Java Server Pages

Java Server Pages Jeffrey Jongko

Introduction • Java Server Pages (JSP) technology was created by Sun Microsystems and is built on top of Sun’s Java Servlet technology • Java Servlets was Sun’s first answer for developing web-based applications use the Java programming language.

Introduction • Java Servlet technology suffered similar maintenance problems that many traditional web-technologies like CGI had (presentation and logic are combined) • JSP is designed to facilitate development of dynamic web sites by more easily dissociating presentation from application logic • allowing page developers to develop page presentation without interfering with application logic development

JSP Architecture • The purpose of JSP is to provide a declarative, presentation-centric method of developing servlets. • JSP specification is defined as a standard extension on top the Servlet API. • Consequently, it should not be too surprisingly that under the covers, servlets and JSP pages have a lot in common.

JSP Architecture • Typically, JSP pages exist as simple text files and are subject to a translation phase and a request processing phase. • The translation phase is carried out only once, unless the JSP page changes, in which case it is repeated. • The JSP page is transformed into a servlet which subsequently processes all future requests to the page

JSP Architecture

<html> <body> <center> <% String hello = "Hello World"; %> <h1><%= hello %></h1> </center> </body> </html> The following JSP file is saved to a file HelloWorld.jsp It looks like regular HTML will special regions delimited with special markers like “<%” and “%>” which represent JSP features Sample JSP file

JSP Syntax core elements • There are four basic core elements in JSP • comments • declarations • expressions • scriptlets

JSP Comments • There are 2 types of JSP comments • HTML comment • Hidden comment • HTML Comment Syntax  • Hidden Comment Syntax <%-- comment --%> • A JSP HTML comment is a comment that is sent to the client (appears on the page data) • JSP expressions (seen later) can be included inside an HTML comment

<html> <body> <%@ page import="java.util.*" %> <%@ page import="java.text.*" %>  <% String hello = "Hello World"; %> <h1><%= hello %></h1> <%-- This will not appear --%> </body> </html> DATA RECEIVED BY CLIENT: <html> <body>  <h1>Hello World</h1> </body> </html> Sample JSP Comments

JSP Declarations • JSP Declarations are used to define variables and methods that are visible to the whole page • variables are translated into an instance variable in the compiled servlet • JSP Declaration Syntax <%! variable / method declaration %> • Variables declared in this way are not thread-safe. The JSP page has to be declared as single-threaded if thread safety is needed for these variables.

<%! int counter = 0; boolean isSameString(String a, String b) { return a.equals(b); } %> <html> <body> bjlee and hjk is <%= (isSameString(“bjlee”,”hjk”)) ? ”” : ”not” %> same string </body> </html> OUTPUT ON CLIENT: <html> <body> bjlee and hjk is not same string </body> </html> Sample JSP Declaration

JSP Expressions • Scripting language expression that is evaluated and converted into a String for insertion into the output page • JSP Expression Syntax <%= variable / method call %>

<html> <body> <%@ page import="java.util.*" %> <%@ page import="java.text.*" %>  <% String hello = "Hello World"; %> <h1><%= hello %></h1> <%-- This will not appear --%> </body> </html> DATA RECEIVED BY CLIENT: <html> <body>  <h1>Hello World</h1> </body> </html> Sample JSP Expressions

JSP Scriptlets • Scripting language code fragment that is run within the service() method of the compiled servlet • variables declared within a scriptlet are local unlike those declared by JSP Declarations • JSP Scriptlet Syntax <% scripting language code %>

<%@ page import=“java.util.*”%> <html><body bgcolor=“white”> <% String name = “Byung Joon Lee”; StringTokenizer st= new StringTokenizer(name, “ “); while ( st.hasMoreTokens() ) { %> <%= st.nextToken() %> <BR> <% } %> </body></html> OUTPUT ON CLIENT: <html><bodybgcolor=“white”> Byung<BR> Joon<BR> Lee<BR> </body></html> Sample JSP Scriptlet

<%@page %> Directive • <%@page %> directive defines attributes that apply to a whole JSP page • Example of some attributes: [ language=“java”] [ extends=“package.class”] [ import=“{package.class | package.*}, ...”] [ session=“true|false”] [ isThreadSafe=“true|false”] [ info=“text”] [ errorPage=“relativeURL”] [ isErrorPage=“true|false”]

Implicit Objects • Data is passed to JSP pages from the outside via HTTP POST or GET • This data is accessed via an implicit object • the name of this object is called request of type javax.servlet.ServletRequest • implicit objects are accessed via the scriptlets • Other implicit objects exist such as • response of type javax.servlet.ServletResponse • pageContext • session • application

Other JSP features • Ability to access JavaBean components using JSP tags, e.g. • <jsp:useBean> • <jsp:setProperty> • <jsp:getProperty> • This allows access to JavaBean objects without the use of scriptlets/Java code

Other JSP features • Ability to extend the usable JSP tags using a custom tag library • this is used to reduce the number of scriptlets on the page by encapsulating their logic behind tags. • Both these features reduce the need for people with actual Java language experience which is needed for coding scriptlets • allows for the development of presentation (JSP page) and the actual business logic (JavaBeans) to developed separately

Sites • Some sites that use Java Server Pages • http://www.sun.com • http://www.friendster.com

References • http://java.sun.com/products/jsp/ • http://www.swpark.pe.kr/lecture/jsp.pdf • http://developer.java.sun.com/developer/onlineTraining/JSPIntro/ • http://archive.coreservlets.com

Java Server Pages