CSCI A348/A548 Lecture Notes 27 Fall 1999

This is going to be an involved assignment. At the end you will have seen a lot of essential Java elements in action, and will have solved in the process your homework 8 assignment and a great deal (more than 90%) of your semester group project. For this project teams could merge, creating larger teams, if you think it helps you.

I start by creating all the code in a directory

/u/dgerman/November/apache_1.3.9/htdocs/hw8

tucotuco.cs.indiana.edu% ls -l
total 4
-rw-r--r--   1 dgerman  students      708 Nov 30 12:09 ChatServer.java
-rw-r--r--   1 dgerman  students     1254 Nov 30 09:07 HttpChatApplet.java
-rw-r--r--   1 dgerman  students      645 Nov 30 10:13 HttpMessage.java
tucotuco.cs.indiana.edu% pwd
/nfs/paca/home/user1/dgerman/November/apache_1.3.9/htdocs/1201
tucotuco.cs.indiana.edu% 

HttpChatApplet.java looks like this:

import java.applet.*;
import java.awt.*;
import java.io.*;
import java.net.*;
import java.util.*;

public class HttpChatApplet extends Applet implements Runnable {

  TextArea  text;
  Label     label;
  TextField input;
  Thread    thread;
  String    user; 

  public void init() 
  {
    URL codebase = getCodeBase(); 

    user = getParameter("user"); 
    if (user == null) user = "anonymous"; 

    text = new TextArea(); 
    text.setEditable(false); 

    label = new Label("Say something: "); 

    input = new TextField(); 
    input.setEditable(true); 

    setLayout(new BorderLayout()); 
    Panel panel = new Panel(); 
    panel.setLayout(new BorderLayout()); 
    add("Center", text);
    add("South",  panel); 
    panel.add("West",   label); 
    panel.add("Center", input); 

  } 

  public void start() 
  { 
    thread = new Thread(this); 
    thread.start(); 
  } 

  String getNextMessage() 
  {
    String nextMessage = null; 
    while (nextMessage == null) {
      try {
        URL             url  = new URL(getCodeBase(), "/servlet/ChatServer");  
        HttpMessage     msg  = new HttpMessage(url);  
        InputStream     in   = msg.sendGetMessage(); 
        DataInputStream data = new DataInputStream(
                                 new BufferedInputStream(
                                   in)); 
        nextMessage          = data.readLine(); 
      } catch (Exception e) { 
        try { Thread.sleep(5000); } 
        catch (InterruptedException ignored) { }  
      } 
    }
    return nextMessage + "\n"; 
  } 

  public void run() { 
    while (true) 
      text.appendText(getNextMessage()); 
  } 

  public void stop() { 
    thread.stop(); 
    thread = null; 
  } 

  void broadcastMessage(String message) {
    message = user + ": " + message; 
    try {
      URL url          = new URL(getCodeBase(), "/servlet/ChatServer"); 
      HttpMessage msg  = new HttpMessage(url); 
      Properties props = new Properties(); 
      props.put("message", message); 
      msg.sendPostMessage(props); 
    } catch (Exception ignored) { } 
  } 

  public boolean handleEvent(Event event) {
    switch (event.id) {
      case Event.ACTION_EVENT: 
        if (event.target == input) {
          broadcastMessage(input.getText()); 
          input.setText(""); 
          return true; 
	} 
    } 
    return false; 
  } 
}

import java.io.*;
import java.net.*;
import java.util.*;

public class HttpMessage {

  URL    servlet = null; 
  String args    = null; 

  public HttpMessage(URL servlet) 
  {
    this.servlet = servlet; 
  }

  public InputStream sendGetMessage() throws IOException 
  {
    return sendGetMessage(null); 
  } 

  public InputStream sendGetMessage(Properties args) throws IOException 
  { 
    String argString = ""; 

    if (args != null) { argString = "?" + toEncodedString(args); } 

    URL url           = new URL(servlet.toExternalForm() + argString); 
    URLConnection con = url.openConnection();

    con.setUseCaches(false); 
    return con.getInputStream(); 
  } 

  public InputStream sendPostMessage() throws IOException {
    return sendPostMessage(null); 
  } 

  public InputStream sendPostMessage(Properties args) throws IOException 
  {
    String argString = ""; 

    if (args != null) {
      argString = toEncodedString(args); 
    } 

    URLConnection con = servlet.openConnection(); 

    con.setDoInput(true); 
    con.setDoOutput(true); 
    con.setUseCaches(false); 
    con.setRequestProperty(
      "Content-Type", "application/x-www-form-urlencoded"
    ); 

    DataOutputStream out = new DataOutputStream(con.getOutputStream()); 
    out.writeBytes(argString); 
    out.flush(); 
    out.close(); 

    return con.getInputStream(); 
  } 

  private String toEncodedString(Properties args) 
  {
    StringBuffer buf = new StringBuffer();
    Enumeration names = args.propertyNames(); 
    while (names.hasMoreElements()) {
      String name = (String) names.nextElement(); 
      String value = args.getProperty(name); 
      buf.append(URLEncoder.encode(name) + "=" + URLEncoder.encode(value)); 
      if (names.hasMoreElements()) buf.append("&"); 
    } 
    return buf.toString(); 
  } 
}

import java.io.*;
import java.net.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*; 

public class ChatServer extends HttpServlet { 

  MessageSource source = new MessageSource(); 

  public void doGet(HttpServletRequest req, HttpServletResponse res) 
    throws ServletException, IOException 
  { 
    res.setContentType("text/plain"); 
    PrintWriter out = res.getWriter(); 
    out.println(getNextMessage()); 
  }

  public void doPost(HttpServletRequest req, HttpServletResponse res) 
    throws ServletException, IOException 
  { 
    String message = req.getParameter("message");
    if (message != null) broadcastMessage(message); 
    res.setStatus(res.SC_NO_CONTENT);     
  }

  public String getNextMessage() 
  {
    return new MessageSink().getNextMessage(source); 
  }

  public void broadcastMessage(String message) 
  {
    source.sendMessage(message); 
  }
} 

class MessageSource 
  extends Observable 
{
  public void sendMessage(String message) 
  {
    setChanged();
    notifyObservers(message); 
  } 
}

class MessageSink 
  implements Observer 
{
  String message = null; 

  synchronized public void update(Observable o, Object arg) 
  {
    message = (String)arg; 
    notify(); 
  } 
  
  synchronized public String getNextMessage(MessageSource source) 
  {
    source.addObserver(this); 

    while (message == null) {
      try {
        wait(); 
      } catch (Exception ignored) { } 
    } 

    source.deleteObserver(this); 
    String messageCopy = message;
    message = null;
    return messageCopy; 
  } 
}

But how do we distribute the clients?

We need to send the applets to the browsers.

So we create three files which differ in only one place.

First we create Larry.html,

<html>
<applet code=HttpChatApplet
        codebase=/1201
        width=400 height=400>
<param name=user value="Larry"> 
</applet>
</html>

<html>
<applet code=HttpChatApplet
        codebase=/1201
        width=400 height=400>
<param name=user value="Michael">
</applet>
</html>

<html>
<applet code=HttpChatApplet
        codebase=/1201
        width=400 height=400>
<param name=user value="Tony">
</applet>
</html>

• You can be Larry
• You can be Michael
• You can be Tony

What's left?

You need to

• change the applet to AWT 1.1 (optional)
• create a dispatcher servlet that allows the user to specify a name

1. The code for RMI is simpler, closer to the problem to be solved
2. The ancillary compilation phases are much more complicated for RMI
3. For HTTP each file is compiled independently
4. The code for HTTP is slightly more complicated, but only just