Servlet/JSP course chapter 1: Introduction to servlets

Chaper 1 Servlets / JSP Course Introduction to servlets

Servlet / JSP course topics Chapter 0 Introduction to Java Web Development Chapter 1 Introduction to servlets Chapter 2 Introduction to JavaServer Pages Chapter 3 How to use the MVC pattern in a Java Web Application Chapter 4 How to share information in servlets and JSPs Chapter 5 Advanced JSP concepts Chapter 6 How to use JavaBeans with JSP Chapter 7 How to use the JSP Expression Language (EL) Chapter 8 How to use the JSP Standard Tag Library (JSTL) Chapter 9 How to use custom JSP tags Chapter 10 How to access databases in java web applications Chapter 11 How to use JavaMail to send email Chapter 12 How to secure java web applications Chapter 13 How to download files with Servlets Chapter 14 How to work with listeners Chapter 15 How to work with filters

Introduction to servlets Servlet Definition Servlet life cycle Exercise Exercise explanation Methods provided by ServletRequest for retrieving client-sent parameters Life-cycle Servlet Methods

What Is a Servlet? A servlet is a Java class that is used to extend the capabilities of servers that host our web applications accessed under a request-response programming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by web servers. For such applications, Java Servlet technology defines HTTP-specific servlet classes. The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. All servlets must implement the Servlet interface, which defines life-cycle methods. When implementing a generic service, you can use or extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet and doPost, for handling HTTP-specific services. This chapter focuses on writing servlets that generate responses to HTTP requests.

Servlet Life Cycle The life cycle of a servlet is controlled by the container in which the servlet has been deployed. When a request is mapped to a servlet, the container performs the following steps: If an instance of the servlet does not exist, the web container Loads the servlet class. Creates an instance of the servlet class. Initializes the servlet instance by calling the init method. Initialization is covered in Initializing a Servlet. Invokes the service method, passing request and response objects. Service methods are discussed in Writing Service Methods. If the container needs to remove the servlet, it finalizes the servlet by calling the servlet’s destroy method. Finalization is discussed in Finalizing a Servlet.

Exercise 1 Develope the first java web application that uses servlets only Download the file: jspservlet-01.zip Unzip it Import from Eclipse Run it

Exercise 1 Results You should get this result

Exercise 1 Results The web client: join_email_list.html < form action = " addToEmailList " method = "post" > < table cellspacing = "5" border = "0" > < tr > < td align = "right" > First name: </ td > < td >< input type = "text" name = "firstName" ></ td > </ tr > < tr > < td align = "right" > Last name: </ td > < td >< input type = "text" name = "lastName" ></ td > </ tr > < tr > < td align = "right" > Email address: </ td > < td >< input type = "text" name = "emailAddress" ></ td > </ tr > < tr > < td ></ td > < td >< br >< input type = "submit" value = "Submit" ></ td > </ tr > </ table > </ form > The servlet mapped In the web.xml

Exercise 1 Results The web component: AddToEmailListServlet.java . . . protected void doPost( HttpServletRequest request , HttpServletResponse response ) throws ServletException, IOException { // get parameters from the request String firstName = request .getParameter( "firstName" ); String lastName = request .getParameter( "lastName" ); String emailAddress = request .getParameter( "emailAddress" ); // get a relative file name ServletContext sc = getServletContext(); String path = sc.getRealPath( "/WEB-INF/EmailList.txt" ); // use regular Java objects to write the data to a file User user = new User(firstName, lastName, emailAddress); UserIO. add (user, path); . . . The request & Response objects

Exercise 1 Results The web component: AddToEmailListServlet.java . . . // send response to browser response.setContentType( "text/html;charset=UTF-8" ); PrintWriter out = response.getWriter(); out.println( . . . + " <table cellspacing=\"5\" cellpadding=\"5\" border=\"1\">\n" + " <tr><td align=\"right\">First name:</td>\n" + " <td>" + firstName + "</td>\n" + " </tr>\n" + " <tr><td align=\"right\">Last name:</td>\n" + " <td>" + lastName + "</td>\n" + " </tr>\n" + " <tr><td align=\"right\">Email address:</td>\n" + " <td>" + emailAddress + "</td>\n" + " </tr>\n" + " </table>\n" . . . out.close(); . . . The Response to The web client

Exercise 1 Results Result to the response

Methods provided by ServletRequest for retrieving client-sent parameters

Life-cycle Servlet Methods Methods to initialize / service / destroy methods The init(ServletConfig) can be overloaded by the init() method Usually the method service(ServletRequest, ServletResponse) is used internally by the server container

Example of Life-cycle Servlet Methods The web component: AddToEmailListServlet2.java . . . public class AddToEmailListServlet2 extends HttpServlet { // declare an instance variable for the page int globalCount ; // instance variables are not thread-safe public void init () throws ServletException { globalCount = 0; // initialize the instance variable } protected void doPost( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // update global count variable globalCount ++; // this is not thread-safe . . .

Resources To download example code for this chapter go to: http://www.jeetrainers.com

Servlet/JSP course chapter 1: Introduction to servlets

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