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Introduction to Servlets
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THE WEB PROGRAMMING ENVIRONMENT
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HttpServletResponse response) throws ServletException, IOException;
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The service(Request, Response) method in HttpServlet casts the request and response objects into their HTTP-specific counterparts, and then calls service(HttpServletRequest, HttpServletResponse), which examines the request and calls the appropriate doGet(), doPost(), or other method A typical HTTP servlet, then, includes an override to one or more of these subsidiary methods, rather than an override to service()
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The servlet specification allows a servlet engine to unload a servlet at any time This may be done to conserve system resources or in preparation for servlet engine shutdown The servlet engine notifies each loaded servlet this is about to happen by calling its destroy() method By overriding destroy(), you can release any resources allocated during init() Calling destroy() yourself won t actually unload the servlet Only the servlet engine can do this
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Example: Kilometers per Liter to Miles per Gallon Servlet
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Let s look at a simple servlet K2MServlet, shown in the following, is a servlet that creates a fuel efficiency conversion table that expresses kilometers per liter in terms of miles per gallon
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package jspcrservlets; import import import import import javaio*; javatext*; javautil*; javaxservlet*; javaxservlethttp*;
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/** * Prints a conversion table of miles per gallon * to kilometers per liter */
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JSP: The Complete Reference
public class K2MServlet extends HttpServlet { private static final DecimalFormat FMT = new DecimalFormat("#000"); private static final String PAGE_TOP = "" + "<HTML>" + "<HEAD>" + "<TITLE>Fuel Efficiency Conversion Chart</TITLE>" + "</HEAD>" + "<BODY>" + "<H3>Fuel Efficiency Conversion Chart</H3>" + "<TABLE BORDER=1 CELLPADDING=3 CELLSPACING=0>" + "<TR>" + "<TH>Kilometers per Liter</TH>" + "<TH>Miles per Gallon</TH>" + "</TR>" ; private static final String PAGE_BOTTOM = "" + "</TABLE>" + "</BODY>" + "</HTML>" ; public void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { responsesetContentType("text/html"); PrintWriter out = responsegetWriter(); outprintln(PAGE_TOP); for (double kpl = 5; kpl <= 20; kpl += 10) { double mpg = kpl * 2352146; outprintln("<TR>"); outprintln("<TD>" + FMTformat(kpl) + "</TD>"); outprintln("<TD>" + FMTformat(mpg) + "</TD>"); outprintln("</TR>"); } outprintln(PAGE_BOTTOM); } }
4:
Introduction to Servlets
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To start, note the two import statements at the beginning of the program:
import javaxservlet*; import javaxservlethttp*;
These statements identify to the compiler that we ll use classes from the general and HTTP-specific servlet packages import statements are not strictly required, but they make referring to classes possible without specifying their fully qualified names Next, the class declaration:
public class K2MServlet extends HttpServlet
A servlet is required at a minimum to implement the javaxservletServlet interface To simplify servlet writing, the servlet API provides a basic implementation of this interface called GenericServlet It also supplies an HTTP-specific subclass HttpServlet, which is the base class most commonly used for servlets
public void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
Our servlet has no special requirement for startup or termination actions, so it only overrides one method doGet() This will be invoked from the HttpServlet superclass service() method if the request method is GET
responsesetContentType("text/html");
Before writing any results back to the client, we need to specify any HTTP headers we want to send In our case, the only one is Content-Type, which we set to text/html
PrintWriter out = responsegetWriter();
Creating an HTML page consists of writing HTML statements to an output stream associated with the HTTP request This output stream can be obtained from the response object using either its getOutputStream() or getWriter() methods, depending on whether a binary stream or character output is to be written, respectively Important to note is a servlet must chose one or the other of these methods; it cannot call both Because we re writing ordinary HTML, we ll use getWriter() to obtain a character writer
JSP: The Complete Reference
All that remains is to print the text of our HTML table For convenience, we ve coded the page header and footer in the static string variables PAGE_TOP and PAGE_BOTTOM We print the table itself in a loop over the desired range of kilometers per liter
outprintln(PAGE_TOP); for (double kpl = 5; kpl <= 20; kpl += 10) { double mpg = kpl * 2352146; outprintln("<TR>"); outprintln("<TD>" + FMTformat(kpl) + "</TD>"); outprintln("<TD>" + FMTformat(mpg) + "</TD>"); outprintln("</TR>"); } outprintln(PAGE_BOTTOM);
To run the servlet, we first need to compile it For this to be successful, the classes in the servlet API must be in the classpath These classes are typically found in a JAR file distributed with the servlet engine The official JAR file can also be found at the Apache Jakarta Web site http://jakartaapacheorg Next, depending on the servlet engine, it might be necessary to describe the servlet in the Web application deployment descriptor /WEB-INF/webxml For a simple servlet, this might consist only of a <servlet> tag with its child <servlet-name> and <servlet-class> elements In this case, the entry looks like this:
< xml version="10" > <web-app> <servlet> <servlet-name>K2M</servlet-name> <servlet-class>jspcrservletsK2MServlet</servlet-class> </servlet> </web-app>
<servlet> entries in webxml must be coded in a specific position with respect to other elements See 18 for details or examine the web-app_22DTD In most cases, modifying the webxml file requires the servlet engine be restarted before any changes take effect Finally, the servlet can be invoked using a URL of this form:
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