AhmadLang / Java, How To Program, 2004

note that the get request could have been typed directly into the browser's Address or Location field as follows:

http://localhost:8080/jhtp6/welcome2?firstname=Jon

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Error-Prevention Tip 26.4

If an error occurs during the servlet invocation, the log files in the logs directory of the Tomcat installation can help you determine the error and debug the problem.

Software Engineering Observation 26.4

A get request is limited to standard characters, which means that you cannot submit any special characters via a get request. The length of the URL in a get request is limited. For example, the maximum URL length in Internet Explorer is 2,083 characters. Some Web servers might restrict this even more.

Good Programming Practice 26.1

A get request should not be used for sending sensitive data (e.g., a password) because the form data is placed in a query string that is appended to the request URL as plain text and can be intercepted.

[Page 1258 (continued)]

26.6. Handling HTTP post Requests

An HTTP post request is frequently used to send data from an HTML form to a server-side form handler that processes the data. For example, when you respond to a Web-based survey, a post request normally supplies the information you type in the form to the Web server.

Browsers often cache (save on disk) Web pages so they can quickly reload them. If there are no changes between the last version stored in the cache and the current version on the Web, this helps speed up your browsing experience. The browser first asks the server whether the document has changed or expired since the date the file was cached. If not, the browser loads the document from the cache. Thus, the browser minimizes the amount of data that must be downloaded for you to view a Web page. Browsers typically do not cache the server's response to a post request, because the next post might not return the same result. For example, in a survey, many users could visit the same Web page and respond to a question. The survey results could then be displayed for the user. Each new response changes the overall results of the survey.

When you use a Web-based search engine, the browser normally supplies the information you specify in an HTML form to the search engine with a get request. The search engine performs the search, then returns the results to you as a Web page. Such pages are often cached by the browser in case you perform the same search again. As with post requests, get requests can supply parameters as part of the request to the Web server.

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The WelcomeServlet3 servlet in Fig. 26.15 is identical to the servlet in Fig. 26.12, except that it defines a doPost method (lines 1348) rather than a doGet method to respond to post requests. The default functionality of doPost is to indicate a "Method not supported" error. We override this method to provide custom post request processing. Method doPost accepts the same two arguments as doGetan object that implements interface HttpServletRequest to represent the client's request and an object that implements interface HttpServletResponse to represent the servlet's response. As with doGet, method doPost throws a ServletException if it is unable to handle a client's request and throws an IOException if a problem occurs during stream processing.

Figure 26.15. WelcomeServlet3 responds to a post request containing data.

(This item is displayed on pages 1259 - 1260 in the print version)

1// Fig. 26.15: WelcomeServlet3.java

2// Processing post requests containing data.

3import javax.servlet.ServletException;

4import javax.servlet.http.HttpServlet;

5import javax.servlet.http.HttpServletRequest;

6import javax.servlet.http.HttpServletResponse;

7import java.io.IOException;

8import java.io.PrintWriter;

9

10public class WelcomeServlet3 extends HttpServlet

11{

12// process "post" request from client

13protected void doPost( HttpServletRequest request,

14HttpServletResponse response )

15throws ServletException, IOException

16{

17String firstName = request.getParameter( "firstname" );

19response.setContentType( "text/html" );

20PrintWriter out = response.getWriter();

22// send XHTML page to client

12<body>

13<form action = "/jhtp6/welcome3" method = "post" >

14<p><label>

15Type your first name and press the Submit button

16<br /><input type = "text" name = "firstname" />

17<input type = "submit" value = "Submit" />

18</label></p>

19</form>

20</body>

21</html>

[View full size image]

Figure 26.17. Deployment descriptor information for servlet

WelcomeServlet3.

[Page 1261 (continued)]

26.7. Redirecting Requests to Other Resources

Sometimes it is useful to redirect a request to a different resource. For example, a servlet could determine the type of the client browser and redirect the request to a Web page that was designed specifically for that browser. This technique is also used to redirect browsers to an error page when handling a request fails. The RedirectServlet in Fig. 26.18 receives a page parameter as part of a get request, then uses that parameter to redirect the request to a different resource.

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Figure 26.18. Redirecting requests to other resources.

(This item is displayed on pages 1262 - 1263 in the print version)

1 // Fig. 26.18: RedirectServlet.java

2 // Redirecting a user to a different Web page.

3import javax.servlet.ServletException;

4import javax.servlet.http.HttpServlet;

5import javax.servlet.http.HttpServletRequest;

6import javax.servlet.http.HttpServletResponse;

7import java.io.IOException;

8import java.io.PrintWriter;

9

10public class RedirectServlet extends HttpServlet

11{

12// process "get" request from client

13protected void doGet( HttpServletRequest request,

14HttpServletResponse response )

15throws ServletException, IOException

16{

17String location = request.getParameter( "page" );

19if ( location != null )

20{

21if ( location.equals( "deitel" ) )

27// code that executes only if this servlet

28// does not redirect the user to another page

29response.setContentType( "text/html" );

30PrintWriter out = response.getWriter();

31

32// start XHTML document

33out.println( "<?xml version = \"1.0\"?>" );

35out.printf( "%s%s%s" , "<!DOCTYPE html PUBLIC" ,

36" \"-//W3C//DTD XHTML 1.0 Strict//EN\"",

37" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd\">\n" );

39out.println(

40"<html xmlns = \"http://www.w3.org/1999/xhtml\">" );

42// head section of document

43out.println( "<head>" );

44out.println( "<title>Invalid page</title>" );

45out.println( "</head>" );

47// body section of document

48out.println( "<body>" );

49out.println( "<h1>Invalid page requested</h1>" );

50out.println( "<p><a href = " +

51"\"servlets/RedirectServlet.html\">" );

52out.println( "Click here to choose again</a></p>" );

53out.println( "</body>" );

54

55// end XHTML document

56out.println( "</html>" );

57 out.close(); // close stream to complete the page

58} // end method doGet

59} // end class RedirectServlet

Line 17 obtains the page parameter from the request. If the value returned is not null, the nested

if... else if statement at lines 2124 determines whether the value is either "deitel" or "welcome1". If the value is "deitel," the response object's sendRedirect method (line 22) redirects the request to www.deitel.com. If the value is "welcome1," line 24 redirects the request to the servlet of Fig. 26.7.

Note that line 24 does not explicitly specify the jhtp6 context root for our Web application. When a servlet uses a relative path to reference another static or dynamic resource, the servlet assumes the same base URL and context root as the one that invoked the servletunless a complete URL is specified for the resource. So line 24 actually is requesting the resource located at

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http://localhost:8080/jhtp6/welcome1

Similarly, line 51 is actually requesting the resource located at

http://localhost:8080/jhtp6/servlets/RedirectServlet.html

Software Engineering Observation 26.5

Using relative paths to reference resources in the same context root makes your Web application more flexible. For example, you can change the context root without making changes to the static and dynamic resources in the application.

Once method sendRedirect executes, processing of the original request by the RedirectServlet terminates. If method sendRedirect is not called, the remainder of method doGet outputs a Web page indicating that an invalid request was made. The page allows the user to try again by returning to the XHTML document of Fig. 26.19. Note that one of the redirects is sent to a static XHTML Web page and the other is sent to a servlet.

Figure 26.19. RedirectServlet.html document to demonstrate redirecting requests to other resources.

(This item is displayed on page 1264 in the print version)

7<html xmlns = "http://www.w3.org/1999/xhtml">

8<head>

9<title>Redirecting a Request to Another Site</title>

Figure 26.20. Deployment descriptor information for servlet

RedirectServlet.

(This item is displayed on page 1265 in the print version)

[Page 1265]

When redirecting requests, the request parameters from the original request are passed as parameters to the new request. Additional request parameters can also be passed. For example, the URL passed to sendRedirect could contain name-value pairs. New parameters are added to the existing parameters. A new parameter with the same name as an existing parameter takes precedence over the original value. However, all the values are still passed. In this case, the complete set of values for a given parameter name can be obtained by calling method getParameterValues from interface HttpServletRequest. This method accepts the parameter name as an argument and returns an array of strings containing the parameter values in most-recent-to-least-recent-order.