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Chapter 7 Java Server Pages

Chapter 7 Java Server Pages. Objectives. Explain how the separation of concerns principle applies to JSP Describe the operation and life-cycle of a JSP List and describe the JSP element types Explain the operation of a JSP error page Explain the operation of include and forward directives

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Chapter 7 Java Server Pages

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  1. Chapter 7Java Server Pages

  2. Objectives • Explain how the separation of concerns principle applies to JSP • Describe the operation and life-cycle of a JSP • List and describe the JSP element types • Explain the operation of a JSP error page • Explain the operation of include and forward directives • Explain the purpose of a Java Bean and how one can be used from a JSP • Explain the purpose of a taglib and how a tag can be used from a JSP

  3. Separation of Concerns • A basic principle of software design:Software modules should have distinct responsibilities as much as possible input data retrieval calculation output

  4. Separation of ContentFrom Processing (1/2) • A servlet generating an HTML document intermixes Java code and HTML public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException() { PrintWriter out = res.getWriter(); res.setContentType("text/html"); out.println("<html><head><title>" + "Example HTML" + "</title></head><body>";... out.println("</html>"); out.close();}

  5. Separation of ContentFrom Processing (2/2) • Java Server Pages (JSP) allow static HTML content to be separated from dynamic content (created by Java code) within a document <%@ page %><html><head><title>Example Page</title></head><body><h1>Example Page</h1><% String name = req.getParameter("name");%><p>Hello, <%= name %></p>...</html>

  6. JSP Elements • JSP tags belong to one of five element types Declaration: declare variable or procedure Expression: evaluate a Java expression Scriptlet: execute a Java code segment Directive: a JSP compiler command Action Element: interacts with another component

  7. JSP Declaration • A declaration can be used to initialize variables or define procedures for use later in the JSP <%! Date today = new Date(); %> <%! // return the difference between two integers public int diff(int a, int b) { return (a < b) ? (b - a) : (a – b);} %>

  8. JSP Expression • A JSP expression is evaluated and then replaced by that value <%! Date today = new Date(); %> <p>Today is: <%= today.toString() %> </p> Generated HTML: <p>Today is: Mon Jun 15 08:05:00 EST 2009 </p>

  9. JSP Scriptlet • A JSP scriptlet is a short section of Java code that is executed in-line • Scriptlets have access to several pseudovariables: • out HTML output stream • session current HttpSession object • request HTTPRequest object • response HTTPResponse object

  10. JSP Scriptlet • This scriptlet displays the current date and time <% // output the current date and time Date today = new Date(); out.println("<p>Today is: " + date.toString() + " </p>"; %>

  11. JSP Scriptlet • The scriptlet checks to see if a Boolean session attributed named "login" exists <% // check to see if the user is logged in boolean loggedIn = ((Boolean) session.getAttribute("login")).booleanValue(); if (loggedIn) { out.println("<p>Please select an option</p>"; } else { out.println("<p>Please login</p>"; } %>

  12. JSP Scriptlet • The scriptlet maintains a "hit count" <% // get or create hit count Integer hitCount = (Integer) session.getAttribute("hits"); if (hitCount == null) { hitCount = new Integer(0): } // increment and save hit count hitCount = new Integer(hitCount.intValue() + 1); session.setAttribute("hits", hitCount); // display hit count <p>Hit Count: <%= hitCount.intValue() %></p> %>

  13. JSP Life-Cycle • JSP's are actually executed as servlets • The first time a JSP is requested, it is translated to a servlet and compiled • Each subsequent request executes the same servlet JSP Servlet translated upon first request

  14. JSP Life-Cycle

  15. JSP Directive • The page directive appears at the top of each JSP and controls its execution • The page directive also specifies imports <%@ page import="java.util.Date, java.io.*" %>

  16. JSP Errors • Compilation errors and run-time exceptions are logged and then returned as an HTML response ???

  17. JSP Error Page • The proper way to handle errors is to declare an error page • The error page is invoked whenever a JSP compilation or run-time error occurs • The error page can display a standard error message

  18. JSP Error Page • JSP that might throw an error: <%@ page import="java.util.*" errorPage="stderror.jsp" %> ... possible errors ... • stderror.jsp: <%@ page IsErrorPage="true" %> <html><head><title>Error Page</title></head><body><p>That request cannot be completed at this time.</p><p>Please contact your system administrator.</p></body></html>

  19. JSP Action Element • JSP action elements control interactions with other JSP's, including • include: add the contents of another JSP • forward: send a request to another JSP for handling

  20. JSP Include • A JSP include can be used to add standard content to a JSP, such as headers and footers <%@ page %> <jsp:include page="stdheader.jsp" /> ...unique content of this JSP... <jsp:include page="stdfooter.jsp" />

  21. JSP Forward • A JSP forward causes control to be transferred to a different JSP <%@ page %><% // verify that user is logged in Boolean loggedIn = (Boolean) session.getAttribute("login"); if (loggedIn == null || loggedIn.booleanValue() == false) {%><jsp:forward page="login.jsp" /><% }%> the scriptlet is continued after the directive, which is not a Java statement

  22. Java Beans • A Java Bean is a Java class that can be invoked from a JSP • This allows complex processing required by a JSP to be separated from the JSP • A JSP can instantiate an instance of a bean class and invoke its methods

  23. Java Bean Example • Java Bean package bean; public class AdderUtility { public int add(int a, int b) { return a + b; } } • JSP <%@ page %> <jsp:useBean id="adder" class="bean.AdderUtility"scope="session" /> <% int a, b, sum; … int sum = adder.add(a, b); %>

  24. Java Bean Uses • Java Beans are typically used to encapsulate complex application logic, including calculations, database access, or network access Application Java Bean JSP Database Network

  25. JSP Taglibs • A tag library allows programmers to create custom tags to invoke application functions with JSPs • Each custom tag is associated with a Java class that is executed when the tag is evaluated

  26. Taglib Example • JSP code: <tags:boxin border="green"> This code appears in a green box </tags:boxin> tag body content This code appears in a green box

  27. Taglib Example • BoxInTag.java public class BoxInTag extends BodyTagSupport { private String border; public void setBorder(String border) { this.border = border; } public int doStartTag() throws JspTagException { return EVAL_BODY_BUFFERED; } public int doEndTag() throws JspTagException { JspWriter out = pageContext.getOut(); String content = bodyContent.getString(); try { out.println("<div style=\"border-style: solid; border-color: " + border + "\">" + content + "</div>" ); } catch (IOException ioe) { ioe.printStackTrace(); } return EVAL_PAGE; } } returns the tag body content, i.e., the text between the tag opening and closing

  28. Tag Library Definition File (TLD) <?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN" "http://java.sun.com/j2ee/dtd/web-jsptaglibrary_1_2.dtd"> <taglib xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance“ xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd" version="2.0"> <tlib-version>1.0</tlib-version> <description>Show Standings Tag</description> <tag> <name>boxin</name> <tag-class>tags.BoxInTag</tag-class> <description> Draw a box around the content </description> <body-content>tagdependent</body-content> </tag> </taglib>

  29. Review • Java Server Pages • JSP life-cycle • JSP Elements • JSP include and forward directives • Java Beans with JSP • JSP taglibs

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