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CSC 2720Building Web Applications
JavaServer Pages (JSP)The Basics
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
<%-- 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>
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A JSP file interweaved with HTML codes and JSP scriplets
JSP vs. Servlet
JSP (corresponding class) Servlet
jspInit() init()
jspDestroy() destroy()
_jspService(request, response)
service(request, response)
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 PageContext instance 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
<!-- HTML comment --> Treated as template data. Reproduced in the output
Note: <!-- <%= 3+4 %> -->
is produced as <!-- 7 -->
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
<%for (int i=100; i>=0; i--) { %> <%= i %> bottles of beer on the
wall.<br><%}%>
for (int i=100; i>=0; i--) { out.println(i); out.println(" bottles of beer o
n the wall.<br>");}
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.…
JSP
Resulting Servlet
Output appears in browser
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>
Original JSP<H1>Some Heading</H1><%! private String randomHeading() { return("<H2>" + Math.random() + "</H2>"); }%><%= randomHeading() %>
Possible resulting servlet codepublic 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
Original JSP<%! int age1 = 100; %>
<% int age2 = 100; %>
Possible resulting servlet codepublic class xxxx implements HttpJspPage { int age1 = 100;
public void _jspService(HttpServletRequest request, HttpServletResponse response) throws
ServletException, IOException { ...
int age2 = 100; ... }
JSP/Servlet Correspondence (Declaration vs. Sniplet)
Example: JSP Tags + HTML Tags
<h2>Table of Square Roots</h2><table border="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
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