JAVA 中进行网络通信时，通信的程序两端要传输的对象，不仅要序列化，而且这个对象所属的类的名字要完全一样，连包的名字都得一样

　　如上图项目目录，这是一个简易的QQ，客户端登录的时候要传输用户信息到服务器验证，所以两端都会用到User类的对象，但一开始我在Server端的包名是com.qq.server.common，两端的报名字不一致，所以server端卡在了逆序列化那里：

User user=(User)ois.readObject();

　　也就是说server端收到了传输过来的user对象，但是无法解析，当时报的错误有两种，（虽然当时知道了问题出在哪里，不知道为什么会报不同的错误，就在把这种迷惑记录在案的时候突然似乎明白了，在最下面有分析）

1、第一种情况是只有包1、2，没有包3，报下面的错误：（绿色线标出的是出错的代码，即User user=(User)ois.readObject(); ）

java.lang.ClassNotFoundException: com.qq.common.User

at java.net.URLClassLoader.findClass(URLClassLoader.java:381)

at java.lang.ClassLoader.loadClass(ClassLoader.java:424)

at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)

at java.lang.ClassLoader.loadClass(ClassLoader.java:357)

at java.lang.Class.forName0(Native Method)

at java.lang.Class.forName(Class.java:348)

at java.io.ObjectInputStream.resolveClass(ObjectInputStream.java:626)

at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1613)

at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1518)

at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1774)

at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1351)

at java.io.ObjectInputStream.readObject(ObjectInputStream.java:371)

at com.qq.server.model.MyQQServer.<init>(MyQQServer.java:20)

at com.qq.server.view.MyServerFrame.actionPerformed(MyServerFrame.java:37)

at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)

at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)

at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)

at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)

at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)

at java.awt.Component.processMouseEvent(Component.java:6525)

at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)

at java.awt.Component.processEvent(Component.java:6290)

at java.awt.Container.processEvent(Container.java:2234)

at java.awt.Component.dispatchEventImpl(Component.java:4881)

at java.awt.Container.dispatchEventImpl(Container.java:2292)

at java.awt.Component.dispatchEvent(Component.java:4703)

at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)

at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)

at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)

at java.awt.Container.dispatchEventImpl(Container.java:2278)

at java.awt.Window.dispatchEventImpl(Window.java:2750)

at java.awt.Component.dispatchEvent(Component.java:4703)

at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)

at java.awt.EventQueue.access$500(EventQueue.java:97)

at java.awt.EventQueue$3.run(EventQueue.java:709)

at java.awt.EventQueue$3.run(EventQueue.java:703)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)

at java.awt.EventQueue$4.run(EventQueue.java:731)

at java.awt.EventQueue$4.run(EventQueue.java:729)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)

at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)

at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)

at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)

at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)

at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

2、第二种情况是1、2、3包共存，这时还是用的包1、2，所以肯定还是报错的，但是却报下面的错，虽然出错的地方是一样的，但是一直没搞懂为什么上面报的是ClassNotFoundException，而下面报ClassCastException。

java.lang.ClassCastException: com.qq.common.User cannot be cast to com.qq.server.common.User