I am designing a chat server in java. The communication is Http based and not socket based. In the client side I have an applet. In the server side I have a servlet.
Applet: I create a new thread to listen for incoming messages(GET method). The main thread is used to send messages(POST messages).
The partial code is :
public void start() {
System.out.println("Creating new thread");
Thread thread = new Thread(this);
thread.start();
}
private String getNewMessage() {
System.out.println("Inside getNewMessage");
String msg = null;
try {
while(msg == null) {
System.out.println("Trying to listen to servlet");
URL servlet = new URL(getCodeBase(), "NewServlet?mode=msg");
URLConnection con = servlet.openConnection();
con.setUseCaches(false);
DataInputStream din = new DataInputStream(new BufferedInputStream(con.getInputStream()));
msg = din.readUTF();
System.out.println("message read :" + msg);
}
} catch (Exception e) {
e.printStackTrace();
}
return msg + "\n";
}
public void run() {
System.out.println("Inside new thread");
while(true) {
System.out.println("inside first while");
String newMsg = getNewMessage();
chatOutput.append(newMsg);
System.out.println("Appended!!");
}
}
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
String message = chatInput.getText();
chatInput.setText("");
chatOutput.append(message + "\n");
try {
System.out.println("Trying to send msg :" + message);
URL url = new URL(getCodeBase(), "NewServlet");
URLConnection servletConnection = url.openConnection();
servletConnection.setDoInput(true);
servletConnection.setDoOutput(true);
servletConnection.setUseCaches(false);
servletConnection.setRequestProperty("Content-Type", "application/octet-stream");
ObjectOutputStream out = new ObjectOutputStream(servletConnection.getOutputStream());
out.writeObject(message);
out.flush();
out.close();
System.out.println("Message sent!");
} catch (Exception e) {
e.printStackTrace();
}
}
This next code is from the servlet side. it uses the Observable interface to identify and send messages to clients.
public class NewServlet extends HttpServlet {
// getNextMessage() returns the next new message. // It blocks until there is one.
public String getNextMessage() {
// Create a message sink to wait for a new message from the
// message source.
System.out.println("inside getNextMessage");
return new MessageSink().getNextMessage(source);}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
System.out.println("Inside Doget");
response.setContentType("text/plain");
PrintWriter out = response.getWriter();
out.println(getNextMessage());
}
// broadcastMessage() informs all currently listening clients that there
// is a new message. Causes all calls to getNextMessage() to unblock.
public void broadcastMessage(String message) {
// Send the message to all the HTTP-connected clients by giving the
// message to the message source
source.sendMessage(message); }
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
System.out.println("Inside DoPost");
try {
ObjectInputStream din= new ObjectInputStream(request.getInputStream());
String message = (String)din.readObject();
System.out.println("received msg");
if (message != null) broadcastMessage(message);
System.out.println("Called broadcast");
// Set the status code to indicate there will be no response
response.setStatus(response.SC_NO_CONTENT);
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* Returns a short description of the servlet.
* @return a String containing servlet description
*/
@Override
public String getServletInfo() {
return "Short description";
}
MessageSource source = new MessageSource();}
class MessageSource extends Observable {
public void sendMessage(String message) {
System.out.println("inside sendMsg");
setChanged();
notifyObservers(message);
}
}
class MessageSink implements Observer {
String message = null; // set by update() and read by getNextMessage()
// Called by the message source when it gets a new message
synchronized public void update(Observable o, Object arg) {
// Get the new message
message = (String)arg;
// Wake up our waiting thread
notify();
}
// Gets the next message sent out from the message source
synchronized public String getNextMessage(MessageSource source) {
// Tell source we want to be told about new messages
source.addObserver(this);
System.out.println("AddedObserver");
// Wait until our update() method receives a message
while (message == null) {
try { wait(); } catch (Exception ignored) { }
}
// Tell source to stop telling us about new messages
source.deleteObserver(this);
// Now return the message we received
// But first set the message instance variable to null
// so update() and getNextMessage() can be called again.
String messageCopy = message;
message = null;
System.out.println("Returning msg");
return messageCopy;
}
}
As you can see I have included System.out.println("Some message"); in some places. this was just for debugging purposes. In the Java console, I get the following output:
Creating new thread
Inside new thread.
inside first while.
Inside getNewMessage.
Trying to listen to servlet.
On the servlet side, I get the following output in the tomcat logs:
Inside Doget.
inside getNextMessage.
AddedObserver.
After I type a message in the applet, and send it, I get the following output in the Java console:
Trying to send msg :you deR??
Message sent!
But on the servlet side, I dont get anything in the logs.
I used the O'Reily Java Servlet Programming as reference (The observer interface comes from there). But I am not getting any chat communication between two clients. As can be understood from the logs, the doPOST method is not called. What is the reason for this?
