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:

Parameters, which are typically used to convey information between clients and servlets
Object-valued attributes, which are typically used to pass information between the servlet container and a servlet or between collaborating servlets
Information about the protocol used to communicate the request and about the client and server involved in the request
Information relevant to localization

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) {
    Book book = bookDB.getBook(bookId);

You can also retrieve an input stream from the request and manually parse the data. To read character data, use the java.io.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:

Context path: A concatenation of a forward slash (/) with the context root of the servlet’s web application.
Servlet path: The path section that corresponds to the component alias that activated this request. This path starts with a forward slash (/).
Path info: The part of the request path that is not part of the context path or the servlet path.

If the context path is /catalog and for the aliases listed in Table 4-4, Table 4-5 gives some examples of how the URL will be parsed.

Table 4-4 Aliases

Pattern	Servlet
`/lawn/*`	`LawnServlet`
`/*.jsp`	`JSPServlet`

Table 4-5 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:

A query string can explicitly appear in a web page. For example, an HTML page generated by CatalogServlet could contain the link <a href="/bookstore1/catalog?Add=101">Add To Cart</a>. CatalogServlet extracts the parameter named Add as follows:
```
String bookId = request.getParameter("Add");
```
A query string is appended to a URL when a form with a GET HTTP method is submitted. In the Duke’s Bookstore application, CashierServlet generates a form, then a user name input to the form is appended to the URL that maps to ReceiptServlet, and finally ReceiptServlet extracts the user name using the getParameter method.

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:

Retrieve an output stream to use to send data to the client. To send character data, use the java.io.PrintWriter returned by the response’s getWriter method. To send binary data in a MIME body response, use the ServletOutputStream returned by getOutputStream. To mix binary and text data (as in a multipart response), use a ServletOutputStream and manage the character sections manually.
Indicate the content type (for example, text/html) being returned by the response with the setContentType(String) method. This method must be called before the response is committed. A registry of content type names is kept by the Internet Assigned Numbers Authority (IANA) at http://www.iana.org/assignments/media-types/.
Indicate whether to buffer output with the setBufferSize(int) method. By default, any content written to the output stream is immediately sent to the client. Buffering allows content to be written before anything is actually sent back to the client, thus providing the servlet with more time to set appropriate status codes and headers or forward to another web resource. The method must be called before any content is written or before the response is committed.
Set localization information such as locale and character encoding. See Chapter 15, Internationalizing and Localizing Web Applications for details.

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

Status codes, which are used to indicate the reason a request is not satisfied or that a request has been redirected.
Cookies, which are used to store application-specific information at the client. Sometimes cookies are used to maintain an identifier for tracking a user’s session (see Session Tracking).

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 {
                Book bd =
                    bookDB.getBook(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 4-2.

Figure 4-2 Book Details