CSC 2720 Building Web Applications JavaServer Pages (JSP) The Basics.

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".

http://java.sun.com/javaee/5/docs/api/javax/servlet/jsp/PageContext.html

http://java.sun.com/javaee/5/docs/api/javax/servlet/jsp/PageContext.html

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

<%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

http://en.wikipedia.org/wiki/JavaServer_Pages

http://en.wikipedia.org/wiki/JavaServer_Pages

http://www.courses.coreservlets.com/Course-Materials/

http://www.java2s.com/Code/Java/JSP/CatalogJSP.htm

CSC 2720 Building Web Applications JavaServer Pages (JSP) The Basics.

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