Bài giảng Web Technologies and e-Services - Bài 7, Phần 2: Java servlet

Java Servlet Objectives • Explain the nature of a servlet and its operation • Use the appropriate servlet methods in a web application • Code the extraction of environment entries within a servlet • Handle HTML forms within a servlet • Explain the significance of web application state • Explain the purpose and operation of HTTP cookies and their role in state management • Develop java web application with MVC model Free Servlet and JSP Engines (Servlet/JSP Containers) • Apache Tomcat • • IDE: NetBeans, Eclipse • https://netbeans.org/ • https://eclipse.org/ • Some Tutorials: • Creating Servlet in Netbeans: netbeans.php • Java Servlet Example: Compiling and Invoking Servlets • Put your servlet classes in proper location • Locations vary from server to server. E.g., • tomcat_install_dir/webapps/ROOT/WEB-INF/classes • Invoke your servlets (HTTP request) • • Custom URL-to-servlet mapping (via web.xml) Purposes of Web Applications (A single WAR file) • Organization • Related files grouped together in a single directory hierarchy. • HTML files, JSP pages, servlets, beans, images, etc. • Portability • Most servers support Web apps. • Can redeploy on new server by moving a single file. • Separation • Each Web app has its own: • ServletContext, Class loader • Sessions, URL prefix, Directory structure Structure of a Web Application • JSP and regular Web content (HTML, style sheets, images, etc.): • Main directory or a subdirectory thereof. • Servlets: • WEB-INF/classes (if servlet is unpackaged – i.e. in default package) • A subdirectory thereof that matches the package name. • JAR files: • WEB-INF/lib. • web.xml: • WEB-INF • Tag Library Descriptor files: • WEB-INF or subdirectory thereof • Files in WEB-INF not directly accessible to outside clients Example Structure Java Servlets • A servlet is a Java program that is invoked by a web server in response to a request Client Server Platform Web Server Web Application Servlet Java Servlets • Together with web pages and other components, servlets constitute part of a web application • Servlets can • create dynamic (HTML) content in response to a request • handle user input, such as from HTML forms • access databases, files, and other system resources • perform any computation required by an application Java Servlets • Servlets are hosted by a servlet container, such as Apache Tomcat* *Apache Tomcat can be both a web server and a servlet container Server Platform Web Server Servlet Container The servlet container provides a Java Virtual Machine for servlet execution The web server handles the HTTP transaction details Environment For Developing and Testing Servlets • Compile: • Need Servlet.jar. Available in Tomcat package • Setup testing environment • Install and start Tomcat web server • Place compiled servlet at appropriate location Servlet Operation Servlet Methods • Servlets have three principal methods .init() invoked once, when the servlet is loaded by the servlet container (upon the first client request) .service(HttpServletRequest req, HttpServletResponse res) invoked for each HTTP request parameters encapsulate the HTTP request and response .destroy() invoked when the servlet is unloaded (when the servlet container is shut down) Servlet Methods • The default .service() method simply invokes method- specific methods • depending upon the HTTP request method .service() .doGet() .doPost() .doHead() etc. Methods HTTP Requests Comments doGet GET, HEAD Usually overridden doPost POST Usually overridden doPut PUT Usually not overridden doOptions OPTIONS Almost never overridden doTrace TRACE Almost never overridden Methods Servlet Example 1 • This servlet will say "Hello!" (in HTML) package servlet; import javax.servlet.http.*; public class HelloServlet extends HttpServlet { public void service(HttpServletRequest req, HttpServletResponse res) throws IOException { PrintWriter htmlOut = res.getWriter(); res.setContentType("text/html"); htmlOut.println("" + "Servlet Example Output" + "Hello!" + ""); htmlOut.close(); } } Servlet Example 2 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWorld extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { PrintWriter out = response.getWriter(); out.println("Hello World"); } } Servlet Configuration • The web application configuration file, web.xml, identifies servlets and defines a mapping from requests to servlets HelloServlet servlet.HelloServlet</servlet- class> HelloServlet /hello An identifying name for the servlet (appears twice) The servlet's package and class names The pathname used to invoke the servlet (relative to the web application URL) Environment Entries • Servlets can obtain configuration information at run- time from the configuration file (web.xml) • a file name, a database password, etc. • in web.xml: password</env-entry- description> UserId Xy87!fx9* java.lang.String Environment Entries • in the init() method of the servlet: try { Context envCtx = (Context) (new InitialContext()).lookup("java:comp/env"); password = (String) envCtx.lookup("password"); } catch (NamingException e) { e.printStackTrace(); } catch (ClassNotFoundException e) { e.printStackTrace(); } Handling HTML Forms • An HTML form can be sent to a servlet for processing • The action attribute of the form must match the servlet URL mapping HelloServlet /hello Simple Form Servlet User Id: public class HelloServlet extends HttpServlet { public void doPost(HttpServletRequest req, HttpServletResponse res) throws IOException { PrintWriter out = res.getWriter(); res.setContentType("text/html"); String userId = req.getParameter("userid"); out.println("Hello</he ad>" + "Hello, " + userId + "!"); out.close(); } State Management • session: a series of transaction between user and application • session state: the short-term memory that the application needs in order to maintain the session • e.g., shopping cart, user-id • cookie: a small file stored by the client at the instruction of the server Cookies • The Set-Cookie: header in an HTTP response instructs the client to store a cookie Set-Cookie: SESSIONID=B6E98A; Path=/slms; Secure • After a cookie is created, it is returned to the server in subsequent requests as an HTTP request Cookie: header Cookie: SESSIONID=B6E98A Cookie Attributes • Name: the unique name associated with the cookie • Content: value stored in the cookie • Expiration Date: cookie lifetime • Domain: Defines the hosts to which the cookie should be returned • Path: Defines the resource requests with which the cookie should be returned • Secure: if true, cookie is returned only with HTTPS requests Cookie Example • Name: session-id • Content: 104-1898635-929144 • Expiration Date: Monday, June 29, 2009 3:33:30 PM • Domain: .ehsl.org • Path: /slms • Secure: no • This cookie will be returned with all requests matching *.ehsl.org/slms*, through the indicated expiration date Session Management • HTTP is inherently stateless, i.e., there is no memory between transactions • Applications must maintain a session memory if it is required • Cookies are used to identify sessions, by recording a unique session-id State Management Client Cookie [session-id] Server Session Memory session • At the start of a new session, the server sets a new cookie containing the session-id • With each transaction, the client sends the session-id, allowing the server to retrieve the session Session Attributes • The methods session.setAttribute(key, value) session.getAttribute(key) store and retrieve session memory • key is a string; value can be any object • For example, session.setAttribute("userid", userId); String userId = (String)session.getAttribute("userid"); Problem 30 Initial Solution • Develop a number of servlets • Each servlet plays the role of one function (a.k.a business logic) Better Solution: Using MVC • Take business logic out servlets and put them inside Model Example 1: Beer Recommendation JSP page HTML page 34 beer_v1 websrc WEB-INF web.xml result.htmlform.htmlcom BeerExpert .java example web model BeerSelect .java Application Programming Structure Structure of Folder Development WEB-INF beer_v1 classes webapps web.xml result.htmlform.html com BeerExpert .class example web model BeerSelect .class tomcat form.html <form method="POST“ action="SelectBeer.do"> Select beer characteristics Color: light amber brown dark web.xml ServletBeer com.example.web.BeerSelect ServletBeer /SelectBeer.do Servlet BeerSelect – Version 1 public class BeerSelect extends HttpServlet { @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("Beer Selection Advice"); String c = request.getParameter("color"); out.println("Got beer color "+c); } } Application Test Model BeerExpert public class BeerExpert { public List getBrands(String color){ List brands = new ArrayList(); if(color.equals("amber")){ brands.add("Jack Amber"); brands.add("Red Moose"); } else{ brands.add("Jail Pale Ale"); brands.add("Gout Stout"); } return brands; } } 41 Servlet BeerSelect – Version 2 import package com.example.web; protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String c = request.getParameter("color"); BeerExpert be = new BeerExpert(); List result = be.getBrands(c); response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("Beer Selection Advice"); Iterator it = result.iterator(); while(it.hasNext()){ out.print("try: "+it.next()); } } Application Test 43 Current Architecture of the Application Desired Application Architecture Result.jsp Beer Recommendation <% List styles=(List) request.getAttribute("styles"); Iterator it = styles.iterator(); while(it.hasNext()){ out.print("try: "+it.next()); } %> Servlet BeerSelect – Version 3 import package com.example.web; protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String c = request.getParameter("color"); BeerExpert be = new BeerExpert(); List result = be.getBrands(c); request.setAttribute("styles", result); RequestDispatcher view = request.getRequestDispatcher("result.jsp"); view.forward(request, response); } Application Test 48 Review • Java servlets • Servlet methods and operation • HTML forms and servlets • HTTP cookies • Web application state management • Beer Recommendations with MVC model