C05 单例模式 Enum枚举单例（二） 抗反射攻击分析

枚举单例抗反射攻击演示（一）

• NoSuchMethodException 报的是枚举类（EnumInstance ）中没有
  getDeclaredConstructor() 这个方法；
• 意思是EnumInstance枚举类中没有无参构造器；

public enum EnumInstance {

    INSTANCE;

    private Object data;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }

    public static EnumInstance getInstance() {
        return INSTANCE;
    }

}

import java.lang.reflect.Constructor;

public class Test {

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

    public static void reflectionAttack() throws Exception {
        Class objectClass = EnumInstance.class;
        Constructor constructor = objectClass.getDeclaredConstructor();
        constructor.setAccessible(true);
    }

}

输出：

Exception in thread "main" java.lang.NoSuchMethodException: designpattern.creational.singleton.enuminstance.EnumInstance.<init>()
at java.lang.Class.getConstructor0(Class.java:3082)
at java.lang.Class.getDeclaredConstructor(Class.java:2178)
at designpattern.creational.singleton.enuminstance.Test.reflectionAttack(Test.java:14)
at designpattern.creational.singleton.enuminstance.Test.main(Test.java:9)

攻击失败源码分析（一）

• Enum类中只有一个构造器，该构造器有2个参数，所以在用 objectClass.getDeclaredConstructor() 获取Enum的无参构造器时是无法获得的；

  
  Enum类的唯一构造器.png

枚举单例抗反射攻击演示（二）

• 获取到了Enum类的构造器还是无法用反射创建出EnumInstance类的实例；
• 这会报的错不一样，是 IllegalArgumentException: Cannot reflectively create enum objects，说得很明显，不允许以反射的方式创建enum实例；

public class Test {

    public static void main(String[] args) throws Exception {
        reflectionAttack2();
    }

    public static void reflectionAttack2() throws Exception {
        Class objectClass = EnumInstance.class;
        Constructor constructor = objectClass.getDeclaredConstructor(String.class, int.class);
        constructor.setAccessible(true);
        EnumInstance instance = (EnumInstance)constructor.newInstance("", 0);
    }

}

输出：

Exception in thread "main" java.lang.IllegalArgumentException: Cannot reflectively create enum objects
at java.lang.reflect.Constructor.newInstance(Constructor.java:417)
at designpattern.creational.singleton.enuminstance.Test.reflectionAttack2(Test.java:16)
at designpattern.creational.singleton.enuminstance.Test.main(Test.java:9)

攻击失败源码分析（二）

• java.lang.reflect.Constructor类在 newInstance(Object ... initargs) 方法中对待反射的对象的类型针对Enum专门做了判断了，在Java层面禁止了反射创建枚举实例的可能；

  
  不能用反射创建enum实例.png

EnumInstance反编译分析其抗反射攻击原因

• 反编译工具 jad 下载地址：https://varaneckas.com/jad/
• 反编译EnumInstance；

 >.\jad .\EnumInstance.class
Parsing .\EnumInstance.class... Generating EnumInstance.jad

反编译结果

• 很优雅的单例类的写法；
• 私有构造器EnumInstance(String s, int i)；
• getInstance() 方法；
• public static final EnumInstance INSTANCE;
• 在静态代码块中初始化，饿汉模式（可用防御代码抵御反射攻击）；

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   EnumInstance.java

package designpattern.creational.singleton.enuminstance;


public final class EnumInstance extends Enum
{

    public static EnumInstance[] values()
    {
        return (EnumInstance[])$VALUES.clone();
    }

    public static EnumInstance valueOf(String name)
    {
        return (EnumInstance)Enum.valueOf(designpattern/creational/singleton/enuminstance/EnumInstance, name);
    }

    private EnumInstance(String s, int i)
    {
        super(s, i);
    }

    public Object getData()
    {
        return data;
    }

    public void setData(Object data)
    {
        this.data = data;
    }

    public static EnumInstance getInstance()
    {
        return INSTANCE;
    }

    public static final EnumInstance INSTANCE;
    private Object data;
    private static final EnumInstance $VALUES[];

    static 
    {
        INSTANCE = new EnumInstance("INSTANCE", 0);
        $VALUES = (new EnumInstance[] {
            INSTANCE
        });
    }
}

在枚举单例中定义方法

• 定义完的方法要在EnumInstance中定义抽象方法；

public enum EnumInstance {

    INSTANCE {
        protected void printTest() {
            System.out.println("EmumInstance.INSTANCE.printTest()...");
        }
    };

    protected abstract void printTest();

    private Object data;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }

    public static EnumInstance getInstance() {
        return INSTANCE;
    }

}

public class Test {

    public static void main(String[] args) throws Exception {
        EnumInstance instance = EnumInstance.getInstance();
        instance.printTest();
    }

}

输出：

EmumInstance.INSTANCE.printTest()...

反编译带方法的EnumInstance

• 变成抽象类了；

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   EnumInstance.java

package designpattern.creational.singleton.enuminstance.addmethod;

import java.io.PrintStream;

public abstract class EnumInstance extends Enum
{

    public static EnumInstance[] values()
    {
        return (EnumInstance[])$VALUES.clone();
    }

    public static EnumInstance valueOf(String name)
    {
        return (EnumInstance)Enum.valueOf(designpattern/creational/singleton/enuminstance/addmethod/EnumInstance, name);
    }

    private EnumInstance(String s, int i)
    {
        super(s, i);
    }

    protected abstract void printTest();

    public Object getData()
    {
        return data;
    }

    public void setData(Object data)
    {
        this.data = data;
    }

    public static EnumInstance getInstance()
    {
        return INSTANCE;
    }


    public static final EnumInstance INSTANCE;
    private Object data;
    private static final EnumInstance $VALUES[];

    static 
    {
        INSTANCE = new EnumInstance("INSTANCE", 0) {

            protected void printTest()
            {
                System.out.println("EmumInstance.INSTANCE.printTest()...");
            }

        }
;
        $VALUES = (new EnumInstance[] {
            INSTANCE
        });
    }
}