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Writing Service Methods

The service provided by a servlet is implemented in the service method of a GenericServlet, the doMethod methods (where Method can take the value Get, Delete, Options, Post, Put, Trace) of an HttpServlet, or any other protocol-specific methods defined by a class that implements the Servlet interface. In the rest of this chapter, the term "service method" will be 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 fill in the response headers, then retrieve an output stream from the response, and finally write any body content to the output stream. Response headers must always be set before a PrintWriter or ServletOutputStream is retrieved because the HTTP protocol expects to receive all headers before body content. 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:

Table 14 gives some examples of how the URL will be broken down if the context path is /catalog, and the aliases are as listed in Table 13:

Table 13 Aliases 
Pattern
 		Servlet
/lawn/*
 		LawnServlet
/*.jsp
 		JSPServlet

Table 14 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 with 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, also have fields representing HTTP headers such as

In Duke's Bookstore, BookDetailsServlet generates an HTML page that displays information about a book which the servlet retrieves from a database. The servlet first sets response headers: the context 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 client will not see 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.

For filling 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 the Content of Another Resource 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 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>Book Description</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 out 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:

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