January 18, 2002

2077433Can I use a Java application (client) instead of a JSP (JavaServer Page) to invoke a servlet on an application server?

A: Yes, you can invoke a servlet from a Java client by simply using the java.net.URL and java.net.URLConnection classes.

Here's a simple servlet that takes one parameter, username, and prints out Hello username!:

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HelloJavaWorld extends HttpServlet {
    private final static String _USERNAME = "username";
    
    protected void doPost( HttpServletRequest request, HttpServletResponse response )
        throws ServletException, IOException {
            
        PrintWriter out = response.getWriter();
        String username = request.getParameter( _USERNAME );
        
        response.setContentType("text/html");
        out.println("");
        out.println("");
            out.println("HelloWorld");
        out.println("");
        out.println("");
        out.println("");
        out.println("

Hello " + username + "!

");
        out.println("");
        out.println("");
    } 
}

ContactServlet contains a simple main method that creates a connection to the servlet, POSTs a request, and prints out the response:

import java.io.*;
import java.net.*;
public class ContactServlet {
    public static void main( String [] args ) {
        
        try {
            
            URL url = new URL("http://192.168.1.101:8080/javaworld/servlet/HelloJavaWorld");
            URLConnection conn = url.openConnection();
            conn.setDoOutput(true);
            
            BufferedWriter out = 
                new BufferedWriter( new OutputStreamWriter( conn.getOutputStream() ) );
            out.write("username=javaworld\r\n");
            out.flush();
            out.close();
            BufferedReader in = 
                new BufferedReader( new InputStreamReader( conn.getInputStream() ) );
            
            String response;
            while ( (response = in.readLine()) != null ) {
                System.out.println( response );
            }
            in.close();
        }
        catch ( MalformedURLException ex ) {
            // a real program would need to handle this exception
        }
        catch ( IOException ex ) {
            // a real program would need to handle this exception
        }
    }
}

Use ContactServlet as a guide whenever you need to communicate with servlets that accept POSTs. If your servlet has more than one parameter, simply separate each key/value pair with an ampersand (&). Also be sure to pass each value of the key/value pair through URLEncoder.encode. I didn't do so here because the value would have remained unchanged. If your values contain special characters, you must encode them, otherwise the POST will fail!

In "Abstract Classes and Interface Practicum," I presented a reusable framework for key/value pair communication. Instead of reiterating those points here, below I simply present a message and a main method that fits into the framework and communicates with the HelloJavaWorld servlet:

public class HelloJavaWorldMessage extends AbstractMessage {
    private String _username;
    private final static String _KEY = "username";
    
    public HelloJavaWorldMessage( String username ) {
        setUsername( username );
    }
    
    public void setUsername( String username ) {
        _username = username;
    }
    
    protected String[][] values() {
        String [][] values = {
            { _KEY, _username }
        };
        return values;
    }
    
}
public class ContactServlet {
    public static void main( String [] args ) {
        
        Message message = new HelloJavaWorldMessage( "JavaWorld" );
        MessageBus bus  = new HttpMessageBus( "http://192.168.1.101:8080/javaworld/servlet/HelloJavaWorld" );
        
        try
        {
            String response = message.send( bus );
            System.out.println( response );
        }
        catch( BusException ex ) {
            // a real program would need to handle the exception
        }
    }
}

I encourage you to review the framework. It can save time when you're communicating with servers that communicate through key/value pairs.

This story, "Clients can invoke servlets too" was originally published by JavaWorld.