如何防止单例模式被JAVA反射攻击

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单例模式相信大家都知道,用过的人不在少数。之前写过一篇博文《singleton模式四种线程安全的实现》(参见:http://blog.csdn.NET/u013256816/article/details/50427061),讲诉了单例模式的四种写法,并指出占位符模式的写法比较ok,详见如下:

package com.effective.singleton;
public class Elvis

{

private static boolean flag = false;
private Elvis(){

}
private
static class SingletonHolder{

private static final Elvis INSTANCE = new Elvis();

}
public static Elvis getInstance()

{

return SingletonHolder.INSTANCE;

}
public void doSomethingElse()

{
}

}

但这都是基于一个条件:确保不会通过反射机制调用私有的构造器。
这里举个例子,通过JAVA的反射机制来“攻击”单例模式:

package com.effective.singleton;
import java.lang.reflect.Constructor;

import java.lang.reflect.InvocationTargetException;
public class ElvisReflectAttack

{
public static void main(String[] args) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException

{

Class<?> classType = Elvis.class;


Constructor<?> c = classType.getDeclaredConstructor(null);

c.setAccessible(true);

Elvis e1 = (Elvis)c.newInstance();

Elvis e2 = Elvis.getInstance();

System.out.println(e1==e2);

}
}

运行结果:false
可以看到,通过反射获取构造函数,然后调用setAccessible(true)就可以调用私有的构造函数,所有e1和e2是两个不同的对象。
如果要抵御这种攻击,可以修改构造器,让它在被要求创建第二个实例的时候抛出异常。
经修改后:

package com.effective.singleton;
public class ElvisModified

{

private static boolean flag = false;
private ElvisModified(){

synchronized(ElvisModified.class)

{

if(flag == false)

{

flag = !flag;

}

else

{

throw new RuntimeException("单例模式被侵犯!");

}

}

}
private
static class SingletonHolder{

private static final ElvisModified INSTANCE = new ElvisModified();

}
public static ElvisModified getInstance()

{

return SingletonHolder.INSTANCE;

}
public void doSomethingElse()

{
}

}

测试代码:

package com.effective.singleton;
import java.lang.reflect.Constructor;
public class ElvisModifiedReflectAttack

{
public static void main(String[] args)

{

try

{

Class<ElvisModified> classType = ElvisModified.class;


Constructor<ElvisModified> c = classType.getDeclaredConstructor(null);

c.setAccessible(true);

ElvisModified e1 = (ElvisModified)c.newInstance();

ElvisModified e2 = ElvisModified.getInstance();

System.out.println(e1==e2);

}

catch (Exception e)

{

e.printStackTrace();

}

}

}

运行结果:

Exception in thread "main" java.lang.ExceptionInInitializerError

at com.effective.singleton.ElvisModified.getInstance(ElvisModified.java:27)

at com.effective.singleton.ElvisModifiedReflectAttack.main(ElvisModifiedReflectAttack.java:17)

Caused by: java.lang.RuntimeException: 单例模式被侵犯!

at com.effective.singleton.ElvisModified.<init>(ElvisModified.java:16)

at com.effective.singleton.ElvisModified.<init>(ElvisModified.java:7)

at com.effective.singleton.ElvisModified$SingletonHolder.<clinit>(ElvisModified.java:22)

... 2 more

可以看到,成功的阻止了单例模式被破坏。
从JDK1.5开始,实现Singleton还有新的写法,只需编写一个包含单个元素的枚举类型。推荐写法:

package com.effective.singleton;
public enum SingletonClass

{

INSTANCE;
public void test()

{

System.out.println("The Test!");

}

}

测试代码:

package com.effective;
import java.lang.reflect.Constructor;

import java.lang.reflect.InvocationTargetException;
import com.effective.singleton.SingletonClass;
public class TestMain

{
public static void main(String[] args) throws NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException

{

Class<SingletonClass> classType = SingletonClass.class;

Constructor<SingletonClass> c = (Constructor<SingletonClass>) classType.getDeclaredConstructor();

c.setAccessible(true);

c.newInstance();

}

}

运行结果:

Exception in thread "main" java.lang.NoSuchMethodException: com.effective.singleton.SingletonClass.<init>()

at java.lang.Class.getConstructor0(Unknown Source)

at java.lang.Class.getDeclaredConstructor(Unknown Source)

at com.effective.TestMain.main(TestMain.java:22)

由此可见这种写法也可以防止单例模式被“攻击”。
而且这种写法也可以防止序列化破坏单例模式,具体不在举例了,有关序列化以及单例模式被序列化破坏可以参考博文《JAVA序列化》(链接:http://blog.csdn.net/u013256816/article/details/50474678)。
单元素的枚举类型已经成为实现Singleton模式的最佳方法。

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