java - 如何破解单例？

Question

如何打破java中的单例类？单例模式确保一个类只有一个实例，并提供一个全局访问点。

该类的默认构造函数是私有(private)的，这防止了其他类直接实例化该对象。

静态修饰符应用于返回对象的实例方法，因为它使该方法成为无需创建对象即可访问的类级别方法

编辑:

public class Singleton {
    private static Singleton singleInstance;

    private Singleton() {
        System.out.println("Singleton Constructor Running...");
    }
    public static Singleton getInstance() { 
        if (singleInstance == null) { 
            synchronized (Singleton.class) { 
            if (singleInstance == null) { 
                    singleInstance = new Singleton();
                }
            }
        } 
        return singleInstance; 
    }
}

使用反射打破单例

import java.lang.reflect.Constructor;

public class BreakSingletonUsingReflection {
    public static void main(String[] args) {
        Singleton instanceOne = Singleton.getInstance();
        Singleton instanceTwo = null;
        try {
            Constructor[] constructors = Singleton.class.getDeclaredConstructors();
            for (Constructor constructor : constructors) {
                //Below code will destroy the singleton pattern
                constructor.setAccessible(true);
                instanceTwo = (Singleton) constructor.newInstance();
                break;
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println(instanceOne.hashCode());
        System.out.println(instanceTwo.hashCode());
    }
}

Answer 1

1.

import java.lang.reflect.Constructor;


    public class ReflectionSingletonTest {

        public static void main(String[] args) {
            EagerInitializedSingleton instanceOne = EagerInitializedSingleton.getInstance();
            EagerInitializedSingleton instanceTwo = null;
            try {
                Constructor[] constructors = EagerInitializedSingleton.class.getDeclaredConstructors();
                for (Constructor constructor : constructors) {
                    //Below code will destroy the singleton pattern
                    constructor.setAccessible(true);
                    instanceTwo = (EagerInitializedSingleton) constructor.newInstance();
                    break;
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println(instanceOne.hashCode());
            System.out.println(instanceTwo.hashCode());
        }

    }

或

2.

单例类:

package com.singleton.securitymgr;

public class Singleton {

    private static final Singleton INSTANCE = new Singleton();

    private Singleton() {
        System.out.println("Singleton Constructor Running...");
    }

    public static final Singleton getInstance() {
        return INSTANCE;
    }
}

测试类:

package com.singleton.securitymgr;

import java.lang.reflect.Constructor;

public class Test {

    public static void main(String[] args) throws Exception {

        Singleton s = Singleton.getInstance();

        Class clazz = Singleton.class;

        Constructor cons = clazz.getDeclaredConstructor();
        cons.setAccessible(true);

        Singleton s2 = (Singleton) cons.newInstance();
    }
}