Java Server Pages

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Java Server Pages

  1. 1. Java Server Pages Eleonora Ciceri ciceri@elet.polimi.it
  2. 2. Servlets: what we know so far ¤  Interaction with user ¤  A servlet accepts HTTP requests ¤  HTML pages are returned as HTTP responses ¤  Page production ¤  Dynamic parts of the page: ¤  Depend on dynamic states (e.g., request parameters) ¤  Are filled with Java code which, when compiled, produced HTML code ¤  Static parts of the page are printed on the output as they are, without any modification
  3. 3. Servlets: too many out.println… out.println(“<span style=“width:100%;”>”);! out.println(“Select an action:”);! out.println(“<ul>”);! out.println(“<li>Action 1</li>”);! out.println(“<li>Action 2</li>”);! out.println(“<li>Action 3</li>”);! out.println(“</ul>”);! out.println(“</span>”);! Difficult to read! Mixing presentation with content in Java code
  4. 4. Servlets: are they a good idea? ¤  Complex code, not really readable ¤  HTML is mixed up with the Java Servlet code ¤  If HTML is printed, a lot of out.println() are used ¤  Presentation and logic are merged in a unique resource ¤  What if I want to change some of the aspects of the presentation style? Do I have to read and modify also the code used for producing dynamic content? ¤  Errors in the HTML code are difficult to be spotted
  5. 5. Servlet modification Web designer •  Required to understand Java Programmer •  Required to write part of the page layout Problem: “mixing competences” How do we define which are the tasks that need to be developed by each person in the team? •  Train web designers for Java? •  Compromises about page layout?
  6. 6. Java Server Pages (1) ¤  JSP integrates Java instructions in the HTML code. These instructions are used in order to compute parts of the page dynamically ¤  Difference with Java Servlet: a servlet integrates HTML in the Java code, JSP integrates Java code in the HTML ¤  This is an attempt to separate content from presentation ¤  Still some problems: one could integrate huge pieces of Java code… ¤  Advantages: learning is easier, more readable ¤  Disadvantage: still “primitive” solution (JSTL?)
  7. 7. Java Server Pages (2) Servlets Java Server Pages Necessary to •  Write a class •  Write a configuration •  Know methods and syntax Necessary to •  Know HTML + a bit of Java No configuration (auto-compiled)
  8. 8. Java Server Pages (3) Scriptlet •  1+ for each page •  JSP with 0 scriptlet = plain HTML page Template Here: whatever depends on dynamic behaviors (e.g., request parameters, db connections…) •  Modified by programmers Here: fixed page layout, presentation •  Modified by designersStatic parts Dynamic parts
  9. 9. Java Server Pages (4) ¤  A JSP page is called template ¤  The Java code inserted in the template is called scriptlet ¤  Servlets are still useful! ¤  General usage ¤  Servlets are used to contain the logic ¤  JSP pages are used for the presentation of the results
  10. 10. Mixing HTML, JSP and servlets (1) ¤  Example of application ¤  The user inserts the parameters of the query he wants to perform in an HTML form ¤  Results are retrieved from the database and displayed
  11. 11. Mixing HTML, JSP and servlets (2) HTML form •  No dynamic parts •  No need to use server-side scripting Servlet •  Database connection •  Retrieve results JSP page •  Display the results Logic Presentation
  12. 12. JSP interpretation ¤  A Java Server Page does NOT produce HTML code as an output ¤  JSP belongs to the server-side programming techniques, thus it runs on the server WebContent folder Translated into what?
  13. 13. JSP goes to servlets Template HTML code Scriptlets HTML code Interpreted by the server ¤  The template is interpreted by the server in order to generate a page made of HTML code ¤  The browser receives just HTML code ¤  This hides the scripts, hackers would not see them Servlet Automatically generated
  14. 14. How do we produce HTML code?
  15. 15. JSP: behind the scenes (1) ¤  The server automatically creates, compiles, loads and runs a special servlet to generate the page’s content ¤  The static portions of the HTML page are generated by the servlet by using out.println() ¤  The dynamic portions are included directly ¤  Every time there is a change in the JSP page, the servlet is automatically recompiled ¤  First time: longer periods of time in order to create the servlet ¤  Other times: shorter periods of time in order to add pieces to the servlet
  16. 16. JSP: behind the scenes (2) 1)  Start server (load every JSP page in WebContent) WebContent folder 2) Translate (each JSP page is automatically translated into a Servlet) Automatic mapping between the requested page and the created servlet
  17. 17. JSP: behind the scenes (3) 1)  Request to the JSP page (user requires to access to the page) 2) Look for the mapping (the server looks for the mapping between the requested JSP page and the created servlet) 3) Compute the response (the servlet handles the request and returns the response) HTML code
  18. 18. JSP: behind the scenes (4) 1)  Modify the JSP page (with any modification – either on static or dynamic parts) 2) Look for the mapping (the server looks for the mapping between the modified JSP page and the created servlet) 3) Modify the servlet (the update is reflected on the servlet)
  19. 19. Plain HTML or JSP? ¤  Allowed ¤  Create a JSP page with 0+ scriptlets ¤  Suggested ¤  Create a JSP page with 1+ scriptlets ¤  To be avoided ¤  Create a JSP page with 0 scriptlets JSP page with 0 scriptlets (= HTML page) Unaltered HTML code Translation OVERHEAD
  20. 20. Using Java Server Pages ¤  Each block of servlet code is surrounded by a leading <% tag and a closing %> tag ¤  A scriptlet can use predefined variables: ¤  request: the servlet request(HttpServletRequest) ¤  response: the servlet response (HttpServletResponse) ¤  out: the output writer (PrintWriter) ¤  in: the input reader (BufferedReader)
  21. 21. HelloWorld JSP page <html>! <head><title>Hello, World!</title></head>! <body>! !<% ! !if (request.getParameter("name") == null) ! out.println("Hello, World!");! !else! out.println("Hello, " + request.getParameter ("name") + "!"); !! !%>! </body>! </html> Retrieve a parameter from the request (here: query string)
  22. 22. Scope variables (1) Page scope Request scope Session scope Application scope
  23. 23. Scope variables (2) ¤  page: the data can be retrieved from any of the parts of the page ¤  request: the data can be retrieved from all the pages that analyse the same HTTP request (<jspforward>) ¤  session: the data can be retrieved from any template required by the same user (at least until the session is valid) ¤  application: the data can be retrieved from any of the templates of the same web application
  24. 24. Scope variables (3) ¤  Set an attribute of a variable ¤  variable.setAttribute(String name, Object value) ¤  Read an attribute from a variable ¤  Object o = variable.getAttribute(String name)
  25. 25. JSP syntax (1) ¤  Comment: <%-- comment --%> ¤  Expression: <%= expression %> ¤  The expression is computed, translated into a String and then displayed ¤  Declaration: <%! declaration %> ¤  How to define attributes and methods that will be copied in the servlet ¤  Action: <jsp:actionName action /> ¤  XML tag that invokes Web server functionalities
  26. 26. JSP syntax (2) ¤  Directive: <%@ directiveName attrName = “attrValue”%> ¤  Allows to use other servlet aspects ¤  Examples of directives: ¤  <%@ page session=“false”%> disables the session tracking ¤  <%@ page errorPage=“error.jsp”%> sets the page URL that will be used in case of error ¤  <%@ page import=“java.lib.*”%> imports a Java library
  27. 27. HelloWorld JSP: refactored <html>! <head><title>Hello, World!</title></head>! <body>! !Hello, <%=getName(request) %>!! </body>! </html>! ! <%!! private static final String DEFAULTNAME = "World";! private static final String getName(HttpServletRequest request) {! String name = request.getParameter("name");! if (name == null)! !return DEFAULTNAME;! else! !return name;! }! %> JSP declaration
  28. 28. Database connection InsertEmployeeName.html DatabaseConnection.jsp Name Phone John 89398544 Sam 83478534
  29. 29. Database connection: user input <html>! <head><title>Retrieve phone number</title></head>! <body>! !<form method=POST action="DatabaseConnection.jsp">! Insert the name of the employee here:! <input type="text" name="employeeName"/>! <input type="submit"/>! !</form>! </body>! </html>
  30. 30. Database connection: data retrieval (1) <%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>! <%@ page import="java.sql.*" %>! ! <%! String name = request.getParameter("employeeName");! Class.forName("com.mysql.jdbc.Driver");! Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/ test", "root", "");! PreparedStatement statement = connection.prepareStatement("SELECT * FROM employees WHERE name=?");! statement.setString(1,name);! ResultSet resultSet = statement.executeQuery();! %>! ! <html>! <head><title>Phone numbers</title></head>! <body>! Phone numbers for the required user:! <% if (!resultSet.isBeforeFirst()) { %>! There are no numbers!! <% ! }! Check if there are no results Prepare query and execute it
  31. 31. Database connection: data retrieval (2) else {%>! <table>! <tr>! <td><font size="+1">Name</font></td>! <td><font size="+1">Phone</font></td>! </tr>! <% while (resultSet.next()) { %>! <tr>! <td><%= resultSet.getString("name") %></td>! <td><%= resultSet.getString("phone") %></td>! </tr>! <% } %>! </table>! <% } %>! </body>! </html>! ! <%! resultSet.close();! statement.close();! connection.close();! %> Print each tuple in the result set Close connection and statement
  32. 32. Mixing JSP and Servlets NewEmployee.jsp CreateEmployee Name Phone NewEmployeeCreated.jsp Store new employee and retrieve ID Retrieve data about the created object
  33. 33. Insert new employee data <html>! <head><title>Create a new employee</title></head>! <body>! <form method=POST action="CreateEmployee">! !Name: <input type="text" name="name"/>! !<br />! !Phone: <input type="text" name="phone"/>! !<br />! !<input type="submit"/>! </form>! </body>! </html> NewEmployee.jsp
  34. 34. Store the employee in the database (1) public class CreateEmployee extends HttpServlet {! Connection connection;! !! public void init(ServletConfig config) throws ServletException { ! ServletContext context = config.getServletContext(); ! String loginUser = context.getInitParameter("dbUser");! String loginPasswd = context.getInitParameter("dbPassword");! String loginUrl = context.getInitParameter("dbURL"); ! ! try {! Class.forName("com.mysql.jdbc.Driver");! connection = DriverManager.getConnection(loginUrl, loginUser, loginPasswd);! }! catch (ClassNotFoundException ex) {! System.err.println("ClassNotFoundException: " + ex.getMessage());! throw new ServletException("Class not found Error");! }! catch (SQLException ex) {! System.err.println("SQLException: " + ex.getMessage());! }! }! CreateEmployee Get parameters from the context Get the connection
  35. 35. Store the employee in the database (2) public void doPost(HttpServletRequest request, HttpServletResponse response)! !throws IOException, ServletException {! String id = ””;! try {! synchronized(connection) {! String query = "INSERT INTO employees (name, phone) VALUES (?,?)";! PreparedStatement statement = connection.prepareStatement(query);! statement.setString(1, (String) request.getParameter("name"));! statement.setString(2, (String) request.getParameter("phone"));! statement.executeUpdate(); ! statement.close();! ! !! query = "SELECT id FROM employees WHERE name=? AND phone=?";! statement = connection.prepareStatement(query);! statement.setString(1, (String) request.getParameter("name"));! statement.setString(2, (String) request.getParameter("phone"));! ResultSet resultSet = statement.executeQuery();! resultSet.next();! id = resultSet.getString("id");! }! } ! CreateEmployee Insert employee in the database Retrieve the employee ID
  36. 36. Store the employee in the database (3) catch(Exception ex) { ! ex.printStackTrace();! return;! }! response.sendRedirect("NewEmployeeCreated.jsp?id=" + id);! } ! public void destroy() {! try {! if (connection != null){! connection.close();! }! } ! catch (SQLException sqle) {}! }! } CreateEmployee Send redirect to the second JSP page
  37. 37. Report the results (1) <%@ page import="java.sql.*,java.util.*,java.text.*" %>! <%Connection connection = null; ResultSet resultSet = null;! ServletContext context = config.getServletContext(); ! String loginUser = context.getInitParameter("dbUser");! String loginPasswd = context.getInitParameter("dbPassword");! String loginUrl = context.getInitParameter("dbURL");! ! String name = "”; String phone = "”;! ! try {! Class.forName("com.mysql.jdbc.Driver");! connection = DriverManager.getConnection(loginUrl, loginUser, loginPasswd); ! }! catch (ClassNotFoundException ex) {! System.err.println("ClassNotFoundException: " + ex.getMessage());! throw new ServletException("Class not found Error");! }! catch (SQLException ex) {! System.err.println("SQLException: " + ex.getMessage());! }! String id = request.getParameter("id");! Retrieve data about the connection to the database Retrieve the id of the created employee from the request Create database connection NewEmployeeCreated.jsp
  38. 38. Report the results (2) if ((id != null) && (id != "")) { ! try {! String query = "SELECT name,phone FROM employees WHERE id = ?";! PreparedStatement statement = connection.prepareStatement (query);! statement.setInt(1, new Integer(id));! resultSet = statement.executeQuery();! resultSet.next();! name = resultSet.getString("name");! phone = resultSet.getString("phone");! }! catch (Exception e) {! e.printStackTrace();! }! } ! %>! Retrieve data about the inserted employee NewEmployeeCreated.jsp
  39. 39. Report the results (3) <html>! <head><title>New employee created!</title></head>! <body>! New employee created.! <ul>! <li> Name: <%=name %></li>! <li> Phone: <%=phone %></li>! </ul>! </body>! </html> Print the values that were inserted in the database NewEmployeeCreated.jsp
  40. 40. JSP include (1) ¤  A JSP page can be fractioned in parts ¤  Each part could represent a specific functionality ¤  Each part can be reused in other JSP pages Connection.jsp (performs the connection to databases) SearchEmployee.jsp InsertEmployee.jsp
  41. 41. JSP include (2) ¤  How to include (mode 1) ¤  <%@ include file=“path” %> ¤  The code is included when the servlet is created ¤  If the included page is modified, the including pages need to be recompiled ¤  The variables that are present in the included page are visible also in the including page
  42. 42. JSP include (3) ¤  How to include (mode 2) ¤  <jsp:include page=“path” flush=“true”/> ¤  The page is included at request time: no need to recompile ¤  The output of the resource is included in the caller ¤  Caller page and called page are independent, thus in order to pass parameters between them: <jsp:include page=“path”> <jsp:param name=“paramName” value=“paramValue”/> </jsp:include>
  43. 43. JSP redirect vs. JSP forward ¤  The forward action allows one to send the request to another resource ¤  The redirect is performed in a transparent way: the browser is not notified ¤  <jsp:forward page=“/path.jsp”/> ¤  The request variable is identical to the one sent to the source page ¤  The redirect action performed on HttpServletResponse asks to the client to send a new request in order to obtain the new page ¤  response.sendRedirect(String location)
  44. 44. Disadvantages of scriptlets ¤  Writing the scriptlets in the page makes the page difficult to be read and understood by those who don’t know the Java language ¤  Content developers ¤  Designers
  45. 45. So many questions… ¤  Mixing competences ¤  Who edits the Java code? ¤  Who edits the presentation? ¤  How to divide the roles? ¤  XML validation ¤  How can we validate the JSP templates? (No valid XML: it includes no-XML code)
  46. 46. Solution: Java beans ¤  A Java bean is a reusable class with methods and variables that follow some naming conventions, so as to make them easily accessible ¤  Constructor: without parameters ¤  Getter and setter for each attribute ¤  A Java bean is used so as to perform ad-hoc operations ¤  JSP uses natively the Java beans ¤  They can be included in the scopes ¤  Their attributes can be set automatically
  47. 47. Java beans: usage ¤  Inclusion ¤  <jsp:useBean id=“name” scope=“page|request| session|application” class=“className” type=“typeName”/> ¤  Handling parameters ¤  <jsp:setProperty name=“beanName” property=“propertyName” param=“paramName”/> ¤  <jsp:setProperty name=“beanName” property=“propertyName” value=“constant”/> ¤  <jsp:getProperty name=“beanName” property=“propertyName”/> From the request Constant value
  48. 48. A simple Java bean public class HelloBean {! private String name = "World";! private String place = "empty";! !! public void setName(String name) {! this.name = name;! }! public String getName(){! return this.name;! }! public void setPlace(String place) {! this.place = place;! }! public String getPlace() {! return this.place;! }! } Properties of the bean
  49. 49. Accessing to the bean <%@page import="it.polimi.tiw.jsp.examples.beans.HelloBean" %>! <jsp:useBean id="hello" ! class="it.polimi.tiw.jsp.examples.beans.HelloBean">! <jsp:setProperty name="hello" property="name" param="userName"/>! </jsp:useBean>! ! <html>! <head><title>Hello, World!</title></head>! <body>! !Hello, <jsp:getProperty name="hello" property="name"/>!! </body>! </html> Bean name Bean name Name of the property that will be modified Parameter of the query string that will be used to set the property Bean name Name of the property that will be read from the bean
  50. 50. Java beans and session scope ¤  When a bean is stored in the session, it can be accessed from any page that uses that session Java Bean Session Read information
  51. 51. Store bean in session <%@page import="it.polimi.tiw.jsp.examples.beans.HelloBean" %>! <jsp:useBean id="hello" class="it.polimi.tiw.jsp.examples.beans.HelloBean" scope="session">! <jsp:setProperty name="hello" property="name" param="userName"/>! <jsp:setProperty name="hello" property="place" value="Como"/>! </jsp:useBean>! ! <html>! <head><title>Hello, World!</title></head>! <body>! Hello, <jsp:getProperty name="hello" property="name"/>!! <br />! Place (stored in session): <jsp:getProperty name="hello" property="place"/>! <form method=POST action="HelloWorldBeanSession2.jsp">! <input type="submit"/>! </form>! </body>! </html> Retrieve from the query string Set the property equal to a fixed value When instantiated, the bean is stored in the session. •  Properties can be retrieved from the session
  52. 52. Read information from the session <%@page import="it.polimi.tiw.jsp.examples.beans.HelloBean" %>! <jsp:useBean id="hello" ! class="it.polimi.tiw.jsp.examples.beans.HelloBean" scope="session"/>! ! <html>! <head><title>Hello, World!</title></head>! <body>! Name in session: <jsp:getProperty property="name" name="hello"/>! </body>! </html> Retrieve the bean from the session Read the property from the bean
  53. 53. JSP Tag Library
  54. 54. Tag libraries: Custom actions (1) ¤  We can define custom action libraries using the JSP tag extension ¤  A custom action is invoked by using a custom tag in a JSP page ¤  A tag library is a collection of custom tags ¤  The tag libraries are used in order to hide the Java code in the mark-up language ¤  Each tag is associated with a Java class, called tag handler, that implements a specific interface: javax.servlet.jsp.tagext.Tag
  55. 55. Tag libraries: Custom actions (2) ¤  The Tag Library Descriptor (TLD) is an XML file that keeps track of the associations between classes and tags ¤  Example: <%@ taglib uri=“myTagLibrary.tld” prefix=“mtl”%>! <tr>! <td>! Cost of the transaction: <mtl:write scope=“session” name=“transactionValue”/>! </td>! </tr>!
  56. 56. Tag libraries: Advantages ¤  The JSP pages are built out of reusable components ¤  Increase productivity ¤  Separate Java programmers (who will write the tag libraries) and the Web application designers ¤  Features ¤  Configuration using attributes ¤  Access to JSP variables ¤  Communication between different tags
  57. 57. How does it work? Translates into <tag> JSP page Each time <tag> is encountered in the JSP page, it is translated into a call to one of the methods in Tag Handler
  58. 58. How to build a custom tag ¤  In order to build a custom tag: ¤  Define a tag handler, i.e., a Java class that encapsulate the logic of the tag ¤  Define a TLD, i.e., an XML file that stores the coupling between tag handlers and tags ¤  Types of custom tags: ¤  javax.servet.jsp.tagext.Tag: generic tags ¤  javax.servlet.jsp.tagext.BodyTag: tag that write on nested elements
  59. 59. Custom tag: methods (1) ¤  The main methods are doStartTag() and doEndTag() ¤  They are invoked on the tag handler ¤  Between the two invocations the tag handler holds a state that must be preserved ¤  After the doEndTag() call, the tag is available for further invocations Default values for the properties Initialized properties (pageContext, parent, AttSet) Hold state new() setPageContext() setParent() setters() doStartTag() doEndTag() release()
  60. 60. Custom tag: methods (2) ¤  Field summary ¤  EVAL_BODY_INCLUDE: evaluate body into existing out stream ¤  EVAL_PAGE: continue evaluating the page ¤  SKIP_BODY: skip body evaluation ¤  SKIP_PAGE: skip the rest of the page ¤  doStartTag() returns EVAL_BODY_INCLUDE is the tag content has to be executed, otherwise it returns SKIP_BODY ¤  doEndTag() returns EVAL_PAGE is the rest of the page has to be executed and SKIP_PAGE otherwise
  61. 61. Simple tag example: handler (1) public class SimpleTagHandler implements Tag, Serializable {! !private PageContext pc = null;! !private Tag parent = null;! !private String name = null;! !! !public void setPageContext(PageContext pc) {! ! !this.pc = pc;! !}! !! !public void setParent(Tag parent) {! ! !this.parent = parent;! !}! !! !public void setName(String name) {! ! !this.name = name;! !} Declaration of page context Attributes Attribute setters Page context setters
  62. 62. Simple tag example: handler (2) !public int doStartTag() throws JspException {! try {! if (name != null)! ! !pc.getOut().write("Hello, " + name + "!");! else! ! !pc.getOut().write("You didn't enter your name");! }! catch (IOException e) {! throw new JspTagException("An IO exception occurred");! }! return SKIP_BODY;! !}! !public int doEndTag() throws JspException {! return EVAL_PAGE;! !}! !public void release() {! pc = null;! parent = null;! name = null;! !}! } When the tag is evaluated, printing the name on the output is requested Once the tag is evaluated, the body does not have to be evaluated Then, the page evaluation is continued Then the tag is released, all the properties are set to an undefined value
  63. 63. Simple tag example: TLD <taglib>! <tlibversion>1.0</tlibversion>! <jspversion>1.1</jspversion>! <shortname>MyTagLibrary</shortname>! <info>My Tag Library</info>! <tag>! <name>simple</name>! <tagclass>it.polimi.tiw.jsp.taglib.handlers.SimpleTagHandler</ tagclass>! <bodycontent>empty</bodycontent>! <info>This tag prints a name</info>! <attribute>! <name>name</name>! <required>false</required>! </attribute>! </tag>! </taglib> The name that will be used to reference the tag The attribute that can be used in the tag. It is not required, i.e., if not specified a default behavior will be applied
  64. 64. Simple tag example: JSP page <%@ taglib uri="/WEB-INF/MyTagLibrary.tld" prefix="mtl" %>! <html>! !<head>! ! !<title>Simple Tag Example</title>! !</head>! !<body>! ! !Name: <mtl:simple name="TIWuser"/>! !</body>! </html> Include the tag library and assign the prefix mtl to it Use the tag by specifying the attribute name
  65. 65. Tags and variables ¤  It is possible to make the tag create and store variables either in the page context or in the tag body <star:secondtag> <p align="center">Date value retrieved from JSP Tag : <%= time %></p> </star:secondtag>
  66. 66. Java Server Pages Application example
  67. 67. Forum application ¤  Users register to the forum ¤  Several topics are available ¤  A message is related to a topic ¤  By selecting a topic one can visualize the list of messages related to that topic ¤  Users can: ¤  Write a new message related to a specific topic ¤  Write a new message that answers to another message
  68. 68. Datamodel
  69. 69. Data model - SQL CREATE TABLE user ( oid INTEGER NOT NULL PRIMARY KEY, username VARCHAR(255), password VARCHAR(255), email VARCHAR(255)); CREATE TABLE message( oid SERIAL NOT NULL PRIMARY KEY, title VARCHAR(255), body TEXT, date DATE, parentmessageoid INTEGER, useroid INTEGER, topicoid INTEGER); CREATE TABLE topic( oid INTEGER NOT NULL PRIMARY KEY, name VARCHAR(255), description VARCHAR(255));
  70. 70. Application structure topicList.jsp messageList.jsp topicID messageDetails.jsp messageID newMessage.jsp includeinclude CreateMessage.java TopicList.java Bean TopicBean.java
  71. 71. Topic bean public class TopicBean {! private String oid;! private String name;! private String description;! !! public TopicBean(String oid, String name, String description) {! !this.setOid(oid);! !this.setName(name);! !this.setDescription(description);! }! ! // getters, setters… !! } Topic data TopicBean.java
  72. 72. Topics retrieval (1) public class TopicList extends HttpServlet {! private Connection connection = null;! !! public void init(ServletConfig config) throws ServletException {! try {! Class.forName("com.mysql.jdbc.Driver");! ! ServletContext context = config.getServletContext ();! String url = context.getInitParameter("dbUrl");! String user = context.getInitParameter("dbUser");! String password = context.getInitParameter ("dbPassword");! connection = DriverManager.getConnection(url, user, password); ! ! !! }! …Exceptions here…! }! Connection to the database TopicList.java
  73. 73. Topics retrieval (2) protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {! HttpSession session = request.getSession();! ! !! String query="SELECT oid, name, description FROM topic"; ! PreparedStatement statement;! try {! statement = connection.prepareStatement(query);! ResultSet result = statement.executeQuery();! ! ! !! Vector<TopicBean> vector = new Vector<TopicBean>();! while (result.next())! vector.add(new TopicBean(result.getString("oid"), result.getString("name"), result.getString("description"))); ! ! !! result.close();! statement.close();! ! ! !! session.setAttribute("topicPopulation", vector);! }! catch (SQLException e) {e.printStackTrace();}! ! !! response.sendRedirect("topicList.jsp"); ! !! }! } Store the topic data in a vector of beans Store the beans in the session
  74. 74. Visualize the list of topics ...! <h3>Topics</h3>! <ul>! <% ! Vector<TopicBean> beans = (Vector<TopicBean>) session.getAttribute("topicPopulation");! if (beans == null) { %>! No topics available.! <%}! else! for (int i = 0; i < beans.size(); i++) {! %>! <li>! <a href="messageList.jsp?tid=<%=beans.get (i).getOid() %>”><%=beans.get(i).getName() %></a>:! <%=beans.get(i).getDescription() %>! </li>! <%} %>! </ul> Retrieve the beans containing the topic data from the session Read the data of a specific bean topicList.jsp

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