Java单例模式的4种写法

向右看齐 2023-02-23 08:52 24阅读 0赞

单例模式：

1、懒汉式

public class Handler {
        private Handler(){
            System.out.println("handler create success!");
        }
    
        private static volatile Handler handler;
    
        public static Handler getInstance(){
            if (null == handler){
                synchronized (Handler.class){
                    if (null == handler){
                        handler = new Handler();
                    }
                }
            }
            return handler;
        }
    }

main函数：

public class Main {
        public static void main(String[] args) throws Exception {
            Handler handler = Handler.getInstance();
    
            Constructor<Handler> declaredConstructor = Handler.class.getDeclaredConstructor();
            declaredConstructor.setAccessible(true);
            Handler handler2 = declaredConstructor.newInstance();
            Handler handler3 = declaredConstructor.newInstance();
            System.out.println(handler);
            System.out.println(handler2);
            System.out.println(handler3);
        }
    }

懒汉式在反射的情况下是不安全的！！！

2、饿汉式

public class Handler {
        private Handler(){
            System.out.println("handler create success!");
        }
    
        private static volatile Handler handler = new Handler();
    
        public static Handler getInstance(){
            return handler;
        }
    }

3、内部类的方式

package ljxwtl.cn;
    
    /**
     * @author: wtl
     * @Date: 2020/7/6
     * @Time: 21:06
     * @Description:
     */
    public class Handler {
        private Handler(){
            System.out.println("handler create success!");
        }
    
        public static Handler getInstance(){
            return InnerClass.getInstance();
        }
    
        private static class InnerClass{
            private static volatile Handler handler = new Handler();
            public static Handler getInstance(){
                return handler;
            }
        }
    }

4、枚举类的实现（最安全的）

public enum Handler {
        INSTANCE;
    
        public static Handler getInstance(){
            return INSTANCE;
        }
    }

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Enum类是不能反射的，所以是最优的！！！

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