Much has been written and discussed about Java and its interoperability with RPG legacy applications. With Java spreading faster than office gossip and the AS/400 able to run Java and host Web sites, the content of these articles, white papers, and discussions grows increasingly important. Fortunately, developers concerned with providing Web-based business applications while keeping the back-end business rules of RPG have received good news concerning RPG communication with Java and, subsequently, the Internet.

However, two problems have come out of this information. One is that the technology related to Java seems to be a moving target. Just as you decide which technology to implement, something better and faster comes along. I realize that this is common in the IT industry as a whole, but it occurs even faster with Java. The other problem is that, because so many potential solutions are available to solve RPG-Java interoperability, developers have no time to test which ones might work the best. Therefore, before you make your decision, you should consider things such as portability, speed, and ease of implementation. One tool that fits these criteria is sockets.

Sockets 101

A socket is basically an endpoint, a port that can be addressed in a network. You can use sockets to communicate with other sockets in a communication area. This area can be as large as the Internet or as small as any two points that can connect using TCP/IP. You can even use sockets to communicate between programs on the same machine, something I have done using RPG and Java on the AS/400.

I chose sockets to solve my interoperability dilemma because they meet my selection criteria. First, sockets are portable; they are part of Java’s core API and go everywhere Java goes. No extra toolkit or extensions are necessary to implement them. I even compiled and ran a Java Socket client servlet on the AS/400 to communicate with an RPG Socket host on the same AS/400. I then moved the servlet to a Windows/NT server; the servlet worked just as well with the Socket host on the AS/400. Second, sockets are fast. How fast? Well, results may vary, but I consistently get my desired data in less than a second. As for my third criterion, ease of implementation, you will soon see that implementing Socket classes in a Java program is very easy.

The Example

Figure 1 shows a Java servlet using Socket classes. This is a simple servlet that throws a Web page after sending a request to a socket and receiving an answer back. I’ll touch briefly on what makes this Java code a servlet but won’t go into too much detail about servlets. (Don Denoncourt’s article from the August issue of MC, “Serving Up Host-based Java Apps,” does that nicely.) Label A of Figure 1 shows the import statements required for servlets. The code marked by Label B is the remainder of code that really makes this a servlet.

I’ve created a class called testServlet, which inherits, extends, or is a child of (pick your term) class HttpServlet. The servlet engine calls the doGet method based on the URL request. The doGet method requires two parameters. The first parameter, HttpServletRequest, contains information about the request, and the second parameter, HttpServletResponse, houses the dynamically generated HTML response. The socket client starts at the code in Label C.

A lot is going on here, but you wouldn’t know it from the code. First, you need to declare the Socket objects you are going to use. In my example, I’ve declared the Socket object as testSocket, the PrintWriter object as toServer, and the BufferedReader object as fromServer. The Socket object defines the host and port number; PrintWriter sends requests to the socket; and BufferedReader receives information from the socket. Once the objects are declared, they are instantiated at the three lines marked by Label D. The testSocket object is set to a new socket with the TCP/IP address or host name of the server from which you are requesting information and to which the Socket host code will run. The port number (3004 in this case) is the remote port that listens for the request, and the client uses any local port it can bind to. The toServer object instance of the PrintWriter class contains the request to the socket, and the fromServer object contains information received from the Socket host. The fromServer object is constructed a little differently from testSocket and toServer because Sun recommends wrapping the InputStreamReader into BufferedReader for performance. Who am I to argue with Sun?

Next is to send the request to the socket and wait for the response. Label E denotes the two lines of code that accomplish that. In this example, I am sending the string “My test message” to the Socket host and waiting for a response, which I am storing in the string named temp. The fromServer object waits for the response before going to the next line of code that displays the response on the Web page. The last thing to do in testServlet is to close the socket connections; Label F shows the code for doing that.

The Java Host Socket Class

Now, I’ll take a look at the Java host Socket class. Label G shows the entire myHost class, which is a separate class and not part of the Socket client servlet. I’ve coded both the Socket host and the Socket client classes as part of testServlet.java. When you compile this Java code, you create two classes: testServlet.class and myHost.class. You don’t have to do it this way; I did it for simplicity. What’s interesting about coding them in the same source member is that the two classes may run on different machines. (You’ll have to excuse my excitement, but, for an RPG-learning-Java programmer like me, this is pretty cool.)

OK, back to myHost. You first declare ServerSocket and Socket classes, instantiating the ServerSocket class to port 3004. The accept method waits for a connection request from a client to the host and port of your server (in this example, port 3004). When you request and successfully establish a connection, the accept method returns a new Socket object bound to a new local port. The server can then communicate with the client over this new socket and continue to listen for client connection requests on the ServerSocket bound to port 3004. After the request is accepted, it is read by using the readLine method, which returns the request to the clientData object. In my example, the request reads “My Servlet Test Page.” You then append the string “- Success” to the

clientData object and send that string back to the client through a new port. Finish by closing the socket connections.

To run the servlet and have it communicate successfully with a host socket, you must first run the myHost class on the AS/400. You can run myHost interactively by using the RUNJVA or JAVA command, or you can submit the RUNJVA command to batch by using Submit Job (SBMJOB). As soon as myHost is running on the AS/400, you can load your servlet to whatever Web application server you’re using. I used WebSphere on the AS/400 for my example. When the servlet is loaded, bring up your browser and type the URL to your server, followed by the /servlet/ directive and name of your servlet (i.e., http://MyServer/servlet/testServlet). If all goes well, you should see a Web page similar to the one in Figure 2 with the message “My Test Servlet Page - Success.”

A Recipe for E-business Success

By understanding how to implement sockets in RPG and Java, you possess a powerful tool that allows interoperability between the two. (See “TCP/IP Socket Programming in RPG? Cool!” MC, February 1999.) Add servlets and their dynamic Web pages into the mix, and you’re well on your way to a full-fledged e-business solution that takes advantage of your legacy applications on the AS/400. For more information on the Socket classes available in Java, visit Sun’s tutorial site at www.java.sun.com/docs/books/tutorial/networking/Sockets/index.html.

Author’s Note: I would like to thank Java programmer extraordinaire Will Houck of Boise Cascade Office Products, who provided me with much-needed code examples, some Java training, and a little HTML.

References

• “Lesson: All About Sockets,” The Java Tutorial: www.java.sun.com/docs/ books/tutorial/networking/sockets/index.html
• ”Serving Up Host-based Java Apps,” Don Denoncourt, MC, August 1999
• “TCP/IP Socket Programming in RPG? Cool!” Jim D. Barnes, MC, February 1999

Related Material

OS/400 Sockets Programming V4R4 (SC41-5422-03, CD-ROM QB3ANN03)

A B

C

D

E

import javax.servlet.http.*;
import javax.servlet.*;

import java.io.*;
import java.net.*;
import java.lang.String.*;

public class testServlet extends HttpServlet {

//Handle the HTTP GET method by building a simple web page.
public void doGet (HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException, UnknownHostException {

// Set the content type first
response.setContentType("text/html");

PrintWriter htmlOut = response.getWriter();
String title = "My Servlet Test Page";
String temp = null;

Socket testSocket = null;
PrintWriter toServer = null;
BufferedReader fromServer = null;

testSocket = new Socket("YourServer", 3004);
toServer = new PrintWriter(testSocket.getOutputStream(), true);
fromServer = new BufferedReader(new InputStreamReader(
testSocket.getInputStream()));

// Write the response headings
htmlOut.println("" +

"

" + title + "

toServer.println("My test message");
temp = fromServer.readLine();

}

class myHost {
public static void main(String[] args) throws IOException,

UnknownHostException,
ClassNotFoundException {

ServerSocket hostSocket = new ServerSocket(3004);
Socket clientSocket = null;
String clientData = null;
String hostData = null;

clientSocket = hostSocket.accept();

PrintWriter toClient = new PrintWriter(
clientSocket.getOutputStream(), true);
BufferedReader fromClient = new BufferedReader(
new InputStreamReader(
clientSocket.getInputStream()));

clientData = fromClient.readLine();
hostData = clientData + " - Success!";

toClient.println(hostData);

toClient.close();
fromClient.close();
clientSocket.close();
hostSocket.close();
}

}

htmlOut.println("

" + temp + "

toServer.close();
fromServer.close();
testSocket.close();

F

G

}

Figure 1: Servlets dynamically construct HTML to communicate with Web users and can communicate with legacy applications through Java's simple Sockets API.

Figure 2: A servlet responds to remote requests from URLs and HTML form submissions.