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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. - PowerPoint PPT Presentation
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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
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 <!--- this is a comment -->
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>
Directive Elements: Include
Example: jsp page name = includexample.jsp
<%@ page import="java.util.*" %><%= (new java.util.Date() ).toLocaleString() %>
Directive Elements: Include
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() %>
Scripting elements: Expressions
Example
<html>
<body>
Current time is: <%= new java.util.Date() %>
</body>
</html>
Scripting elements: Expressions
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)
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 %>
Scripting elements: Scriptlets
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
Scripting elements: 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 { %> <!-- closing the scriptlet before the tags start -->
... tags follow ...
<% } %> <!-- reopening the scriptlet to close the language block -->
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
Scripting elements: Scriptlets
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
Scripting elements: Scriptlets
<% if (Math.random() < 0.5) { %>
Have a <B>nice</B> day!
<% } else { %>
Have a <B>lousy</B> day!
<% } %>
Scripting elements: Scriptlets
In background, JSP container compiles the JSP page into Java code.. converting the HTML snippets into java code to out.print statements
---
Scripting elements: Scriptlets
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
Scripting elements: Scriptlets
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 “ ;”
Scripting elements: Declarations
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>
Scripting elements: Declarations
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