CSC 2720 Building Web Applications

CSC 2720Building Web Applications JavaServer Pages (JSP) TheBasics

The JSP Framework • JavaServer Pages (JSP) is a Java technology that allows Java code and certain pre-defined actions to be embedded into static content. • JSPs are compiled into Java Servlets by a JSP compiler. • Architecturally, JSP can be viewed as a high-level abstraction of servlets that is implemented as an extension of the Servlet API. Ref: Wikipedia

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 <%-- This is a comment in JSP --%> <%@page contentType="text/html" pageEncoding="UTF-8"%> <% String param = request.getParameter("ParamName"); %> <html> <head> <title>My first JSP page </title> </head> <body> Parameter value is <%= param %>. </body> </html> A JSP file interweaved with HTML codes and JSP scriplets

JSP vs. Servlet • A JavaServer page can be viewed a Java class written in "different language". • A JavaServer page is first compiled into a Servlet class and then into Java byte code. • It has the same lifecycle as a servlet (init, serve, destroy) Static contents, <% … %>, and <%= … %> elements in a JSP Page are translated into Java codes and placed in _jspService()

JSP Syntax • A JavaServer Page may be broken down into the following pieces: • Static data such as HTML • JSP scripting elements and variables • JSP actions • JSP directives • Custom tags with correct library

Predefined Variables (Implicit Objects) • These objects are automatically made available in a JSP page (they are declared as local variables in _jspService()) • request • The HttpServletRequest object • response • The HttpServletResponse object • out • The stream (of type JspWriter) used to send output to the client • application • An instance of ServletContext. It is the object obtained by invoking getServletContext(). We can use this object to share data among all servlets and JSP pages in the same application.

Predefined Variables (Implicit Objects) • session • The HttpSession object associated with the request (unless disabled with the session attribute of the page directive) • page • The servlet itself (for self reference). • pageContext • A PageContextinstance that contains data associated with the whole page. • config • An instance of ServletConfig. It is the object obtained by calling getServletConfig(). • exception • Represent an exception not caught by the application code in an "Error handling JSP page".

Implicit objects and their corresponding class • application: javax.servlet.ServletContext • config: javax.servlet.ServletConfig • exception: java.lang.Throwable • out: javax.servlet.jsp.JspWriter • page: java.lang.Object • pageContext: javax.servlet.jsp.PageContext • request: javax.servlet.ServletRequest • response: javax.servlet.ServletResponse • session: javax.servlet.http.HttpSession

JSP Elements • Three types of JSP elements: • Directive elements • Scripting elements • Action elements • JSP Elements have two forms: • the XML form • the <% … %> alternative form

Comments • Comments <%-- JSP comment --%> • Discarded by JSP container • Won't appear in the output  • Treated as template data. • Reproduced in the output • Note:  is produced as

Scripting Elements • Allow you to insert Java code in JSP pages. • Three types: • Expressions <%=Expression%> • Scriplets <%Java code %> • Declarations <%!Declaring methods or instance variables%>

Expressions • Format: <%= Java Expression %> • Result • The element is replaced by the evaluated result of the expression in the output. • Examples • Current time: <%= new java.util.Date() %> • Your hostname: <%= request.getRemoteHost() %> • 3 * 4 + 5 = <%= 3 * 4 + 5 %> • XML-compatible syntax • <jsp:expression>Java Expression</jsp:expression>

Scriptlets • Format:<% Java Code %> • Result • Code is inserted verbatim into servlet's _jspService() • Examples • <% String queryData = request.getQueryString();out.println("Attached GET data: " + queryData); %> • <% response.setContentType("text/plain"); %> • XML-compatible syntax • <jsp:scriptlet>Java Code</jsp:scriptlet>

JSP/Servlet Correspondence • Scriptlets in JSP: <%= foo() %> <% bar(); %> • Possible resulting servlet code: public void _jspService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Initialization code ... out.println(foo()); bar(); ... } No ';' after foo() as it represents an expression and not a statement

JSP Scriptlets Example JSP <% for (int i=100; i>=0; i--) { %> <%= i %> bottles of beer on the wall.<br> <% } %> Output appears in browser 100 bottles of beer on the wall. 99 bottles of beer on the wall. 98 bottles of beer on the wall. 97 bottles of beer on the wall. 96 bottles of beer on the wall. 95 bottles of beer on the wall. 94 bottles of beer on the wall. … for (int i=100; i>=0; i--) { out.println(i); out.println(" bottles of beer on the wall.<br>"); } Resulting Servlet

Example Using JSP Scriptlets <HTML> <HEAD> <TITLE>Color Testing</TITLE> </HEAD> <% String bgColor = request.getParameter("bgColor"); boolean hasExplicitColor; if (bgColor != null) hasExplicitColor = true; else { hasExplicitColor = false; bgColor = "WHITE"; } %> <BODY BGCOLOR="<%= bgColor %>"> … </BODY></HTML>

Declarations • Format:<%! Java Code %> • Result • Code is inserted verbatim into servlet's class definition, outside of any existing methods • Use this to introduce instance/static variables and methods • Examples • <%! private int someField = 5; %> • <%! private void someMethod(...) {...} %> • XML-compatible syntax • <jsp:declaration>Java Code</jsp:declaration>

JSP/Servlet Correspondence • Original JSP <H1>Some Heading</H1> <%! private String randomHeading() { return("<H2>" + Math.random() + "</H2>"); } %> <%= randomHeading() %> • Possible resulting servlet code public class xxxx implements HttpJspPage { private String randomHeading() { return("<H2>" + Math.random() + "</H2>"); } public void _jspService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ... out.println("<H1>Some Heading</H1>"); out.println(randomHeading()); ... }

JSP/Servlet Correspondence (Declaration vs. Sniplet) • Original JSP <%! int age1 = 100; %> <% int age2 = 100; %> • Possible resulting servlet code public class xxxx implements HttpJspPage { int age1 = 100; public void _jspService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ... int age2 = 100; ... }

Example: JSP Tags + HTML Tags <h2>Table of Square Roots</h2> <tableborder="2"> <tr> <td><b>Number</b></td> <td><b>Square Root</b></td> </tr> <% for (int n=0; n<=100; n++) { %> <tr> <td><%=n%></td> <td><%=Math.sqrt(n)%></td> </tr> <% } %> </table>

References • Wikipedia: JavaServer Pages http://en.wikipedia.org/wiki/JavaServer_Pages • Free Tutorial (Java, JSP, Java Servlets) http://www.courses.coreservlets.com/Course-Materials/ • Sample JSP codes http://www.java2s.com/Code/Java/JSP/CatalogJSP.htm

CSC 2720 Building Web Applications