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Create the client program (the servlet) - Java

One of the greatest advantages which Jserv brings to Apache is ability to leverage the large number of API's available to Java. Remote Method Invocation (RMI) delivers several significant benefits to the servlet solution. The primary benefit of using RMI with servlets is that it significantly expands the variety of datasources which Apache can serve to the browser. Furthermore, RMI's simplifies code on the client side of the RMI connection (the servlet), and also allows for load distribution.

  1. Using RMI with Apache Jserv
  2. Design the Remote Interface
  3. Create the client program (the servlet)
  4. Conclusion
By: Richard Yumul
Rating: starstarstarstarstar / 5
January 21, 2000

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Enabling the servlet (Booklet.java) to talk to the remote Directory service is a fairly straightforward procedure. Just like any other RMI client, the servlet instantiates the remote object with a call to Naming.lookup(). After the remote object is instantiated, you can work with the object as if it were local.

The init() and doGet() methods are the particularly interesting methods in the Booklet servlet.

public class Booklet extends HttpServlet { private Directory dir; private String url; public void init(ServletConfig config) throws ServletException { super.init(config); if (System.getSecurityManager() == null ) { System.setSecurityManager(new RMISecurityManager()); String rmiRegistryHost = getInitParameter("rmiRegistryHost"); if (rmiRegistryHost == null) { rmiRegistryHost = "localhost"; } try { String name = "//" + rmiRegistryHost + ":4000/addressBook"; this.dir = (Directory) Naming.lookup(name); } catch (Exception e) { e.printStackTrace(); } } } public void doGet (HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException { res.setContentType("text/html"); PrintWriter out = res.getWriter(); this.url = req.getRequestURI(); String cmd = req.getParameter("cmd"); printHeader(out); printForm(out); try { if (cmd.equals("search")) { out.println("<h4>Search Results:</h4>"); printResults(dir.search(req.getParameter("query")), out); } else if (cmd.equals("add")) { out.println("<h4>New Entry:</h4>"); printEntry( dir.getEntry( dir.addEntry( new ClientEntry( req.getParameter("name"), req.getParameter("email") ) )), out); } else if (cmd.equals("get")) { out.println("<h4>Directory Entry:</h4>"); printEntry(dir.getEntry(req.getParameter("key")), out); } else if (cmd.equals("everybody")) { out.println("<h4>Complete Directory Listing:</h4>"); printResults(dir.getEntries(), out); } } catch (RemoteException e) { out.print("<h3>Error with the RMI Transport</h3>"); } catch (NullPointerException e) { out.println("Search for an entry, create an entry, " + "or get an entry using the forms on the left."); e.printStackTrace(); } printFooter(out); } // rest of servlet methods.... }
Upon initialization, the servlet first checks the SecurityManager and sets it if needed. Next, the servlet checks for the initialization parameter named "rmiRegistryHost", the name of the computer hosting the RMI registry to which the Directory service is bound. To specify the rmiRegistryHost, in your servlet zone's properties file, insert this line:

If this arguement is not defined, the getInitParameter() method will return null and the rmiRegistryHost will be set to the localhost. Then the init() method obtains the Directory service with Naming.lookup(), and stores it in a private variable, keeping it around for reuse. If the remote service was obtained inside a service method, like doGet(), the servlet would have to obtain the remote object with every request, an expensive procedure where repeating it should be minimized. Instead, one instance of the remote directory service will be reused for the lifetime of the servlet.

The doGet() method, then services the incoming requests to the servlet. It examines the parameters passed to it from the HTML form and then calls the appropriate methods on the remote Directory service.

Nothing particularly special needs to be done in compiling the client classes; just use the basic java compiler, javac.

$ javac client/*.java

> javac client\*.java

Make the JAR files

Now we're going to bundle our classes into JAR files. While not an absolute requirement, this eases the deployment step as well as distribution. At the command prompt, type:

$ jar -cvf client.jar client/*.class directory/*.class server/AddressBook_Stub.class
on Windows operating systems the slashes change to backslashes:

> jar -cvf client.jar client\*.class directory\*.class server\AddressBook_Stub.class
The "c" tag tells the jar tool to create a new file. The "v" tag turns on verbose output, so you can see exactly what the jar tool is doing. The "f" tag tells the jar tool that you will specify the name of the jar file to be created, which is the argument following the "f" tag, client.jar. The next arguments specify which files should go into the jar file.

On the client side, the only the stub class is absolutely necessary for RMI communication to work.

Create the server jar file:

$ jar -cvf server.jar server/*.class directory/*.class

> jar -cvf server.jar server\*.class directory\*.class
On the server side, both stub and skeleton classes generated by rmic are necessary.

Deploy the classes in Jserv

So far, the steps in this article have followed the procedures described in Sun's RMI tutorial or the Javadoc. The only really Jserv specific part is the deployment. Installation of the client servlet is fairly straight forward:

  • Copy the client.jar file on to your webserver.
  • Add the jar file to Jserv's global classpath. This is done in the jserv.properties file. Add the line:

Of course, this is assuming you already have a servlet zone set up. If the RMI registry is going to be running on a different computer, then specify the rmiRegistryHost parameter for the servlet as described above. (If you need help with setting up Apache Jserv, check out Ari Halberstadt's Using Apache Jserv 1.0, http://www.servletcentral.com/1999-01/jserv.dchtml.)

Now restart your Apache Jserv server to ensure that the client.jar file is accessible and the configuration changes take effect. Typically, an "apachectl restart" will do it.

Run it!

Now that the client has been deployed, the all that's left to do is to run the demo. First fire up the server (making sure that the server.jar file is in your classpath...):

java -classpath=$CLASSPATH:/path/to/server.jar \ -Djava.security.policy=java.policy server.AddressBook

java -classpath=%CLASSPATH%;\path\to\server.jar -Djava.security.policy=java.policy server.AddressBook
The server should start, printing status messages to the console.

$ java server.AddressBook Creating RMIRegistry... Creating Address Book... Address Book created ... Populating AddressBook... Added entry: Jeff Added entry: Leonardo Added entry: Jose Alexis Added entry: Walter Added entry: Chris
The -Djava.security.policy=java.policy parameter tells the Java virtual machine to use the security rules specified in the java.policy file. The file tells the JVM to accept all socket connections on ports above 1024. This parameter is necessary otherwise other machines won't be able to connect to the rmiregistry.

In a web browser, go to the url which maps to the client.Booklet servlet. For a typical setup, this might be http://yourhost/servlets/client.Booklet. If you want to abbreviate the URL & not refer to the servlet by it's package, in your zone.properties file, you could add:

This creates an alias for the client.Booklet servlet; it's nick name is "booklet". Now you can access the Booklet servlet with a URL something like, http://yourhost/servlets/booklet, instead.

The servlet will return a form to your browser which will allow you to interact (search, add entries, select an entry, get all entries) with the AddressBook through the HTML form.

As you interact with the AddressBook via the servlet, status messages on what the AddressBook is doing should appear in the console in which it was started.

... All entries retrieved... Added entry: John Retrieved entry: John Queried on:Leo

>>> More Java & J2EE Articles          >>> More By Richard Yumul

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