

• 双重校验锁

  在同步的情况下仍然能高效

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;  
    }  
}