以下程式碼是最普通的雙重鎖的單例實現形式

package com.edu.character02;

import java.io.Serializable;

/**
 * <p>
 * 雙重鎖
 * </P>
 *
 * @作者 five-five
 * @建立時間 2020/8/5
 */
public class SingleTon implements Serializable {
    private static SingleTon instance;

    public static SingleTon getInstance() {
        if (instance == null
 
) {
            synchronized (SingleTon.class) {
                if (instance == null) {
                    instance = new SingleTon();
                }
            }
        }
        return instance;
    }

    private SingleTon() {
    }
}

下面是兩種破壞單例的方案

1.通過反射的形式

package com.edu.character02;

 
import java.lang.reflect.Constructor;

/**
 * <p>
 * 反射破壞單例
 * </p>
 *
 * @作者 five-five
 * @建立時間 2020/8/5
 */
public class DamageSingleTonByReflect {
    public static void main(String[] args) throws Exception {
        SingleTon s1 = SingleTon.getInstance();
        Class<? extends SingleTon> aClass = s1.getClass();
        Constructor
 
<? extends SingleTon> constructor = aClass.getDeclaredConstructor();
        constructor.setAccessible(true);
        SingleTon singleTon = constructor.newInstance();
        System.out.println(s1);
        System.out.println(singleTon);
        System.out.println(s1.hashCode());
        System.out.println(singleTon.hashCode());
        System.out.println(singleTon == s1);
    }
}

測試結果如圖:

2.通過序列化的形式

package com.edu.character02;

import com.sun.xml.internal.ws.policy.privateutil.PolicyUtils;

import java.io.*;

/**
 * <p>
 *     破壞單例
 * </p>
 * @作者 five-five
 * @建立時間 2020/8/5
 */
public class DamageSingleTonBySerializable {
    public static void main(String[] args) {
        SingleTon s1=null;
        SingleTon s2=SingleTon.getInstance();
        FileOutputStream fos=null;
        ObjectOutputStream oos=null;
        FileInputStream fis=null;
        ObjectInputStream ois=null;
        try {
            fos=new FileOutputStream("SingleTon.obj");
            oos=new ObjectOutputStream(fos);
            oos.writeObject(s2);
            oos.flush();
            oos.close();

            fis=new FileInputStream("SingleTon.obj");
            ois = new ObjectInputStream(fis);
            s1= (SingleTon) ois.readObject();
            ois.close();

            System.out.println(s1);
            System.out.println(s2);
            System.out.println(s1.hashCode());
            System.out.println(s2.hashCode());
            System.out.println(s1==s2);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

測試結果如圖: