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单例模式

单例模式

作者: isjinhao | 来源:发表于2018-07-19 15:54 被阅读0次

    定义

      一个类至多有一个对象实例。这个实例被称为单例。在使用时才初始化被称为懒汉式单例,类一加载就初始化被叫做饿汉式单例。有8种常见实现方式。

    实现

    1. 静态常量法:可用
    public class Singleton {
        private final static Singleton instance = new Singleton();
        private Singleton(){}
        public static Singleton getInstance(){
            return instance;
        }
    }
    
    1. 静态代码块:可用
    public class Singleton {
        private static Singleton instance;
        static { instance = new Singleton(); }
        private Singleton() {}
        public Singleton getInstance() {
            return instance;
            }
    }
    
    1. 静态方法:不可用
    public class Singleton {
        private static Singleton singleton;
        private Singleton() {}
        public static Singleton getInstance() {
            if (singleton == null) 
                singleton = new Singleton();
            return singleton;
        }
    }
    

      线程A进入第7句但还未来得及初始化时,线程B在第5句判断为null,所以也可以进入第6句,有并发问题。

    1. 同步方法:可用但不推荐
    public class Singleton {
        private static Singleton singleton;
        private Singleton() {}
        public static synchronized Singleton getInstance() {
            if (singleton == null)
                singleton = new Singleton();
            return singleton;
        }
    }
    
    1. 同步代码块:不可用
    public class Singleton {
        private static Singleton singleton;
        private Singleton() {}
        public static Singleton getInstance() {
            if (singleton == null) {
                synchronized (Singleton.class) {
                    singleton = new Singleton();
                }
            }
            return singleton;
        }
    }
    

      线程A进入第7句但还未来得及初始化时,线程B在第5句判断为null,虽然得等到A初始化完成,但也可以进入第7句,有并发问题。

    1. 双重检查:可用且推荐用
    public class Singleton {
        private static volatile Singleton singleton;
        private Singleton() {}
        public static Singleton getInstance() {
            if (singleton == null) {
                synchronized (Singleton.class) {
                    if (singleton == null) {
                        singleton = new Singleton();
                    }
                }
            }
            return singleton;
        }
    }
    
    1. 静态内部类:可用且推荐用
    public class Singleton {
        private Singleton() {}
        private static class SingletonInstance {
            private static final Singleton INSTANCE = new Singleton();
        }
        public static Singleton getInstance() {
            return SingletonInstance.INSTANCE;
        }
    }
    
    1. 枚举:可用且推荐用
    public enum Singleton {
        INSTANCE;
        public void whateverMethod() {
            doSomethingHere...
        }
    }
    

    原文来至https://www.cnblogs.com/zhaoyan001/p/6365064.html

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