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
    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)

　　

　　通过这两个错误的不同，也对序列化有了更深的认识，对于网络中传输过来的序列化的对象，我们虽然是从外面接收过来的，但是要想逆序列化，也就是识别出来，达到像是完全在本地操作的目的，通信的两端就要有完全一样的类库（类的完整名字是包括包的名字的，所以包名也得一直）。

　　1、上面第一个错误是因为，在本地没有找到com.qq.common.User这个类（错误提示的很明了，而且这个错误是在server端抛出的，虚拟机在告诉你：我没有在Server中找到这个包。但当时却忽视了“抛错地点”，当时还想呢，那不在Client端明明有一个com.qq.common.User类吗，你为什么还抛出这样的错误提示啊，窃以为这是在一个项目里，Server和Client端在视觉上的离得如此的近，当时就没有在引包这个思路上想，归根结底还是对序列化理解的不透彻啊）。

　　2、理解了1以及序列化，就很容易理解为什么会报上面第二个错误了，因为Server端在本地找到了com.qq.common.User，也就是说虚拟机能识别收到的对象是com.qq.common.User，但不允许逆序列化为com.qq.server.common.User，也就是说Server端虽然收到的是Object对象，但当虚拟机进行向下转型的时候（此时应该会进行逆序列化），发现了问题，也就是说，虚拟机收到了一只兔子，你现在让它“逆序列化并向下转型”为一只猫，它不知道该怎么做了，所以报错：ClassCastException。