【面试】代码默写-DCL单例

1、双重锁检查（DCL）单例模式

public class Singleton {
    
    private static volatile Singleton instance = null;//防止指令重排序，拿到半成品对象
 
    private Singleton() {}
 
    public static Singleton getInstance() {
        if(null == instance) {//避免每次都进行锁竞争，提高执行效率
            synchronized(Singleton.class) {//加锁
                if(null == instance) {//防止多个线程越过第一个if后多次创建对象
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

2、静态内部类单例

public class Singleton {
 
    private Singleton() {}
 
    pirvate static class SingletonHolder {
        public final static Singleton instance = new Singleton();
    }
 
    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }
}