Web Applications with Java/JSP

Now that you have a sense of how the Web application is packaged and deployed, let's turn our attention to the real action in the Web application: the code.

Java is both a programming language and a runtime environment, much like Perl and PHP. In those cases, the compiler generally is invoked when the script is executed, while Java is always compiled beforehand. The Java programming language itself is object-oriented, procedural, block-structured and entirely familiar to anyone who has written in a C-like language. It has a number of explicitly defined primitive data types as well as reference types. All the Java code you write lives within the definition of a class, including servlet code.

Let's take a look at the source code for the GetTasksServlet (Listing 3), which implements the “get-tasks” servlet, which is mapped to the URL /tasks.

package lj.timesheet;

import java.io.IOException;

import java.util.ArrayList;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class GetTasksServlet
    extends BaseServlet
{
    public void doGet(HttpServletRequest request,
                      HttpServletResponse response)
        throws ServletException, IOException
    {
        String username = request.getUserPrincipal().getName();

        try
        {
            List<Client> clients = getClients();

            // Convert client list to lookup table
            Map<Integer,Client> clientMap
                = new HashMap<Integer,Client>(clients.size());

            for(Client client : clients)
                clientMap.put(client.getId(), client);

            request.setAttribute("clients", clients);
            request.setAttribute("clientMap", clientMap);
            request.setAttribute("tasks", getTasks(username));

            getServletContext().getRequestDispatcher("/tasks.jsp")
                .forward(request, response);
        }
        catch (SQLException sqle)
        {
            throw new ServletException("Database error", sqle);
        }
    }

    ...
}

The first line of the file declares the “package” in which the class is defined. Packages help keep code organized and have implications on variable, method, and class scope and visibility. The next set of lines are “imports” that indicate to the compiler which classes will be referenced by this class. Those classes beginning with java. are standard Java classes, while those beginning with javax.servlet are those provided by the Java Servlet Specification. Then, we define a class called GetTasksServlet that extends an existing class called HttpServlet, the basis for all HTTP-oriented servlets. The HttpServlet class defines a number of doXXX methods, where XXX is one of the HTTP methods, such as GET (doGet), POST (doPost), PUT (doPut) and so on. I have overridden the doGet method in order to respond to HTTP GET requests from clients.

The doGet method accepts two parameters: the request and the response, which provide hooks into the resources provided by the Servlet Container and to the information provided by the client for a particular HTTP request. I use two utility methods (defined later in the class) to obtain a list of clients and a list of tasks, and store them in the request object's “attributes”, a location where data can be placed in order to pass them between stages of request processing. You'll see how to access this information next when I cover JSP files for generating content. Finally, I invoke the “request dispatcher's” forward method, which tells the container to forward the request to another resource: tasks.jsp.