Java的类型转换异常（ClassCastException），恐怕是开发中最常见的异常之一，比如你把一个本身为String的对象强行转换成List时，就会抛出此异常。当然，一般情况下这种错误很容易就从异常信息中发现原因并纠正，通常对于此类问题我们的想法就是：class文件相同，即字节码相同，那么实例化产生的对象肯定也会相同类型。但是，存在一些情况，会发生形如“把class1转换成class1却抛出类型转换异常”的情况
先看一个例子，包含三个源文件：MainClass，ClassOne，ClassTwo 。MainClass是程序入口，另外两个类用于测试，代码很简单，如下

.net/syntaxhighlighting/OutliningIndicators/None.gif" alt="" align="top" /> ClassOne.java
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.net/syntaxhighlighting/OutliningIndicators/None.gif" alt="" align="top" /> package  test.jboss.jmx.classCastEx;
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.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif" alt="" align="top" />Class ClassOne ... {
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif" alt="" align="top" /> public void doTest(Object obj)...{
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  ClassTwo c2 = (ClassTwo)obj;
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif" alt="" align="top" /> }
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />
.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif" alt="" align="top" />}
.net/syntaxhighlighting/OutliningIndicators/None.gif" alt="" align="top" />
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.net/syntaxhighlighting/OutliningIndicators/None.gif" alt="" align="top" />
.net/syntaxhighlighting/OutliningIndicators/None.gif" alt="" align="top" />ClassTwo只是一个空类
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.net/syntaxhighlighting/OutliningIndicators/None.gif" alt="" align="top" />MainClass.java
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.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif" alt="" align="top" /> public   class  MainClass  ... {
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif" alt="" align="top" /> /** *//**
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @param args
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @throws MalformedURLException 
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @throws ClassNotFoundException 
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @throws IllegalAccessException 
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @throws InstantiationException 
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @throws NoSuchMethodException 
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @throws InvocationTargetException 
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @throws SecurityException 
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  * @throws IllegalArgumentException 
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif" alt="" align="top" />  */
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif" alt="" align="top" /> public static void main(String[] args) throws MalformedURLException, ClassNotFoundException, InstantiationException, IllegalAccessException, IllegalArgumentException, SecurityException, InvocationTargetException, NoSuchMethodException ...{
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  // ClassOne 和 ClassTwo打包到一个jar中，名为“test.jar”，放在 MainClass 所在项目的根目录下
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  // 注意，ClassOne 和 ClassTwo 不能和MainClass放在一个项目中，后面 有解释
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  // 自定义一个类加载器，从外部导入ClassOne 和 ClassTwo
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  File jar = new File("test.jar");
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  System.out.println(jar.exists());
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif" alt="" align="top" />  URL[] url = ...{jar.toURL()};
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  System.out.println(url[0]);
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  URLClassLoader ucl1 = new URLClassLoader(url);
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  Class classTwo = ucl1.loadClass("test.jboss.jmx.classCastEx.ClassTwo");
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  //查看是否成功地利用反射机制，将ClassTwo导入进来，并显示其在VM中的hash码
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  // getClassLoader()正常情况返回classTwo的类加载器，也就是上面的 ucl1 ，如果不是，则有问题
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  System.out.println("hash of layout1:"+classTwo.hashCode()+"ucl "+classTwo.getClassLoader());
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  //创建一个ClassTwo的实例
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  Object c2_obj = classTwo.newInstance();
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  // 同理，用另一个类加载器（ucl2）载入ClassOne
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  File jar2 = new File("test.jar");
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  System.out.println(jar2.exists());
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif" alt="" align="top" />  URL[] url2 = ...{jar2.toURL()};
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  URLClassLoader ucl2 = new URLClassLoader(url2);
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  Class classOne = ucl2.loadClass("test.jboss.jmx.classCastEx.ClassOne");
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  Object c1_obj = classOne.newInstance();
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  //利用反射，调用ClassOne的 doTest 方法，将上面创建的 c2_obj 作为方法的参数
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif" alt="" align="top" />  Class[] type = ...{Object.class};
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  Method m = classOne.getMethod("doTest", type);
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif" alt="" align="top" />.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif" alt="" align="top" />  Object[] para = ...{c2_obj};
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  m.invoke(c1_obj, para);
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />  
.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif" alt="" align="top" /> }
.net/syntaxhighlighting/OutliningIndicators/InBlock.gif" alt="" align="top" />

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如果按固有的思维，则 c2_obj 传入 doTest方法后，执行 ClassTwo c2 = (ClassTwo)obj; 是没问题的，但是实际运行则会抛出 ClassCastException ，并明确的告诉你 “ClassTwo c2 = (ClassTwo)obj”有问题，为什么呢？

原因在于使用了不同的类加载器载入各个类。其中，main函数中，ucl1载入的是ClassTwo，而ucl2在载入ClassOne时，由于ClassOne内部引用了ClassTwo，ucl1会把ClassTwo也一起加载进来，这样VM就有了两个ClassTwo，分别对应不同的类加载器。对于ClassOne.doTest() 中的“ ClassTwo c2 = (ClassTwo)obj”这句代码，c2 的类型全称是“ucl2加载的test.jboss.jmx.classCastEx.ClassOne”
现在我们应该明白了，之所以会有类型转换异常，是由于类在VM中的签名不仅仅是类的完整包名，还包括载入它的类加载器。上述例子中，由ucl1加载的ClassTwo，作为参数传入由ucl2加载的ClassOne.doTest() 中，自然就与 c2 的类型不符合了，导致无法转换
也许你一般不会用这种“古怪”的方式加载类，通常我们都是把需要的外部类写入项目的classpath，现在的IDE也提供非常方便的手段导入外部类。但是想象一下在应用服务器容器中，你部署的多个应用都可能共享某个jar库的类实例。当重新部署包含该jar的应用时，所有相关的应用都必须刷新一遍，否则很容易出现上述问题

PS:本文参考了《JBOSS 4.0 标准教材》中的内容，书中提供了相应源码解释这个问题，不过比较繁琐，上面的代码是我简化过的，在我的机子上试验成功。请不要将ClassOne 和 ClassTwo  与MainClass放在一个项目中，那样在运行之前就会预先加载项目中所有的类，等于ClassOne 和 ClassTwo都由VM先加载了，就不会出现预期的转换异常了。可以将ClassOne 和 ClassTwo在另一个项目中编写然后打包，放到MainClass所在项目的根目录。

http://blog.csdn.net/wangchengsi/archive/2008/02/21/2110647.aspx