applet to servlet

[Alan Shiers]

Hi there,

I'm having difficulty trying to send a simple serialized object to a servlet via POST. On the client side I don't generate any exceptions at run time, but on the server side, the servlet keeps throwing a NullPointerException on the readObject() method of the ObjectInputStream. I don't understand why that is. I've been checking my code with numerous examples from other websites, most notably:

http://www.j-nine.com/pubs/applet2servlet/index.htm

In comparison, my code appears to be the same. Can you help me figure this out, please? My applet code and servlet code are below.

Alan

 import java.io.*;
import java.text.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class TableDataServlet extends HttpServlet {
 

    public void doGet(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException
    {
      doPost(request,response);
    }
    public void doPost(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException
    {
        ObjectInputStream inputFromApplet = null;
        TableData td = null;        
        PrintWriter out = null;
        BufferedReader inTest = null;
        String[][] data = null;
        
        try
        {  
          // get an input stream from the applet
  inputFromApplet = new ObjectInputStream(request.getInputStream());
    
          // read the serialized data from applet        
          td = (TableData) inputFromApplet.readObject();//<<<This is the culprit!
        
          inputFromApplet.close();
          data = td.getData();          
          String firstname = data[0][1];
          // send back a confirmation message to the applet
          out = new PrintWriter(response.getOutputStream());
          response.setContentType("text/plain");
          out.println("confirmed " + firstname);
          out.flush();
          out.close();
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }       
            
    }
 
    class TableData implements Serializable
    {
      String[][] data = null;
      public TableData(String[][] newData)
      {
        if(newData != null)
           data = newData;
      }
      public String[][] getData()
      {
        return data;
      }
      public void setData(String[][] newData)
      {
        if(newData != null)
           data = newData;
      }
    }
}
 
public class TableForm extends JApplet implements ActionListener
{
  ...
  public void actionPerformed(ActionEvent e)
  {
    ...
    if(e.getSource() == submit)
    {
      ...
      //Load up a 2D array with data from JTable
      String[][] data = new String[RowCount][headers.length];
      for(int i = 0; i < RowCount; i++)//rows
      {
        for(int j = 1; j < headers.length; j++)//columns
        {
          data[i][j - 1] = (String)dm.getValueAt(i,j);
        }
      }
 
      //TableData exist on both client-side and server-side.      
      TableData td = new TableData(data);
      
      try
      {
        // connect to the servlet
        String location = processingServlet;
        URL testServlet = new URL( location );
        URLConnection servletConnection = testServlet.openConnection(); 
 
        //inform the connection that we will send output and accept input
        servletConnection.setDoInput(true); 
        servletConnection.setDoOutput(true);
 
        //Don't use a cached version of URL connection.
        servletConnection.setUseCaches (false);
        servletConnection.setDefaultUseCaches (false);
 
        //Specify the content type that we will send binary data
        servletConnection.setRequestProperty ("Content-Type", "application/octet-stream");
 
        //send your data to the servlet        
       ObjectOutputStream outputToServlet = new ObjectOutputStream(servletConnection.getOutputStream());
       
       outputToServlet.writeObject(td);        
       outputToServlet.flush();        
       outputToServlet.close();
       
       //now wait for a response from the servlet
       BufferedReader in = new BufferedReader(new InputStreamReader(servletConnection.getInputStream()));
       
       String line = "";
       //statusTextArea is a JTextArea object in the applet
       while((line = in.readLine()) !=  null)
       {
         statusTextArea.append(line + "\n");
       }
       in.close();
     }
     catch(IOException ioe)
     {
       ioe.printStackTrace();
     }      
    }
  }
}
 

[Ben Souther]

I believe you will need to send the size of your post in the headers.

I did all the excercises in this chapter a few years ago and was able to send objects up.
http://csajsp-chapters.corewebprogramming.com/CSAJSP-Chapter17.pdf

It's been a while though.

[Alan Shiers]

Hi there,

I actually have the book Core Servlets and Java Server Pages where in Chapter 17 Marty Hall describes a way to send a String over a POST request using a ByteArrayOutputStream. In his example he sets the URLConnection method setRequestProperty(...) to set the "Content-Type" with "application/x-www-form-urlencoded". This is fine for his example since he is only sending string data over the wire. Therefore, he is able to set the setRequestProperty(...) method earlier to "Content-Length" and the length of the string. That's an easy thing to do when dealing with strings. This doesn't apply in my case, however.

In my case, I am sending a class named "TableData" which is just acting as a wrapper for a 2D array. In my case, I need to set the method setRequestProperty(...) to "Content-Type" with "application/octet-stream" because I'm sending an object over the wire. If I am still required to set the "Content-Length" property, then how does one do that with an object like TableData? In the case of sending serialized objects to a servlet, I have not seen any examples where they set the "Content-Length" property.

Alan

[Ben Souther]

In that book, he also shows:
1.) How to send binary objects from an applet to a servlet.
2.) How to send a serialized java object from the servlet to the applet.

For me, it wasn't too hard to put the pieces together and make an applet that sends serialized java objects to the server.

Again, it's been a while since I've done it but I'm fairly certain, you have to state the size explicitly.

[Alan Shiers]

I see no evidence of determining the size of an object for purposes of setting the "Content-Length" property. On Page 456 Marty Hall does it using a String object, which is easy, but I'm sending a serialized class that acts as a wrapper for a 2D array. If I have to provide a size for the object, how is it done?

Alan

[Ben Souther]

I may still have the project on my home machine. I'll look tonight and see. If I do, I can send it to you. I know I had it working and, because we're using the same book, the code is probably very similar to yours.

-Ben

[Ben Souther]

While looking to see if I had a copy of that app on my machine, I ran across this. In addition to the core servlets book, this article (and it's code samples) was helpful to me when I was doing this.

http://www-128.ibm.com/developerworks/java/library/j-tunnel/

[Ben Souther]

And, in looking at the sample, it appears you were correct, they don't explicitly send the size of the object.

Sorry for any inconvenience that post might have caused.

-Ben

[Carl Trusiak]

JavaRanch actually has a good workable example fo this in JavaRanch Common Classes The two classes are HTTP for the client code and ObjectServlet for server side.

[Alan Shiers]

Ok Ben,

Now we're making headway...I went to the ibm site you sited and borrowed some of their code for the servlet for refining purposes. I intoduced a BufferedInputStream into the mix as seen below and made use of an instanceof check on the object being retrieved.

This time when I ran everything, I still get an error on the readObject() method, but this time it is no longer NullPointerException. This time its EOFException. I thought at first that it was perhaps because I wasn't calling flush() on the client side. But I am.

Please advise,

Alan Shiers

***************************************

public class TableDataServlet extends HttpServlet {
 

    public void doGet(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException
    {
      doPost(request,response);
    }
 
    public void doPost(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException
    {
        ObjectInputStream inputFromApplet = null;
        TableData td = null;        
        PrintWriter out = null;
        String[][] data = null;
        
        try
        {  
          // get an input stream from the applet
  inputFromApplet = new ObjectInputStream(new BufferedInputStream(request.getInputStream()));
          System.out.println("Attempting to read Object...");  
          // read the serialized data from applet
          Object o = inputFromApplet.readObject();
          if(o != null && (o instanceof TableData))
          {        
            td = (TableData) o;
            System.out.println("Got Object");
            inputFromApplet.close();
            data = td.getData();          
            String firstname = data[0][1];
            // send back a confirmation message to the applet
            out = new PrintWriter(response.getOutputStream());
            response.setContentType("text/plain");
            out.println("confirmed " + firstname);
            out.flush();
            out.close();
          }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }       
            
    }
 
    class TableData implements Serializable
    {
      String[][] data = null;
      public TableData(String[][] newData)
      {
        if(newData != null)
           data = newData;
      }
      public String[][] getData()
      {
        return data;
      }
      public void setData(String[][] newData)
      {
        if(newData != null)
           data = newData;
      }
    }
}

TOMCAT LOG:

...
Attempting to read Object...
java.io.EOFException
at java.io.ObjectInputStream$BlockDataInputStream.peekByte(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at TableDataServlet.doPost(TableDataServlet.java:31)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:237)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:157)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:214)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:825)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:731)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:526)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Unknown Source)

[Ben Souther]

Which is line #31?

[Alan Shiers]

Hi Ben,

You seemed to have abandoned me. I still haven't resolved my issue. I've been conversing on another forum where it was suggested I refine my TableData class to include the readObject() and writeObject() methods for serialization. I did that, as you can see below, but it didn't help any. I'm still getting NullPointerException on the readObject() method in the servlet.

Can you collaborate with your buddies and ask around what the problem might be? I really need to get this worked out. Maybe I've inadvertantly discovered a bug what with all the changes having been made with JDK1.5 and Tomcat 5.5?

Please help,

Alan

class TableData implements Serializable
  {
      transient String[][] data = null;
      public TableData(String[][] newData)
      {
        if(newData != null)
           data = newData;
      }
      public String[][] getData()
      {
        return data;
      }
      public void setData(String[][] newData)
      {
        if(newData != null)
           data = newData;
      }
 
      private void writeObject(ObjectOutputStream out)throws IOException
      {
        out.defaultWriteObject();
        int elements = data.length;
        int columns = data[0].length;
        out.writeInt(elements);
        out.writeInt(columns);
        for(int i = 0; i < elements; i++)
        {
           for(int j = 0; j < columns; j++)
           {
              writeString(data[i][j], out);
           }
        }
      }
 
      private void readObject(ObjectInputStream in)throws IOException,ClassNotFoundException
      {
        in.defaultReadObject();
        int elements = in.readInt();
        int columns = in.readInt();
        data = new String[elements][columns];
        for(int i = 0; i < elements; i++)
        {
           for(int j = 0; j < columns; j++)
           {
              data[i][j] = readString(in);
           }
        }
      }
 
      private void writeString(String s, ObjectOutputStream out)throws IOException
      {
        int n = s.length();
        out.writeInt(n);
        out.writeBytes(s);
      }
 
      private String readString(ObjectInputStream in)throws IOException
      {
        int n = in.readInt();
        byte[] b = new byte[n];
        in.read(b);
        return new String(b); 
      }
 
    }

Line 31 in the servlet is here:

public class TableDataServlet extends HttpServlet {
 

    public void doGet(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException
    {
      doPost(request,response);
    }
    public void doPost(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException
    {
        ObjectInputStream inputFromApplet = null;
        TableData td = null;        
        PrintWriter out = null;
        String[][] data = null;
        
        try
        {  
          // get an input stream from the applet
  inputFromApplet = new ObjectInputStream(new BufferedInputStream(request.getInputStream()));
          System.out.println("Attempting to read Object...");  
          // read the serialized data from applet
          Object o = inputFromApplet.readObject(); //<<< THIS IS THE CULPRIT
          if(o != null && (o instanceof TableData))
          {        
            td = (TableData) o;
            System.out.println("Got Object");
            inputFromApplet.close();
            data = td.getData();          
            String firstname = data[0][1];
            // send back a confirmation message to the applet
            out = new PrintWriter(response.getOutputStream());
            response.setContentType("text/plain");
            out.println("confirmed " + firstname);
            out.flush();
            out.close();
          }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }       
            
    }
  }

[Ben Souther]

I didn't abandon, I asked a question.
There are too many people out there doing this sucessfully (me at one point) for it to be a bug.

I don't have much time these days to try to debug this for you but I found the link to the article that I used to get my own project running.
http://www-128.ibm.com/developerworks/java/library/j-tunnel/

Let me know if it helps.
[ February 04, 2005: Message edited by: Ben Souther ]

[Alan Shiers]

I've already checked out http://www-128.ibm.com/developerworks/java/library/j-tunnel/

and encorporated some of their code in with mine earlier. That still leaves my problem unresolved. For the hell of it, I decided to do a simpler test of moving a String object from applet to servlet. Code below.

The test worked! Yeah! So I know the system works to this degree. Now, why won't the system work when I try to pass it my TableData object?

Alan

APPLET:

package forms;
 
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import java.util.*;
import java.applet.*;
import java.io.*;
import java.net.*;
 
public class StringForm extends JApplet implements ActionListener
{
  boolean isStandalone = false;
  BorderLayout borderLayout1 = new BorderLayout();
  String processingServlet = "";
  JButton submit = new JButton("Submit");
  final static String data = new String("Alan Shiers");  
 
  //Get a parameter value
  public String getParameter(String key, String def) {
    return isStandalone ? System.getProperty(key, def) :
        (getParameter(key) != null ? getParameter(key) : def);
  }
 
  //Construct the applet
  public StringForm() {
  }
 
  //Initialize the applet
  public void init()
  {
    try {
      processingServlet = this.getParameter("action","");
    }
    catch (Exception e) {
      e.printStackTrace();
    }
    
    try {
      jbInit();
    }
    catch (Exception e) {
      e.printStackTrace();
    }
  }
 
  //Component initialization
  private void jbInit() throws Exception
  {
    this.setSize(new Dimension(100,100));
    Container container = getContentPane();
    container.setLayout(new FlowLayout());
    
    submit.addActionListener(this);    
    container.add(submit);
       
  }
 
  public void actionPerformed(ActionEvent e)
  {
    if(e.getSource() == submit)
    {
      try
      {
        // connect to the servlet
        String location = processingServlet;
        URL testServlet = new URL( location );
        URLConnection servletConnection = testServlet.openConnection();
 
        if (servletConnection instanceof HttpURLConnection)
        {
          ((HttpURLConnection)servletConnection).setRequestMethod("POST");
        }
        else
        {
          System.out.println("this connection is NOT an HttpUrlConnection connection");
          return;
        } 
 
        //inform the connection that we will send output and accept input
        servletConnection.setDoInput(true); 
        servletConnection.setDoOutput(true);
 
        //Don't use a cached version of URL connection.
        servletConnection.setUseCaches (false);
        servletConnection.setDefaultUseCaches (false);
 
        //Specify the content type that we will send binary data
        servletConnection.setRequestProperty ("Content-Type", "application/octet-stream");
 
        //send your data to the servlet
       System.out.println("Sending data to servlet..." + "\n");       
       ObjectOutputStream outputToServlet = new ObjectOutputStream(servletConnection.getOutputStream());
              
       outputToServlet.writeObject(data);        
       outputToServlet.flush();        
       
       System.out.println("Data has been sent." + "\n"); 
       //now wait for a response from the servlet
       BufferedReader in = new BufferedReader(new InputStreamReader(servletConnection.getInputStream()));
       System.out.println("Waiting for response..." + "\n"); 
       String line = "";
       while((line = in.readLine()) !=  null)
       {
         System.out.print(line + "\n");
       }
       
       System.out.println("Communication ended.");
 
       //Lastly, close the streams
       outputToServlet.close();
       in.close();
 
       //Once we have confirmation, we can simply request
       //the next page in a Step by Step process using:
       //this.getAppletContext().showDocument(URL u, "_self");
 
       
     }
     catch(IOException ioe)
     {
       ioe.printStackTrace();
     }      
    }
  }
}
 

SERVLET:

import java.awt.*;
import java.io.*;
import java.text.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class StringObjectServlet extends HttpServlet {
 

    public void doGet(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException
    {
      doPost(request,response);
    }
    public void doPost(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException
    {
        ObjectInputStream inputFromApplet = null;
        PrintWriter out = null;
        String data = null;
        
        try
        {  
          // get an input stream from the applet
  inputFromApplet = new ObjectInputStream(new BufferedInputStream(request.getInputStream()));
          System.out.println("Attempting to read Object...");  
          // read the serialized data from applet
          Object o = inputFromApplet.readObject();
          if(o != null && (o instanceof String))
          {        
            data = (String) o;
            System.out.println("Got Object");
            inputFromApplet.close();
                      
            // send back a confirmation message to the applet
            out = new PrintWriter(response.getOutputStream());
            response.setContentType("text/plain");
            out.println("confirmed " + data);
            out.flush();
            out.close();
          }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }           
    }    
}

[Alan Shiers]

Yeah! Woopie! I'm dancing in the streets! Come on everybody, Let's boogie down! Do people still say that?

I found the answer! Here's a lesson for you all to learn so listen up closely. You'll want to keep this in mind when dealing with others trying to perform object serialization from an applet to a servlet.

The reason I kept getting EOFException and NullPointerException messages is because in both the applet and servlet I defined the class TableData as an inner class in each. When I compiled both the applet and servlet, each had its own serialized rendition of the TableData class. After compiling, they must have been incompatible. But I fixed that by making TableData.java in its own file.

Now everything works perfectly!

Thanks for all your help,

Alan