Applet Servlet Connection & Communication

[Liz Costelloe]

Hi,

I'm trying to save some information from an applet to a mySQL database using a Servlet. I can access the Servlet fine from a JSP but I can't get an Applet to connect to the servlet at all.

The code is as follows, any suggestions would be gratefully received.

Thanks in advance.

  
 
[B]Listener Code:[/B]
 
public void mouseClicked(MouseEvent event) {
   
int button = event.getButton();
    
    if(button == event.BUTTON1){
    
    Point p = event.getPoint();
    
    if(buttonArea.checkBounds(p.x, p.y)){
    
    applet.doSave();
    }
    
    }
    }
 

[B] Applet Code: [/B]
 

private URLConnection getServletConnection() throws MalformedURLException, IOException {
URL urlServlet = new URL(getCodeBase(), "Save");
URLConnection con = urlServlet.openConnection();
con.setDoInput(true);
con.setDoOutput(true);
con.setUseCaches(false);
con.setRequestProperty("Content-Type","application/x-java-serialized-object");
return con;
}

public void doSave() {

try {
String input = "hello";
URLConnection con = getServletConnection();
PrintStream out = new PrintStream(con.getOutputStream());
out.println("some text to be saved"); 
out.flush();
out.close();
} catch (Exception ex) {
ex.printStackTrace();
}
}

 

[B] Servlet Code: [/B]
 
public void doPost( HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException {

System.out.println("SaveServlet doPost()");

try {
InputStream in = request.getInputStream();
ObjectInputStream inputFromApplet = new ObjectInputStream(in);
String echo = (String) inputFromApplet.readObject();
System.out.println(echo);
} catch (Exception e) {
e.printStackTrace();
}
 
response.sendRedirect("./index.html");

}
 

[Ulf Dittmer]

I can't get an Applet to connect to the servlet at all

What does this mean - what happens when the code is run? Are there any exceptions?

Since you use getCodeBase, the servlet seems to be hosted on the same server as the applet (which is a precondition for this to work at all), but is the servlet really listening at the URL where the applet is hosted?

[Liz Costelloe]

Hi Ulf,

Thanks for your reply.

What does this mean - what happens when the code is run? Are there any exceptions?

The applet code executes in full without throwing any errors. On the servlet side nothing happens, the doPost method is not invoked (and no errors occur).

but is the servlet really listening at the URL where the applet is hosted?

Terribly sorry, but I'm not sure exactly what you mean by this, I'm new to applets and servlets.

Yes, the applet and servlet are both hosted on the same server.

The servlet has responded to requests from JSPs but I'm not sure if this answers your question.

[Ulf Dittmer]

By "listening to the URL" I mean that a servlet is configured to respond to particular URLs (the "servlet-mapping" elements in the web.xml file). It would be a very unusual setup to map a servlet to a directory that contains files to be served. That seems to be what you are doing with the "getCodebase" call in the URL constructor.

Can you invoke the servlet manually through the browser?

Another things to check are the web server access logs, to make sure that the request reaches the server.

[Liz Costelloe]

Hi Ulf,

I do think I have the servlet configured to respond to http://an.ip.address:8080/structures/Save. The web.xml file is included below and I am able to invoke the servlet manually through a web browser.

I have tried looking through the web server (JBoss) logs but I'm not sure where the access logs are stored in JBoss. If you happen to know this, I would be grateful if you could point me in the right direction - I will keep looking of course, but it would save me some time.

Many thanks.

    
<?xml version="1.0" encoding="UTF-8"?>
 
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd">
 
<web-app >
   <distributable/>
 
   <servlet>
      <servlet-name>Login</servlet-name>
      <display-name>Login Servlet</display-name>
      <description><![CDATA[Servlet for Logging into the tutorial]]></description>
      <servlet-class>structures.web.LoginServlet</servlet-class>
 
   </servlet>
   <servlet>
      <servlet-name>Save</servlet-name>
      <display-name>Save Servlet</display-name>
      <description><![CDATA[Servlet for saving an experiment]]></description>
      <servlet-class>structures.web.SaveServlet</servlet-class>
   </servlet>
 
 
   <servlet-mapping>
      <servlet-name>Login</servlet-name>
      <url-pattern>/Login</url-pattern>
   </servlet-mapping>
   <servlet-mapping>
      <servlet-name>Save</servlet-name>
      <url-pattern>/Save</url-pattern>
   </servlet-mapping>
 
   <ejb-ref >
      <description><![CDATA[Reference to the Structures EJB]]></description>
      <ejb-ref-name>ejb/Structures</ejb-ref-name>
      <ejb-ref-type>Session</ejb-ref-type>
      <home>structures.interfaces.StructuresHome</home>
      <remote>structures.interfaces.Structures</remote>
   </ejb-ref>
 
</web-app>

[ May 15, 2006: Message edited by: Liz Costelloe ]

[Ulf Dittmer]

Your web app is named "structures". But in which directory is the applet - is it in the top-level directory? Only then would getCodeBase return the proper directory. It might be a good idea to print out the URL which the applet is trying to access; I'm not quite sure what the "spec" parameter does.

Can't help you with JBoss - I never used it.

[Liz Costelloe]

Hey Ulf,

Quick line just to say thanks for all your help!! At long last I have this applet and servlet communicating. In the end I got out a TCP viewer and noticed that there were a lot of 'Java' connections stuck waiting for responses so I extended my code like so:

  
public void doSave() {
 
try {
String input = "hello"; //inputField.getText();
URLConnection con = getServletConnection();
OutputStream outstream = con.getOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(outstream);
oos.writeObject(input);
oos.flush();
oos.close();
 
[B]InputStream instr = con.getInputStream();
ObjectInputStream inputFromServlet = new ObjectInputStream(instr);
String result = (String) inputFromServlet.readObject();
inputFromServlet.close();
instr.close(); [/B]
 
} catch (Exception ex) {
ex.printStackTrace();
}
 
}
 
  

As a result communication was established There were a few errors along the way (Permissions and an EOF error) but now all is working smoothly.

Thanks again for all your help!!
[ May 24, 2006: Message edited by: Liz Costelloe ]