转自:
http://m.blog.csdn.net/article/details?id=50525335
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:
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package com.effective.singleton;
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public class Elvis
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{
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private static boolean flag = false;
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private Elvis(){
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}
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private static class SingletonHolder{
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private static final Elvis INSTANCE = new Elvis();
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}
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public static Elvis getInstance()
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{
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return SingletonHolder.INSTANCE;
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}
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public void doSomethingElse()
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{
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}
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}
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:
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package com.effective.singleton;
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import java.lang.reflect.Constructor;
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import java.lang.reflect.InvocationTargetException;
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public class ElvisReflectAttack
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{
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public static void main(String[] args) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException
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{
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Class<?> classType = Elvis.class;
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Constructor<?> c = classType.getDeclaredConstructor(null);
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c.setAccessible(true);
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Elvis e1 = (Elvis)c.newInstance();
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Elvis e2 = Elvis.getInstance();
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System.out.println(e1==e2);
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}
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}
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:
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private ElvisModified(){
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synchronized(ElvisModified.class)
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{
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if(flag == false)
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{
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flag = !flag;
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}
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else
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{
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throw new RuntimeException("单例模式被侵犯!");
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}
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}
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}
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private static class SingletonHolder{
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private static final ElvisModified INSTANCE = new ElvisModified();
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}
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public static ElvisModified getInstance()
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{
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return SingletonHolder.INSTANCE;
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}
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public void doSomethingElse()
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{
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}
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}
测试代码:
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package com.effective.singleton;
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import java.lang.reflect.Constructor;
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public class ElvisModifiedReflectAttack
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{
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public static void main(String[] args)
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{
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try
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{
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Class<ElvisModified> classType = ElvisModified.class;
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Constructor<ElvisModified> c = classType.getDeclaredConstructor(null);
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c.setAccessible(true);
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ElvisModified e1 = (ElvisModified)c.newInstance();
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ElvisModified e2 = ElvisModified.getInstance();
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System.out.println(e1==e2);
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}
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catch (Exception e)
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{
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e.printStackTrace();
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}
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}
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}
结果:
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Exception in thread "main" java.lang.ExceptionInInitializerError
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at com.effective.singleton.ElvisModified.getInstance(ElvisModified.java:27)
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at com.effective.singleton.ElvisModifiedReflectAttack.main(ElvisModifiedReflectAttack.java:17)
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Caused by: java.lang.RuntimeException: 单例模式被侵犯!
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at com.effective.singleton.ElvisModified.<init>(ElvisModified.java:16)
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at com.effective.singleton.ElvisModified.<init>(ElvisModified.java:7)
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at com.effective.singleton.ElvisModified$SingletonHolder.<clinit>(ElvisModified.java:22)
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... 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:
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package com.effective.singleton;
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public enum SingletonClass
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{
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INSTANCE;
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public void test()
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{
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System.out.println("The Test!");
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}
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}
测试代码:
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package com.effective;
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import java.lang.reflect.Constructor;
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import java.lang.reflect.InvocationTargetException;
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import com.effective.singleton.SingletonClass;
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public class TestMain
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{
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public static void main(String[] args) throws NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException
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{
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Class<SingletonClass> classType = SingletonClass.class;
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Constructor<SingletonClass> c = (Constructor<SingletonClass>) classType.getDeclaredConstructor();
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c.setAccessible(true);
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c.newInstance();
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}
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}
运行结果:
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Exception in thread "main" java.lang.NoSuchMethodException: com.effective.singleton.SingletonClass.<init>()
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at java.lang.Class.getConstructor0(Unknown Source)
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at java.lang.Class.getDeclaredConstructor(Unknown Source)
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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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