Writing Service Methods

The service provided by a servlet is implemented in the service method of a GenericServlet, in the doMethod methods (where Method can take the value Get, Delete, Options, Post, Put, or Trace) of an HttpServlet object, or in any other protocol-specific methods defined by a class that implements the Servlet interface. In the rest of this chapter, the term service method is used for any method in a servlet class that provides a service to a client.

The general pattern for a service method is to extract information from the request, access external resources, and then populate the response based on that information.

For HTTP servlets, the correct procedure for populating the response is to first retrieve an output stream from the response, then fill in the response headers, and finally write any body content to the output stream. Response headers must always be set before the response has been committed. Any attempt to set or add headers after the response has been committed will be ignored by the web container. The next two sections describe how to get information from requests and generate responses.

Getting Information from Requests

A request contains data passed between a client and the servlet. All requests implement the ServletRequest interface. This interface defines methods for accessing the following information:

For example, in CatalogServlet the identifier of the book that a customer wishes to purchase is included as a parameter to the request. The following code fragment illustrates how to use the getParameter method to extract the identifier: 

String bookId = request.getParameter("Add");
if (bookId != null) {
  BookDetails book = bookDB.getBookDetails(bookId);

You can also retrieve an input stream from the request and manually parse the data. To read character data, use the BufferedReader object returned by the request's getReader method. To read binary data, use the ServletInputStream returned by getInputStream.

HTTP servlets are passed an HTTP request object, HttpServletRequest, which contains the request URL, HTTP headers, query string, and so on.

An HTTP request URL contains the following parts: 

http://[host]:[port][request path]?[query string]

The request path is further composed of the following elements:

If the context path is /catalog and for the aliases listed in Table 11-5, Table 11-6 gives some examples of how the URL will be parsed.

Table 11-5 Aliases 
Pattern
Servlet
/lawn/*
LawnServlet
/*.jsp
JSPServlet

Table 11-6 Request Path Elements 
Request Path
Servlet Path
Path Info
/catalog/lawn/index.html
/lawn
/index.html
/catalog/help/feedback.jsp
/help/feedback.jsp
null

Query strings are composed of a set of parameters and values. Individual parameters are retrieved from a request by using the getParameter method. There are two ways to generate query strings:

Constructing Responses

A response contains data passed between a server and the client. All responses implement the ServletResponse interface. This interface defines methods that allow you to:

HTTP response objects, HttpServletResponse, have fields representing HTTP headers such as the following:

In Duke's Bookstore, BookDetailsServlet generates an HTML page that displays information about a book that the servlet retrieves from a database. The servlet first sets response headers: the content type of the response and the buffer size. The servlet buffers the page content because the database access can generate an exception that would cause forwarding to an error page. By buffering the response, the servlet prevents the client from seeing a concatenation of part of a Duke's Bookstore page with the error page should an error occur. The doGet method then retrieves a PrintWriter from the response.

To fill in the response, the servlet first dispatches the request to BannerServlet, which generates a common banner for all the servlets in the application. This process is discussed in Including Other Resources in the Response. Then the servlet retrieves the book identifier from a request parameter and uses the identifier to retrieve information about the book from the bookstore database. Finally, the servlet generates HTML markup that describes the book information and then commits the response to the client by calling the close method on the PrintWriter. 

public class BookDetailsServlet extends HttpServlet { 
   public void doGet (HttpServletRequest request,
      HttpServletResponse response)
      throws ServletException, IOException {
    // set headers before accessing the Writer
    response.setContentType("text/html");
    response.setBufferSize(8192);
    PrintWriter out = response.getWriter();

    // then write the response
    out.println("<html>" +
      "<head><title>+
      messages.getString("TitleBookDescription")
      +</title></head>");

    // Get the dispatcher; it gets the banner to the user
    RequestDispatcher dispatcher =
      getServletContext().
      getRequestDispatcher("/banner");
    if (dispatcher != null)
      dispatcher.include(request, response);

    // Get the identifier of the book to display
    String bookId = request.getParameter("bookId");
    if (bookId != null) {
      // and the information about the book
      try {
        BookDetails bd =
          bookDB.getBookDetails(bookId);
        ...
        // Print the information obtained
        out.println("<h2>" + bd.getTitle() + "</h2>" +
        ...
      } catch (BookNotFoundException ex) {
        response.resetBuffer();
        throw new ServletException(ex);
      }
    }
    out.println("</body></html>");
    out.close();
  }
}

BookDetailsServlet generates a page that looks like Figure 11-2.

Bppk Details Screen

Figure 11-2 Book Details