设计模式 - 单例模式（枚举式）

package com.mmall.concurrency.example.singleton;
import com.mmall.concurrency.annoations.Recommend;
import com.mmall.concurrency.annoations.ThreadSafe;
/**
 * 枚举模式：最安全
 */
@ThreadSafe
@Recommend
public class SingletonExample7 {
    // 私有构造函数
    private SingletonExample7() {
    }
    public static SingletonExample7 getInstance() {
        return Singleton.INSTANCE.getInstance();
    }
    private enum Singleton {
        INSTANCE;
        private SingletonExample7 singleton;
        // JVM保证这个方法绝对只调用一次
        Singleton() {
            singleton = new SingletonExample7();
        }
        public SingletonExample7 getInstance() {
            return singleton;
        }
    }
}