C05 单例模式 Enum枚举单例（二） 抗反射攻击分析

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);
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；

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；
这次报的错不一样，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);
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) 方法中针对需要被反射创建的对象的类型，专门针对 Enum 做了判断，在 Java 层面禁止了反射创建枚举实例的可能；

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）；
// 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);
        $VALUES = (new EnumInstance[] {
            INSTANCE
在枚举单例中定义方法
定义完的方法要在 EnumInstance 中定义抽象方法；
public enum EnumInstance {
    INSTANCE {
        protected void printTest() {
            System.out.println("EmumInstance.INSTANCE.printTest()...");
    protected abstract void printTest();
    private Object data;
    public Object getData() {
        return data;
    public void setData(Object data) {
        this.data = data;
    public static EnumInstance getInstance() {
        return INSTANCE;
public class Test {
    public static void main(String[] args) throws Exception {
        EnumInstance instance = EnumInstance.getInstance();
        instance.printTest();
EmumInstance.INSTANCE.printTest()...
反编译带方法的EnumInstance
变成抽象类了，所以要把定义好的方法声明成抽象方法；
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   EnumInstance.java
package designpattern.creational.singleton.enuminstance.addmethod;
import java.io.PrintStream;
public abstract class EnumInstance extends Enum
    public static EnumInstance[] values()
        return (EnumInstance[])$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