单例几种实现

喜欢ヅ旅行 2022-10-01 05:42 178阅读 0赞

### 突然想到了我第一次面试单例都不太会写，索性这次写一部分 ###

#### 1 ####

package singlePattern;
    
    /**
     * 饿汉式
     */
    public class Singleton1 {
        private static Singleton1 instance = new Singleton1();
        private  Singleton1() {
        }
        public static Singleton1 getInstance() {
            return instance;
        }
    
        public static void main(String[] args) {
            Singleton1 instance = Singleton1.getInstance();
        }
    }
    
    复制代码

#### 2 ####

package singlePattern;
    
    /**
     * 饿汉式
     */
    public class Singleton2 {
        private static Singleton2 instance = null;
    
        static {
            instance = new Singleton2();
        }
    
        private Singleton2() {
        }
    
        public static Singleton2 getInstance() {
            return instance;
        }
    }
    
    复制代码

#### 3 ####

package singlePattern;
    
    /**
     * 懒汉式，但是这种线程不安全，不存在可用性
     */
    public class Singleton3 {
    
        public static Singleton3 instance = null;
        private Singleton3() {}
    
        public static Singleton3 getInstance() {
            if (instance == null) {
                instance = new Singleton3();
            }
            return instance;
        }
    
    }
    
    复制代码

#### 4 ####

package singlePattern;
    
    /**
     * 懒汉式，线程相对安全，效率低，每次getInstance都需要持锁
     */
    public class Singleton4 {
        private static Singleton4 instance = null;
    
        private Singleton4() {}
    
        public static synchronized Singleton4 getInstance() {
            if (instance == null) {
                instance = new Singleton4();
            }
            return instance;
        }
    }
    
    复制代码

#### 5 ####

package singlePattern;
    
    /**
     * 懒汉式，但是线程不安全
     */
    public class Singleton5 {
    
        private static Singleton5 instance = null;
        private Singleton5() {}
        private static Singleton5 getInstance() {
            if (instance == null) {
                synchronized (Singleton5.class) {
                    instance = new Singleton5();
                }
            }
            return instance;
        }
    }
    
    复制代码

#### 6 ####

package singlePattern;
    
    /**
     * 懒汉式，双重校验锁
     */
    public class Singleton6 {
        private static Singleton6 instance = null;
        private Singleton6(){}
    
        public static Singleton6 getInstance() {
            if (instance == null) {
                synchronized (Singleton6.class) {
                    if (instance == null) {
                        instance = new Singleton6();
                    }
                }
            }
            return instance;
        }
    }
    
    复制代码

#### 7 ####

package singlePattern;
    
    /**
     * 内部类，通过静态类初始化时进行instance的创建，类似饿汉式，但是在初始化时别的线程
     * 无法干扰，而且也可以在形式上有懒加载的感觉
     */
    public class Singleton7 {
        private Singleton7(){}
    
        private static class SingletonHolder {
            private static Singleton7 instance = new Singleton7();
        }
    
        public static Singleton7 getInstance() {
            return SingletonHolder.instance;
        }
    }
    
    复制代码

#### 8 ####

package singlePattern;
    
    /**
     * 枚举
     */
    public enum Singleton8 {
        instance;
    
        private Singleton8(){}
    
        public void method(){}
    }
    
    复制代码

#### 现在都很推荐用枚举，建议多了解，明天写个相关的博客 ####

转载于:https://juejin.im/post/5cd186dcf265da034e7ea3a2