Architecture of JSPs

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Transcript Architecture of JSPs 

Java WWW Week 2
JSP structure and models
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Format of lecture:
Assignment context
JSP models
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JSPs calling other JSPs i.e. breaking up work
Parameter passing
JSPs with Add SQL examples
Summary
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Slide 1
Java WWW Week 2
JSP Built-in Objects
JSPs have access to several objects that we
can use when processing a request from a
browser and then generate a dynamic
response.
There are eight in all:
request
out
response
pagecontext
session
application
config
page
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Java WWW Week 2
Built-in objects
 You will see these objects being used in the
examples provided during the module
coverage of JSPs.
 Request objects methods allow us to access
information about:
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The user
HTTP headers
Clients machine
Request URL and parameters.
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Java WWW Week 2
Built-in objects
Response objects methods allow access to:
Setting HTTP headers.
Set cookies.
Encode session information into URLs.
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Java WWW Week 2
Built-in objects
Out object - has methods to generate and
control output to the browser.
PageContext object - has page attributes.
Page object - represents the servlet instance
i.e. the object generated from having compiled
the JSP.
Application object - used for extracting
information about the lifecycle of the Servlet.
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Java WWW Week 2
The Structure of JSPs
 An initial selection of tags to get to know (Scriptlet
Tags = v. important!!!).
<% //embed Java code %>
 Used for embedding blocks of Java code into JSPs.
 Scriptlets provide control structures like decision (if
and switch) and iteration control structures (for and
while).
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Java WWW Week 2
Structure of JSPs
 Declaration Tags – also important.
<%! // Variable declaration - method declaration %>
 Used for declaring variables and methods. For example:
<%! HttpSession ses = null; %>
<%! public String whereFrom(HttpServletRequest req)
{
ses = req.getSession();
return req.getRemoteHost();
}
%>
<% out.print("Hi there, I see that you are coming in from "); %>
<%= whereFrom(request) %>
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Structure of JSPs
Expression Tags.
<%= java expression %>
Used for inserting the value of a Java
expression into JSPs.
e.g.
<td><%= rs.getString("LastName") %></td>
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Java WWW Week 2
Structure of JSPs
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Directive Tags – used to provide information to the JSP server.
There are three directives:
 page
 include
 taglib
We will only review the page and include directive.
Page Directive - used to set properties of the JSP. E.g. what
packages it imports from the Java library or is an error page.
<%@ page page_attributes %> e.g.
<%@ page import="java.sql.*" %>
<%@ page errorPage=“myErrorPage.jsp" %>
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Catching Errors – myErrorPage.jsp
<%@ page isErrorPage="true" %>
<HTML>
<HEAD><TITLE>My Error Page</TITLE></HEAD>
<BODY>
<H2>Exception Information</H2>
<TABLE>
<tr><td>Exception Class: </td><td><%= exception.getClass() %></td></tr>
<tr><td>Message: </td><td><%= exception.getMessage() %></td></tr>
</TABLE>
</BODY>
</HTML>
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Java WWW Week 2
JSP Action Tags
 Action tags are used to perform actions when a JSP page is requested
by a browser.
 There are six JSP action tags:
 useBean
 setProperty
 getProperty
 Include
 Forward
 plugin
 We will review two of them in during the module:
 include
 forward
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Slide 11
Java WWW Week 2
include action and directive
 Actions and Directives – there is a difference:
 Actions are elements that can create and access programming
language objects and affect the output stream
 include Action.
<jsp:include page= “Relative_URL” />
 Used to include the output of another JSP
 include Directive.
<%@ include file=“Relative_URL_to_file” %>
 Used to include the text of another file
 There are examples of both on the Tomcat server
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include example
<body>
<%@ page buffer="5" autoFlush="false" %>
<p>In place evaluation of another JSP which gives
you the current time:
<jsp:include page="clock.jsp" flush="true">
<jsp:param name="type" value="small">
</jsp:include>
</html>
See: http://java.sun.com/products/jsp/tags/12/syntaxref1214.html
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Action or Directive?
 Well both have their pros and cons and, as always, it just depends on
what your requirements are. The benefits of using the <jsp:include/>
ACTION are:
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Guarantees automatic recompilation,
Smaller class sizes,
Can make use of parameters (treated like form parameters)
JSP expressions can be used in attribute values
 One of the main benefits of using the include DIRECTIVE is that local
page variables can be shared between the two files. It also has slightly
better run time efficiency and doesn't restrict output buffering.
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Java WWW Week 2
forward Action
<jsp:forward page=“Relative_URL” />
 Used to forward the request to another JSP
 This will forward the current request object to the specified file
 When a forward tag is encountered, the JSP engine doesn't process the
remainder of the current JSP file
 Anything that has been written into the output buffer by the current JSP file
will be lost
 The following example tests the virtual memory and either forwards to
another .jsp or another html page - see code screen grab on next page…
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Java WWW Week 2
forward Action example
<%
double freeMem = Runtime.getRuntime().freeMemory();
double totlMem = Runtime.getRuntime().totalMemory();
double percent = 100*freeMem/totlMem;
if (percent < 50.0) {
%>
<jsp:forward page="/jsp/forward/one.jsp"/>
<% } else { %>
<jsp:forward page="two.html"/>
<% } %>
</html>
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Forward model
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Forward model
Client Request
HTML Form
HTTP Server - Tomcat
The JSP parses the input from
the client form (and optionally
updates the Database)
JSP
(optional)
SQL
HTML Form
JSP
Response
(optional)
DATABASE
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Demos in server folder
 CRUD examples
 Create(add);Retrieve(view/get);
 Adding information to a database
 Demo of a add JSP (addtousers.jsp)
 Retrieving (getting) information from the Guestbook
database with JSPs
 Demo of a simple Get JSP (SimpleGetUserExample.jsp)
 Demo of a user input version of get
(GetUserUsingInputExample.jsp)
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Slide 18
Java WWW Week 2
addtousers.jsp
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Slide 19
Java WWW Week 2
GetUserUsingInputExample.jsp
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Slide 21
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Summary
You have learnt more about the structure of
JSPs
Learning from examples is good
You have seen an Add something JSP
You have seen a more sophisticated view
something JSP (compared to last week with the
simple guest listing)
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Slide 23