Re: Applet Hangs when submitting data to servlet

Knute Johnson <>
Sun, 02 Sep 2007 21:16:12 -0700
ILPTAB wrote:

I am currently supporting an interactive web site for a state agency.
Recently we started getting reports from SOME of the users that the
applet hangs when they attempt to submit data to the servlet running
on our web server. We have not made any changes to the java code so
the only thing I can figure is that it must be happening as a result
of an update to the java JRE or Windows. Note that I said it's
occurring with SOME of the users. Other people seem to be having no
problem at all with it.

To help illustrate the problem, I put together a small test server and
applet that demonstrates the problem. I have included the source code
for both parts below. On the applet side, it hangs when it comes to a
line that attempts to instantiate the ObjectInputStream class and
retrieve the input stream from the applet.

ObjectInputStream inobj = new ObjectInputStream(s.getInputStream());

Since the applet locks up, I am unable to give you any error messages
in the java console. In fact the browser gets so locked up that it
requires the user to close the session from the Windows task manager.

On the Servlet side it hangs at the line that attempts to read the
data stream from the socket connection.

sSearchField_ = in.readLine();

The error messages generated at the server are as follows:

Java.lang.NullpointerException at ServerSide.testLookup(ServerSide.54)


at Source)

Any help in resolving this problem would be greatly appricated.

Thank you.

Here is the source code for both the Servlet and the applet.

/* Class Name:

    Copyright ? 2007 Steve Rulison

    Version 1.0.0

    Purpose: This java applet will display relevent information
pertaining to
                this examplet

import javax.swing.*;
import javax.swing.event.*;
import javax.swing.event.ListSelectionListener;
import javax.swing.event.ListSelectionEvent;
import javax.swing.table.*;
import java.awt.*;
import java.awt.event.*;
import java.applet.*;

import java.util.*;
import java.lang.*;

public class AppletSide extends JApplet
    implements ActionListener, KeyListener, MouseListener, Serializable
    private static final long serialVersionUID = 1;

    private JButton theSubmitButton_, theClearButton_;
    private Socket s;
    private String host, ServerName_;
    private TestData td;
    private JPanel appletPanel, resultPanel, buttonPanel,
            socketPanel, headingPanel;
    private Font f1 = new Font("Sansserf", Font.BOLD, 28);

    private JLabel socketConnectionLabel = new JLabel("");
    private JTextField resultField1, resultField2, resultField3,
resultField4, resultField5;

    private java.awt.List inquirylist;

    public AppletSide()
        headingPanel = new JPanel();
        headingPanel.setLayout(new FlowLayout(FlowLayout.CENTER));

        //Initialize String variables for output.

    public void init()
        // Note: The getCodeBase().getHost() methods must be invoked from
        // init() method.
        ServerName_ = getCodeBase().getHost();

        System.out.println("Test Applet Version");
    }//End of init.

    //Establish a socket connection with the server.
    private boolean createSocket(String pServerName)
        boolean SocketConnectSuccessful = false;
            int iPort = 554;
            s = new Socket(pServerName, iPort);
            SocketConnectSuccessful = true;
            socketConnectionLabel.setText("Socket connection successfully
established with host " + pServerName + " on port " + iPort);
        catch(Exception e)
            SocketConnectSuccessful = false;
        }//End of catch block.
        return SocketConnectSuccessful;
    }//End of method createSocket.

    //This method is called when a socket connection could not be
    private void socketConnectionRefused()
        socketConnectionLabel.setText("Unable to establish a connection with
host. Please contact you network administrator.");
    }//End of method socketConnectionRefused.

    //Setup search Inquiry labels.
    private void searchSetup()
        setLayout(new BorderLayout());

        appletPanel = new JPanel();
        appletPanel.setLayout(new GridLayout(0,1));

        JLabel titleLabel = new JLabel("Test Page");


        JPanel inquiryPanel = new JPanel();
        inquiryPanel.setLayout(new FlowLayout(FlowLayout.LEFT));

        JLabel inquiryLabel = new JLabel("Make Selection: ");

        inquirylist = new java.awt.List();
        inquirylist.add("Test 1");
        inquirylist.add("Test 2");
        inquirylist.add("Test 3");;




        socketPanel = new JPanel();
        socketPanel.setLayout(new FlowLayout(FlowLayout.LEFT));

        buttonPanel = new JPanel();
        buttonPanel.setLayout(new FlowLayout(FlowLayout.CENTER));

        theSubmitButton_ = new JButton("Submit");

        theClearButton_ = new JButton("Clear");

        add(BorderLayout.CENTER, appletPanel);

    }//End of method searchLabelSetup.

    //Setup result labels.
    private void resultSetup()
        resultPanel = new JPanel();
        resultPanel.setLayout(new GridLayout(5,2));

   JLabel resultLabel1 = new JLabel("Field 1: ", JLabel.RIGHT);
        JLabel resultLabel2 = new JLabel("Field 2: ", JLabel.RIGHT);
        JLabel resultLabel3 = new JLabel("Field 3: ", JLabel.RIGHT);
        JLabel resultLabel4 = new JLabel("Field 4: ", JLabel.RIGHT);
        JLabel resultLabel5 = new JLabel("Field 5: ", JLabel.RIGHT);

        resultField1 = new JTextField("Default value 1", 5);
        resultField2 = new JTextField("Default value 2 ",5);
        resultField3 = new JTextField("Default value 3",5);
        resultField4 = new JTextField("Default value 4",5);
        resultField5 = new JTextField("Default value 5",5);



    }//End of method resultLabelSetup.

    // Display the fields that have been populated with data received
from the
    // server application.
    private void displayResults()
    }//End of method displayResults.

    //Clears fields of data
    private void clearFields()
        resultField1.setText("Default value 1");
        resultField2.setText("Default value 2");
        resultField3.setText("Default value 3");
        resultField4.setText("Default value 4");
        resultField5.setText("Default value 5");
    }//End of method clearFields.

    //Reset the applet to allow the user to perform a new search.
    private void resetSearch()
        this.setCursor(new Cursor(Cursor.WAIT_CURSOR));
        this.setCursor(new Cursor(Cursor.DEFAULT_CURSOR));
    }//End of method resetSearch.

    //Perform Input/Output operations with the server.
    private void serverIO()
        this.setCursor(new Cursor(Cursor.WAIT_CURSOR));

            PrintWriter out =
                new PrintWriter(
                    new BufferedWriter(
                        new OutputStreamWriter(

            System.out.println("Check point 1 @ serverIO method.");
            // Thread.currentThread().sleep(5000);
            System.out.println("Check point 2 @ serverIO method.");
            //Read data back from server - Inputstream
            ObjectInputStream inObj = new
            System.out.println("Check point 3 @ serverIO method.");
            System.out.println("Check point 4 @ serverIO method.");

            td = (TestData) inObj.readObject();

        catch(Exception e)
            System.out.println("Check point 1 - Exception: " + e);

        }//End of try/catch block

        this.setCursor(new Cursor(Cursor.DEFAULT_CURSOR));
    }//End of method ServerIO

    //Event handling method.
    public void actionPerformed(ActionEvent evt)
        if(evt.getSource() == theClearButton_)
        else if(evt.getSource() == theSubmitButton_)

                // searchLabelSetup();
    }//End of method actionPerformed

    //Methodsfor handling for keyboard events
    public void keyPressed(KeyEvent evt){}
    public void keyReleased(KeyEvent evt){}
    public void keyTyped(KeyEvent evt)
        // System.out.println(" " + (int)evt.getKeyChar() );

        if(evt.getSource() == theClearButton_)
        else if(evt.getSource() == theSubmitButton_)
    }//End of method keyTyped.

    public void mouseClicked(MouseEvent evt)
        if(evt.getClickCount() == 2)
    public void mouseExited(MouseEvent event){}
    public void mouseEntered(MouseEvent event){}
    public void mouseReleased(MouseEvent event){}
    public void mousePressed(MouseEvent event){}

}//End of class ApplSide.

class TestData implements Serializable
    private static final long serialVersionUID = 1;

    /* Test Data */
    public String sField_1, sField_2, sField_3, sField_4, sField_5;
    public boolean bRecFound;

}//End of class ServerTestData.

 - - - - - - - - - - - - - - - - -
- - - -

/* Class name: ServerSide

    Author: Steven Rulison

    Purpose: Demonstrate the servlet side

import java.util.*;

public class ServerSide extends Thread
    private TestData td;
    private Socket s;
    private String sSearchField_;

    public static void main(String[] Args)
            ServerSocket ss = new ServerSocket(554);

            int spawnCnt = 0;
                Socket s = ss.accept();

                System.out.println("Spawning " + spawnCnt);

                Thread servSide = new Thread(new ServerSide(s), "serverside");
        }catch(IOException e)
    }//End of main.

    public ServerSide(Socket s_)
        s = s_;
    }//End of Constructor.

    private void testLookup(String sSearch)
        td = new TestData();
        td.bRecFound = true;
        if(sSearch.trim().equals("Test 1"))
            td.sField_1 = "Alpha";
            td.sField_2 = "Beta";
            td.sField_3 = "Gamma";
            td.sField_4 = "Delta";
            td.sField_5 = "Epsilon";
        else if(sSearch.trim().equals("Test 2"))
            td.sField_1 = "AAA";
            td.sField_2 = "BBB";
            td.sField_3 = "CCC";
            td.sField_4 = "DDDD";
            td.sField_5 = "EEEE";
        else if(sSearch.trim().equals("Test 3"))
            td.sField_1 = "111";
            td.sField_2 = "222";
            td.sField_3 = "333";
            td.sField_4 = "444";
            td.sField_5 = "555";
    }// End of method testLookup.

    public void run()
            //Setup Input Stream.
            BufferedReader in =
                new BufferedReader( new InputStreamReader(s.getInputStream()));

            // Setup output Stream.
            ObjectOutputStream ObjOut = new

            //Perform intial read...
            sSearchField_ = in.readLine();


System.out.println("The sSearchField_ variable = " + sSearchField_);



        catch(Exception e)
    }//End of run method
}//End of ServerSide class...

    This is the class template that will be used to create the object
that will
    be passed through to the ObjectInput/Output - stream to the lookup
class TestData implements Serializable
    private static final long serialVersionUID = 1;

    /* Test Data */
    public String sField_1, sField_2, sField_3, sField_4, sField_5;
    public boolean bRecFound;


}//End of class ServerTestData.

I may have done you a bad turn here. I think your problem is because
you aren't creating an ObjectOutputStream on the socket before you
create your ObjectInputStream. I think they have to be matched (at
least if you are going to do input) on the socket and the
ObjectOutputStream has to be created first. This is a known source of
hangs. I was too hasty reading your post the first time.


Knute Johnson
email s/nospam/knute/

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