爲什麼Java servlet不能發送一個對象？

Question

我用下面的方法來從一個servlet發送的對象：

public void doGet(HttpServletRequest request,HttpServletResponse response) throws IOException 
    { 
    String Full_URL=request.getRequestURL().append("?"+request.getQueryString()).toString(); 

    String Contact_Id=request.getParameter("Contact_Id"); 
    String Time_Stamp=Get_Date_Format(6),query="select from "+Contact_Info_Entry.class.getName()+" where Contact_Id == '"+Contact_Id+"' order by Contact_Id desc"; 

    PersistenceManager pm=null; 
    try 
    { 
     pm=PMF.get().getPersistenceManager(); 

     // note that this returns a list, there could be multiple, DataStore does not ensure uniqueness for non-primary key fields 
     List<Contact_Info_Entry> results=(List<Contact_Info_Entry>)pm.newQuery(query).execute(); 

     Write_Serialized_XML(response.getOutputStream(),results.get(0)); 
    } 
    catch (Exception e) { Send_Email(Email_From,Email_To,"Check_License_Servlet Error [ "+Time_Stamp+" ]",new Text(e.toString()+"\n"+Get_Stack_Trace(e)),null); } 
    finally { pm.close(); } 
    } 

    /** Writes the object and CLOSES the stream. Uses the persistance delegate registered in this class. 
    * @param os The stream to write to. 
    * @param o The object to be serialized. 
    */ 
    public static void writeXMLObject(OutputStream os,Object o) 
    { 
    // Classloader reference must be set since netBeans uses another class loader to loead the bean wich will fail in some circumstances. 
    ClassLoader oldClassLoader=Thread.currentThread().getContextClassLoader(); 
    Thread.currentThread().setContextClassLoader(Check_License_Servlet.class.getClassLoader()); 

    XMLEncoder encoder=new XMLEncoder(os); 
    encoder.setExceptionListener(new ExceptionListener() { public void exceptionThrown(Exception e) { e.printStackTrace(); }}); 
    encoder.writeObject(o); 
    encoder.flush(); 
    encoder.close(); 

    Thread.currentThread().setContextClassLoader(oldClassLoader); 
    } 

    private static ByteArrayOutputStream writeOutputStream=new ByteArrayOutputStream(16384); 

    /** Writes an object to XML. 
    * @param out The boject out to write to. [ Will not be closed. ] 
    * @param o The object to write. 
    */ 
    public static synchronized void writeAsXML(ObjectOutput out,Object o) throws IOException 
    { 
    writeOutputStream.reset(); 
    writeXMLObject(writeOutputStream,o); 
    byte[] Bt_1=writeOutputStream.toByteArray(); 
    byte[] Bt_2=new Des_Encrypter().encrypt(Bt_1,Key); 
    out.writeInt(Bt_2.length); 
    out.write(Bt_2); 
    out.flush(); 
    out.close(); 
    } 
    public static synchronized void Write_Serialized_XML(OutputStream Output_Stream,Object o) throws IOException { writeAsXML(new ObjectOutputStream(Output_Stream),o); } 

在接收端的代碼如下所示：

File_Url="http://"+Site_Url+App_Dir+File_Name; 
    try 
    { 
    Contact_Info_Entry Online_Contact_Entry=(Contact_Info_Entry)Read_Serialized_XML(new URL(File_Url)); 
    } 
    catch (Exception e) 
    { 
    e.printStackTrace(); 
    } 

    private static byte[] readBuf=new byte[16384]; 

    public static synchronized Object readAsXML(ObjectInput in) throws IOException 
    { 
    // Classloader reference must be set since netBeans uses another class loader to load the bean which will fail under some circumstances. 
    ClassLoader oldClassLoader=Thread.currentThread().getContextClassLoader(); 
    Thread.currentThread().setContextClassLoader(Tool_Lib_Simple.class.getClassLoader()); 

    int length=in.readInt(); 
    readBuf=new byte[length]; 

    in.readFully(readBuf,0,length); 
    byte Bt[]=new Des_Encrypter().decrypt(readBuf,Key); 
    XMLDecoder dec=new XMLDecoder(new ByteArrayInputStream(Bt,0,Bt.length)); 
    Object o=dec.readObject(); 
    Thread.currentThread().setContextClassLoader(oldClassLoader); 
    in.close(); 
    return o; 
    } 

    public static synchronized Object Read_Serialized_XML(URL File_Url) throws IOException { return readAsXML(new ObjectInputStream(File_Url.openStream())); } 

但我不能讓從物體在接收端的Java應用程序，爲什麼？該錯誤消息是這樣的：

java.lang.ClassNotFoundException: PayPal_Monitor.Contact_Info_Entry 
Continuing ... 
java.lang.NullPointerException: target should not be null 
Continuing ... 
java.lang.NullPointerException: target should not be null 
Continuing ... 
java.lang.NullPointerException: target should not be null 
Continuing ... 

Answer 1

java.lang.ClassNotFoundException: PayPal_Monitor.Contact_Info_Entry 

即類應該是也存在於接收側的運行時類路徑。