Java单例模式实现的几种方法

小灰灰 2022-05-05 12:30 231阅读 0赞

package offer;
    
    public class Test02 {
    
        /**
         * 01 饿汉模式  线程安全
         */
        public static class Singleton{
            private final static Singleton INSTANCE = new Singleton();
    
            private Singleton (){}
    
            public static Singleton getInstance(){
                return INSTANCE;
            }
        }
    
        /**
         * 02  懒汉模式  非线程安全
         */
        public static class Singleton2{
            private static Singleton2 instance = null;
    
            private Singleton2(){
    
            }
    
            public static Singleton2 getInstance(){
                if (null == instance ){
                    instance = new Singleton2();//非线程安全
                }
                return instance;
            }
    
        }
    
        /**
         * 03 懒汉模式  线程安全 使用synchromized关键字修饰 效率低
         */
        public static class Singleton3{
            private  static Singleton3 instance = null;
            private Singleton3(){}
            public static synchronized Singleton3 getInstance (){
                if (null == instance){
                    instance = new Singleton3();
                }
                return instance;
            }
        }
    
        /**
         * 04 饿汉模式 线程安全  使用静态代码块 注意static的顺序
         */
        public static class Singleton4 {
            private static Singleton4 instance = null;
            static {
                instance = new Singleton4();
            }
    
            private Singleton4(){
    
            }
    
            public static Singleton4 getInstance(){
                return instance;
            }
        }
    
        /**
         * 05 静态内部类方式
         */
        public static class Singleton5{
            private final static class SingletonHolder{
                private static final Singleton5 INSTANCE = new Singleton5();
            }
    
            private Singleton5(){}
    
            public static Singleton5 getInstance(){
                return SingletonHolder.INSTANCE;
            }
        }
    
        /**
         * 06 枚举形式1
         */
        public enum Singleton6{
            INSTANCE;
            public void anyMethod(){
    
            }
        }
    
        /**
         * 07  枚举形式2  推荐
         */
        public static class EnumSingleton{
            private EnumSingleton(){}
    
            public static EnumSingleton getInstance(){
                return Singleton.INSTANCE.getInstance();
            }
    
            private static enum Singleton{
                INSTANCE;
    
                private EnumSingleton singleton;
    
                private Singleton(){
                    singleton = new EnumSingleton();
                }
    
                public EnumSingleton getInstance(){
                    return singleton;
                }
            }
    
        }
    
        /**
         * 07 懒汉模式 线程安全 双重检验
         */
        public static class Singleton7{
            //使用volatile防止代码重排序，读到未初始化完成的instance对象
            private volatile static Singleton7 instance = null;
    
            private Singleton7(){
    
            }
    
            public static Singleton7 getInstance(){
                if(null == instance){
                    synchronized(Singleton7.class){//局部使用同步方法，提高效率
                        instance = new Singleton7();
                    }
                }
                return instance;
            }
    
        }
    
        // test
        public static void main(String[] args){
            System.out.println(Singleton.getInstance()== Singleton.getInstance());
            System.out.println(Singleton2.getInstance() == Singleton2.getInstance());
            System.out.println(Singleton3.getInstance() == Singleton3.getInstance());
            System.out.println(Singleton4.getInstance() == Singleton4.getInstance());
            System.out.println(Singleton5.getInstance() ==  Singleton5.getInstance());
            System.out.println(Singleton6.INSTANCE == Singleton6.INSTANCE);
            System.out.println(EnumSingleton.getInstance() == EnumSingleton.getInstance());
            System.out.println(Singleton7.getInstance() == Singleton7.getInstance());
        }
    }

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