DT211/3 Internet Application Development

DT211/3 Internet Application Development

Introduction to Java Server Pages (JSP)

Introduction

• First – need to know the various “java” related terms:

J2EE, J2SE, JDK, JRE, JSP,JSTL, Java Servlets,Tomcat, Apache etc…..

Introduction – J2EE

• Sun Microsystems supply the Java 2 Enterprise Edition(J2EE) platform, enabling developers to develop java based enterprise applications

• J2EE is a standard set of technologies and APIs• (note: J2SE is the standard edition of the java platform but is

not geared at large enterprise environments with distributed systems)

• J2EE includes the following components:

JavaServerPages

Servlets Javabeans

JDBC JNDIJavaMessaging

Introduction – J2EE

• Since J2EE is a specification, vendors create productsthat support the J2EE specifcation e.g. Apache, IBMWebSphere, BEA Weblogic.

• From a web perspective, the J2EE applications that areparticularly relevant are:

Java Server Pages

Java servlets

Java Beans

Can be used on its ownor with beans/servlets to create a webapplication

Can be used on its ownor with JSP/beans to create a webapplication

More complex web applications may use all 3

Introduction – JDK

• The (JDK) Java Development Kit is the collective name for the various development components supplied in the J2EE (or J2SE).

• The Java Runtime Environment (JRE) is consists of the components required to run a java application

Introduction to JSP• Java Server Pages – A technology for developing web

pages that include dynamic content

• Created by SUN microsystems

• ‘Equivalent’ of Microsoft’s Active Server Pages technology

• Pages have a file extension of .JSP

• JSPs enable web developers to enhance HTML code by adding special JSP elements that are processed by the server

• Will use JSP v2.0

Advantages of JSP

•JSP pages are pre-compiled Fast

•Part of the Sun’s J2EE - Can be used with other types of java technologies, such as servlets - flexible

•JSP is a specification multiple vendors support it commercially stable/not ‘fly by night’ technology

•Easy to develop: HTML can be easily maintained with JSP. Relatively minimal programming skills required.

•JSP page code is not visible on client – only generated HTML is visible

Running JSP pages

To develop and run JSP pages, need: - Java Developer Kit (JDK which is part of J2EE) or higher AND a Web server that contains a JSP Container

-JSP Containers are not automatically included with all Web servers

-Examples of web servers that contain a JSP container are Apache Tomcat and Blazix.

Not automatically included with all web servers

JSP Container

-The JSP Container intercepts all client request for JSP pages

-First time a JSP page is requested, the Container converts the page into a java program called a Servlet and compiles -- Translation phase

-For all follow-on request, the Container processes each request by invoking the appropriate servlet and then generating the response - Request processing phase

Q: What happens if the JSP page is changed?

JSP Containers

How a JSP page is processed by server

First time through, JSP is translated to a servlet

After, container goes directly to the servlet in the request processing phase

If JSP page is changed, servlet is re-compiled

JSP and Apache

•Apache project have a sub project called Jakarta(see http://jakarta.apache.org/index2.html)

•Jakarta project produces

- Tomcat web server (nicknamed Catalina)- Tomcat webserver incorporates JSP container (nicknamed Jasper)

Software Versions

JSP technology developing rapidly

New version contain major new capabilities

Always note the JSP container version you are working with, and check functionality supported (on apache website)

This course using

Apache Tomcat Version 5

JSP 2

JSLT 1.1

Servlet 2.4

To run a JSP

Using apache Tomcat…

Create your web application directory

Create the subset WEB-INF directory (won’t run without this)

Put JSP page into web application directory

Call from the browser

http://localhost:8080/webapp/somename.jsp

To run a JSP

In the background –

Tomcat will retrieve the JSP page from the web server

If it’s the first time JSP page has been called/run or if page has changed, Tomcat will compile the JSP page (into a servlet) - .java and .class placed in /work directory.

- subsequent calls to page will be faster as no compile needed

JSP page will be presented back to the user

Simplest JSP.. Helloworld

• Prints out message to user when JSP is loaded..

• Tomcat demo..

Another Simple JSP example

<html> <%@ page import="java.util.Date" %><head> <title>JSP DATE EXAMPLE</title></head><body bgcolor=#ffffff>

<h1>JSP DATE</h1> <h2> The current date is <%= new Date() %>. </h2> </body></html>

Prints out the current date

Readability of JSP pages

Always always always ensure that code is:

INDENTED COMMENTED CONTAINS AN ID BLOCK

1) Indented - to reflect level of the code

<html> <head>ajsdlkfjads

etc 2) Well commented. Comments in JSP appear as

<%-- calculate the sum of x and z here --%>

Comments in HTML appear as 

HTML comments will be visible by view source in browser, JSP comments won’t.

3) Titled with an ID block:

At the top of each JSP page, should have an ID block explaining who write the script, date, script name, description and any revisions. Can use either JSP or HTML comments (depending on whether users should be able to see the ID block)

<%-- ********************************************* *** Script name: addition.jsp *** *** Author: Keith Morneau *** *** Date: July 7th 2006 *** *** Desciption: whatever it does.. *** *** Revisions: *** *** August 8th 2006: added subroutine *** ************************************************--%> etc

Readability of JSP pages

JSP techniques

Directive elements

Action elements and Java Standard Tags Library

Scripting elements (java)

JSP provides variety of techniques to enable dynamic processing:

Java Beans

In this topic

JSP Pages: Directive Elements

• Directive Elements - Used to provide information about the general set-up of the page. e.g. inclusion of header files, name of page to report errors, whether session tracking is required etc

• Always enclosed between <%@ …… %>

• Syntax: <%@ directivename attribute = “value”, attribute = “value” …. %>

JSP Pages: Directive Elements

• There are Three directives available to use:

– <%@ page ….. >

– <%@ include …… %>

– <%@ taglib ….. %>

Directive Elements

•Each directive has a set of associated attributes (similar to some tags in HTML)

•Usually placed at top of JSP file but doesn’t have to be

Example: <%@ page import="java.util.*, java.lang.*" %>

Full list of attributes available at: http://java.sun.com/products/jsp/syntax/1.1/syntaxref11.html

Directive Elements: Page

Page directive - defines attributes that apply to an entire JSP page.

Examples

<%@ page contentType = “text/html” %><%@ page language = “java” %><%@ page errorPage="error.jsp" %>

List of attributes includes

<%@ page [ language="java" ][ import="{package.class | package.*}, ..." ][ session="true|false" ][ isThreadSafe="true|false" ] *multiple threads allowed or not [ info="text" ] *gives info about the page to administration

[ errorPage="relativeURL" ][ contentType="mimeType [ ;charset=characterSet ]" | "text/html ; charset=ISO-8859-1" ][ isErrorPage="true|false" ] *Specifies whether exception object available or not

%>

Directive Elements: Page

Directive Elements: Include

Include directive - Includes a static file in a JSP file attranslation time

Syntax<%@ include file="relativeURL" %>

The included file can be an HTML file, a JSP file, a text file, or a code file written in the Java programming language

Useful for including repetitive HTML content across a web site – headers, navigation bars, footers etc

Useful for common logic – e.g. date displays

<html><head><title>An Include Test</title></head><body bgcolor="white"><font color="blue">The current date and time is<%@ include file="date.jsp" %></font></body></html>


Example: jsp page name = includexample.jsp

<%@ page import="java.util.*" %><%= (new java.util.Date() ).toLocaleString() %>


Example (continued) jsp page name = date.jsp

When includexample.jsp is run, displays as

The current date and time areAug 30, 2006 2:38:40

Includedinto includexample.jspfrom previous page

Directive Elements: Taglib

Taglib directive - Defines a tag library and prefix for the custom tags used in the JSP page.

Syntax

<%@ taglib uri="URIToTagLibrary“ prefix="tagPrefix" %>

Example: <%@ taglib uri=“http://java.sun.com/jstl/core“ prefix=“c" %>

More on taglib directives later when we use Java Standard Tag Library (JSTL)

Directive Elements: summary

Three directives: page, include, taglib

Used to define general set-up information about the JSP page

By themselves, don’t “do” anything

At least one used in most JSP pages

Directive elements

Action elements and JSTL

Scripting elements

Java Beans

JSP dynamic processing

Done

Scripting elements

Developers in JSP can insert java code directly into a JSP pages using scripting elements

The code is executed when the page is requested

Should be used with extreme care:

• Too much code difficult to maintain

• Difficult for HTML programmers

• More suitable for simple applications with small development team

Q: How do you specify that the language being used in page by scripting elements is java?

A: Page directive language attribute

Scripting elements

Three types of scripting elements:

1.Expressions: The expression syntax <%= ... %> defines a scripting language expression .. “result”

2.Scriptlets: The scriptlet syntax <% ... %> can handle declarations, expressions, or any other type of code fragment valid in the page scripting language. When you write a scriptlet, end the scriptlet with %> before you switch to HTML, text, or another JSP tag

3. Declarations: The declaration syntax <%! ... %> declares variables or methods.

Scripting elements: Expressions

Expressions:

• Contains any valid java expression in the JSP page

• Used to output dynamic values directly onto web page (e.g. result of a calculation, dates)

• Enclosed between <% and %>

• output as a string

Syntax <%= expression %> e.g. <% = 1+1%>

Expressions - examples

• E.gs: any valid java expression

<%= Math.sqrt(2) %><%= items[i] %><%= a + b + c %><%= new java.util.Date() %>


Example

<html>

<body>

Current time is: <%= new java.util.Date() %>

</body>

</html>


Note:

• Evaluated at run time. Result is added to the response body and output directly to web page

• Can use an expression within a line of text, whether or not it is tagged with HTML

• Must be a valid java expression

• No “;” required at end of expression (unlike scriptlets)


1. Expressions

2.Scriptlets

3. Declarations

Scripting elements

Scripting elements: Scriptlets

Scriplets are java code fragments with a JSP page

• Enables more complex functionality than expressions

• Java code is placed between <% and %> characters (just like expressions, but without the = sign at the start of the sequence.)

• Can have any number of valid java code statements

• Scriptlet contains Java code that is executed every time the JSP is invoked

• Syntax: <% code %>


Example – jsp page that outputs numbers 1 to 10 onto a web pagehtml>

<head> <title>Count to 10 in JSP scriptlet</title> </head> <body><% for(int i=1;i<=10;i++){%><%=i%><br/><%}%> </body></html>

Expression used to output to page. Can’t put HTML Tags into the scriptlet. Can onlycontain valid java code

Scriptlets


Example – jsp page that outputs numbers 1 to 10 onto a web page – Output in browser will be:..

Count to 10 in JSP

12345678910

Scriplets: mixing scriplets with HTML and other tags

• When you mingle scripting elements with HTML and JSP tags, you must always end a scripting element before you start using tags and then reopen the scripting element afterwards, like this:

<% } else { %> 

... tags follow ...

<% } %> 

At first, this may look a bit strange, but it ensures that the scripting elements are transformed correctly when the JSP source file is compiled


Example 2 –jsp page that outputs “Hello! The time is now Wed Sep 03 13:17:58 BST 2006

Your machine's address is 127.0.0.1” onto a web page <HTML>

<BODY>

<%

java.util.Date date = new java.util.Date();

%>

Hello! The time is now

<%

out.println( date );

out.println( "<BR>Your machine's address is " );

out.println( request.getRemoteHost());

%>

</BODY>

</HTML>

Note: could have put this line outside the scriptletas HTML

Note:

Any text, HTML tags, or JSP elements you write must be outside the scriptlet

Readability: Mixture of Tags and Java code can be difficultto read – especially for HTML developers


<% if (Math.random() < 0.5) { %>

Have a <B>nice</B> day!

<% } else { %>

Have a <B>lousy</B> day!

<% } %>


In background, JSP container compiles the JSP page into Java code.. converting the HTML snippets into java code to out.print statements

---


if (Math.random() < 0.5)

{

out.println("Have a <B>nice</B> day!");

}

else

{

out.println("Have a <B>lousy</B> day!");

}

JSP code on previous page will be converted in background to the following java code..

<TABLE BORDER=2><% for ( int i = 0; i < n; i++ ) { %> <TR> <TD>Number</TD> <TD><%= i+1 %></TD> </TR> <% }%></TABLE>

Example 3 – fragment of JSP page that generates a table in HTML containing numbers 1 to n

Note: would need to supply int variable n before it will work…

HTML within “for” loop is output n times



1. Expressions

2. Scriptlets

3.Declarations

Scripting elements

Scripting elements: Declarations

Declaration: Declares a variable or method that can be used throughout the JSP page

Examples

<%! int i = 0; %><%! int a, b, c; %><%! Circle a = new Circle(2.0); %>

Syntax

<%! declarations; %> Note the “ ;”


Declarations don’t generate any output onto the page by themselves- usually used with expressions and/or scriptlets

Examples

<%! private int accessCount; %>

Accesses to page since server reboot: <%= ++accessCount %>

Prints the number of times the current page has been requested since the server was booted

Example – Declares method getDate()<%@ page import="java.util.*" %>

<HTML>

<BODY>

<%!

Date theDate = new Date();

Date getDate()

{

System.out.println( "In getDate() method" );

return theDate;

}

%>

Hello! The time is now <%= getDate() %>

</BODY>

</HTML>


MethodgetDate()returns a Dateobject

Declaration

• Declarations (between <%! and %> tags) contain one or more variable or method declarations that end or are separated by semicolons:

• <%! int i = 0; %><%! int a, b; double c; %><%! Circle a = new Circle(2.0); %>

You must declare a variable or method in a JSP page before you use it in the page. The scope of a declaration is usually a JSP file, but if the JSP file includes other files with the include directive, the scope expands to cover the included files as well.

Declarations

• Warnings!

• Declaring variables in a JSP page using declarations can cause synchronisation problems.. Compiled servlets see these as variables which may be shared across all users using the JSP page.

• On previous date example, Date stays the same when the page is reloaded.. Because only a single instance of the page (and there of the variable theDate is available). Better to use local variables within declared methods or scriptlets

DT211/3 Internet Application Development

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