Internet Technologies 6 - Servlets I F. Ricci 2010/2011 Content - PowerPoint PPT Presentation

Internet Technologies 6 - Servlets I F. Ricci 2010/2011

Content  Basic Servlets  Tomcat  Servlets lifecycle  Servlets and forms  Reading parameters  Filtering text from HTML-specific characters  Reading headers  Sending compressed content  Differentiating among browsers  Referer Most of the slides were made available by www. coreservlets.com

Servlet Roles  Read the explicit data sent by the client  Read the implicit HTTP request data sent by the browser  Generate the results  Send the explicit data (i.e., the document) to the client  Send the implicit HTTP response data

HelloWorld import java.io.*; import javax.servlet.*; import javax.servlet.http.*; /** Very simplistic servlet that generates plain text. * <P> * Taken from More Servlets and JavaServer Pages * from Prentice Hall and Sun Microsystems Press, * http://www.moreservlets.com/. * &copy; 2002 Marty Hall; may be freely used or adapted. */ public class HelloWorld extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { PrintWriter out = response.getWriter(); out.println("Hello World"); } code }

Servlet Architecture HTTP Response HTTP Servlet Container Request Java Virtual Machine (JVM) Servlet 1 Servlet 2 Client Web Web Server Servlet n

Installing and Running Tomcat  Tomcat is distributed as a ZIP archive http:// tomcat.apache.org  unzip the download file, for instance into a root- level directory: C:\apache-tomcat-6.0.16  To run Tomcat you'll need to tell it where to find your J2SE SDK installation  Set the JAVA_HOME environment variable to C:\Program Files\Java\jdk1.6.0_04  To run Tomcat :  open a command window  change directory to Tomcat's bin directory  Type startup

Tomcat Directory Structure Tomcat binaries: startup, shutdown All jar libraries: e.g. servlet-api.jar Configuration files: e.g. when you build your application in Netbeans a new file is added to indicate where is deployed (e.g., C:\apache- tomcat-6.0.16\conf \Catalina\localhost \coresjsp.xml) Directories of the web applications deployed here, possibly generated by .war files deployed here (with a web.xml file)

Catalina Base  Go to netbeans: tools>servers to look at these details

web.xml <?xml version="1.0" encoding="ISO-8859-1"?> <!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd"> <web-app> <servlet> <servlet-name>bob</servlet-name> <servlet-class>HelloWorld</servlet-class> </servlet> <servlet-mapping> <servlet-name>bob</servlet-name> <url-pattern>/HelloWorld</url-pattern> </servlet-mapping> </web-app>

Compiling and deploying  Create a directory in \webapps called hello  Create a directory in \webapps\hello called WEB-INF and then a subdirectory classes where to put the sources and the compiled files  Set the classpath  C:\>set CLASSPATH=\apache-tomcat-6.0.16\common \lib\servlet-api.jar  Compile  C:\>javac HelloWorld.java  Deploy the web.xml file in C:\apache- tomcat-6.0.16\webapps\hello\WEB-INF  Or use an IDE (Netbeans) – separating sources ( web directory) and deployment ( build directory).

Starting Tomcat  /bin/startup.bat or startup.sh  Point Browers to http://localhost:8080 should see default page  All the Docs are there on the default page!  Check out the examples pages, good tutorials

Basic Servlet Structure  Here's the outline of a basic servlet that handles GET and POST requests in the same way: import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class SomeServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { PrintWriter out = response.getWriter(); // Use "out" to send content to browser } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }

HelloWorld HTML import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloServlet2 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String docType = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " + "Transitional//EN\">\n"; out.println(docType + "<HTML>\n" + "<HEAD><TITLE>Hello (2)</TITLE></HEAD>\n" + "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1>Hello</H1>\n" + "</BODY></HTML>"); } code }

Some Simple HTML-Building Utilities public class ServletUtilities { public static final String DOCTYPE = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " + "Transitional//EN\">"; public static String headWithTitle(String title) { return(DOCTYPE + "\n" + "<HTML>\n" + "<HEAD><TITLE>" + title + "</TITLE></HEAD>\n"); } ... code }  Don’t go overboard  Complete HTML generation packages usually work poorly  The JSP framework is a better solution

HelloServlet3: Packages and Utilities package coreservlets; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloServlet3 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String title = "Hello (3)"; out.println(ServletUtilities.headWithTitle(title)+ "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1>" + title + "</H1>\n" + "</BODY></HTML>"); code } }

The Servlet Life Cycle ( Servlet Interface)  init  Executed once when the servlet is first loaded Not called for each request  service  Called in a new thread by server for each request  Dispatches to doGet, doPost, etc. Do not override this method!  doGet, doPost, doXxx  Handles GET, POST, etc. requests  Override these to provide desired behavior  destroy  Called when server deletes servlet instance Not called after each request. javadoc

Example of usage of init  If a servlet get a request for a url with the http header  if-Modified-Since: Mon, 12 Nov 2010 18:00:00 GMT  It can check if something has really changed the output after that date – if not the servlet sends back a reply:  HTTP/1.1 304 Not Modified  And the browser shows the cashed url  The servlet must only implement the following method to know when it has been modified the last time:  public long getLastModified(HttpServletRequest request)  The app server will call it when a client requests the servlet  And will send back a result only if the page was modified (according to the value returned by the method) after Mon, 12 Nov 2010 18:00:00 GMT

Code  The init method set the time the page was modified public void init() throws ServletException { // Round to nearest second (i.e, 1000 milliseconds) modTime = System.currentTimeMillis()/1000*1000; for(int i=0; i<numbers.length; i++) { numbers[i] = randomNum(); } }  An then overwrite the method that tells when the page was modified public long getLastModified(HttpServletRequest request) { return(modTime); } LotteryNumbers code

Calls to the servlet The page was not modified The page was modified after after 9:00:00 GMT 8:00:00 GMT

Calls to the servlet  You can do the same experiments with a telnet connection (e.g., using putty)

HTML Form <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD><TITLE>A Sample Form Using GET</TITLE></HEAD> <BODY> <H2 ALIGN="CENTER">A Sample Form Using GET</H2> <FORM ACTION="http://localhost:8080/coresjp/ServletForm"> <CENTER> First name: <INPUT TYPE="TEXT" NAME="FirstName" VALUE=""><BR/> Last name: <INPUT TYPE="TEXT" NAME="LastName" VALUE=""><P> <INPUT TYPE="SUBMIT"> </CENTER> </FORM> </BODY> </HTML> form

Reading form data in servlets  request.getParameter(“FirstName")  Returns URL-decoded value of first occurrence of FirstName parameter in query string  Works identically for GET and POST requests  Returns null if no such parameter is in query data  request.getParameterValues(“FirstName")  Returns an array of the URL-decoded values of all occurrences of FirstName parameter in query string  Returns a one-element array if param is not repeated  Returns null if no such parameter is in the query  request.getParameterNames() or request.getParameterMap()  Returns Enumeration or Map of request parameters  Usually reserved for debugging.