Tutorials : Struts meets Swing (1) :

Struts meets Swing (1)

Introduction

Jakarta Struts is a framework that facilitates building servlet applications based upon the Model-View-Controller (MVC) design paradigm. Most Struts applications use a browser as the client, but the framework is actually open enough to allow other client types. In this first article of two, I'll take the browser application from my JavaBoutique article "Coding your second Jakarta Struts Application", and prepare it for adding a Swing client to it--with only a few changes to the code in the original browser application. In the second article we'll see how the coding of the Swing application is done.

So this article is about connecting a Swing client to an existing servlet application. If you're planning a new Java application that has to service both a web browser and a Swing client you should consider other architectural alternatives too, for example EJB's, or web services, since they may offer simpler interfaces or additional advantages--all depending on your application's needs. It's outside the scope of this article to compare architectures. But if you're interested in the topic you may find some good opinions at Sun's discussion forum.

The Struts architecture

Before we dig down into the technical stuff, let's consider how a browser communicates with a Struts application. This will reveal the requirements for our Swing application.

The entry to a Struts application is through a servlet that accepts GET or POST requests.
The "action" that the servlet must perform is given through the URL, for example a "list" action through: http://myserver/project/list.do
The Java classes used to handle the action are specified in the Struts configuration file struts-config.xml.
The output from Struts is constructed by jsp-pages, whose names are also specified in the Struts configuration file.

So the first challenges for a non-browser client is to be able to make a request to a servlet, and to receive and handle an answer from a jsp-page.

The URLConnection class

The servlet request is easy to code in Java using the URLConnection class. It's quite a funny class with a somewhat strange design--if you want to look into the details I'd recommend you reading "Dodge the traps hiding in the URLConnection class".

To send a request for the "list" action mentioned above, and receive and print out the response you may code like this:

URL url = new URL("http://myserver/project/list.do");
    URLConnection conn = url.openConnection();
    
    BufferedReader in = 
       new BufferedReader(new InputStreamReader(conn.getInputStream()));
    String line;  
    while ((line = in.readLine()) != null) {
       System.out.println(line);
    }

- Figure 1: Request, receive, print -

Normally you'll also have to send some data with your request--like when a browser sends data from an HTML form. These data must be sent before you read the response:

URL url = new URL("http://myserver/project/list.do");
    URLConnection conn = url.openConnection();
    conn.setDoOutput(true);
    conn.setRequestProperty("user-agent","SWING");  
    
    BufferedWriter out =
       new BufferedWriter(new OutputStreamWriter(conn.getOutputStream()));
    out.write("name1=value1&name2=value2");
    out.flush();
    out.close();
    
    String c = conn.getHeaderField("Set-Cookie");
    BufferedReader in = 
       new BufferedReader(new InputStreamReader(conn.getInputStream()));
    String line;  
    while ((line = in.readLine()) != null) {
       System.out.println(line);
    }

- Figure 2: Request, write, receive, print -

The example also shows how you can write and read HTTP header information. A complete working example can be seen here .

The example in figure 1 corresponds to HTTP method GET, and figure 2 to method POST. You may also use method GET to transfer data to the servlet, but then you'll have to add the data to the URL, like this:

URL url = new URL("http://myserver/project/list.do?name1=value1&name2=value2");

Identifying the client

So now we know how to send and request data, but what about the data we receive? If the response is meant for a browser, then it'll contain HTML--and maybe also JavaScript and stylesheets. For our new Swing client we only need the "raw" data, and nothing that has to do with the presentation of the data. This brings up a new question: how does the Struts application recognize whether the client is a web browser or a Swing client--or something else?

One solution would be to add a parameter to each request giving the identification of the client. I've chosen to use the user-agent header field to identify the Swing client--see the example in figure 2.

When the Struts application is ready to send data back to the client, it'll therefore have to retrieve the value of the user-agent field, and then select the jsp-page that matches the client. This would typically take place where the Struts Action class sets the forward information:

String client = (String)request.getHeader("user-agent");
    // Forward control to the list page
    if (client.equals("SWING")) 
     return (mapping.findForward("swinglist"));
    else return (mapping.findForward("list"));

The definition of "list" and "swinglist" are placed in Struts' configuration file:

<forward name="list" path="/list.jsp"/>
    <forward name="swinglist" path="/swinglist.jsp"/>

list.jsp is the file that will construct the page for a web browser, and swinglist.jsp must do the same for our Swing client.

Before we look at how we construct the swinglist.jsp file, let's look at the demo application, which will be used throughout the article.

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