java在通过socket接收序列化对象时获取ClassNotFoundException

Question

首先也是最重要的是，我使用的对象（书）应该是好的：

public class Book implements Serializable  {

    private static final long serialVersionUID = 1L;

    private String  name;
    private String  publisher;
    private int     numbOfPages;
    private int     id;

服务器和客户端都有（复制粘贴，无需进一步修改）。所以绝对不是这样

服务器：

ArrayList<Book> library = this.sql_db.getAllBooks();
    try {
        ObjectOutputStream objOut = new ObjectOutputStream(
                this.client.getOutputStream());
        objOut.writeObject(library);
        objOut.flush();
    }

客户：

ArrayList<Book> library = new ArrayList<Book>();

    try {
        ObjectInputStream objIn = new ObjectInputStream(Client.socket.getInputStream());
        //library = (ArrayList<Book>) objIn.readObject(); // This is where the stack trace is pointing to
        Object o = objIn.readObject(); // well actually here, since I commented last line and added this.
    }

同样，这本书在两个程序中都是一样的，并且在同一个软件包中

堆栈跟踪指向我的注释文本，但以防万一，这里是：

java.lang.ClassNotFoundException: db.Book
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at java.io.ObjectInputStream.resolveClass(Unknown Source)
at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
at java.io.ObjectInputStream.readClassDesc(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at java.util.ArrayList.readObject(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at client.Client.requestAllBooks(Client.java:39)
at handler.HandleUserRequest.<init>(HandleUserRequest.java:19)
at run.Main.main(Main.java:8)