Embeds Java code In HTML tags When used well Simple way to generate dynamic web-pages When...

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Transcript of Embeds Java code In HTML tags When used well Simple way to generate dynamic web-pages When...

Page 1: Embeds Java code  In HTML tags  When used well  Simple way to generate dynamic web-pages  When misused (complex embedded Java)  Terribly messy.
Page 2: Embeds Java code  In HTML tags  When used well  Simple way to generate dynamic web-pages  When misused (complex embedded Java)  Terribly messy.

Embeds Java code In HTML tags When used well

Simple way to generate dynamic web-pages When misused (complex embedded Java)

Terribly messy (and may violate OaOO) Keep the embedded Java simple

Use external helper classes (Beans?)

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JSP Document

.java file

.class file

.class file ready to run

Response Document

Translation

Compilation

Reinitialization

Subsequent User Requests

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A JSP page is translated into a Java Servlet

And then compiled On Tomcat, the compilation happens the

first time a page is requested First request can be very slow! Afterwards, just as fast as a Servlet

(because it is then a servlet)

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<html><head> <title> Hello JSP </title>

</head><body> <p> Hello World: <%= new java.util.Date() %></p></body></html>

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This extract shows the part that produces the output – compare it with the JSP:

out = pageContext.getOut(); _jspx_out = out;

out.write("<html>\r\n"); out.write("<head> "); out.write("<title> Hello JSP "); out.write("</title> "); out.write("</head>\r\n"); out.write("<body> \r\n"); out.write("<p> Hello World:\r\n "); out.print( new java.util.Date() ); out.write("\r\n"); out.write("</p>\r\n"); out.write("</body>\r\n"); out.write("</html>\r\n");

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So far we’ve seen literals: E.g. <html> Copied straight to output (to browser)

And expressions: E.g. <%= new java.util.Date() %> Return a value included in the output

Also have: Directives, Declarations and Scriptlets

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Instructions to the compiler Examples:

Include another page (compile-time) <%@ include file="header.jsp" %> <jsp:include page="page.jsp" flush="true" /> What is the difference ???

Import some Java packages (comma sep.) <%@ page import=“java.util.Collection”%>

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These are tags that start with the word jsp as in the previous example

These are mainly for handling form beans as will be shown later apart from 3 tags.

<jsp:forward, to forward user to another page <jsp:forward page="/servlet/login" />

<jsp:plugin, to import plugin as a java applet in the client browser. <jsp:plugin type=applet code="Molecule.class"

codebase="/html"> <jsp:include, to include the result of a jsp

page

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Used to declare variables and methods Go in the declaration section of the

Servlet Can then be used later in the JSP page Note the syntax Examples:

<%! int count = 0 %> <%! double sqr(double x) { return x * x; } %>

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These are sections of Java code embedded in the page

Unlike expressions, they do not return a value

But may write directly to the page Get the writer: response.getWriter()

They go in the service method of the servlet

Get executed each time a page is requested

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Demonstrates much of the above<%! int n = 0; %>

Page accessed: <%= ++n %> times

<% if ( (n % 10) == 0 ) {

n = 0;

}

%>

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Application Config Out Request Response Session

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Each JSP page has access to two special objects

The Request object carries information passed by the HTTP request (e.g. made by the browser)

This includes any submitted form data The Response object is used to pass

information back to the Client (browser) E.g. response.getWriter() provides an

output stream for direct writing to the client

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JSP makes form handling easy Can use request.getParameter() to get

submitted values Or can define a JavaBean to grab the

values semi-automatically. We’ll see this in action later with a simple

example

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Global object: request, session

<% session.putValue(“username”, request.getParameter(“UserName”)) %>

<html><body>Thanks for giving us your name, <%= session.getValue(“username”) %>

</body></html>

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The form can specify whether data is supplied via a GET or POST request

POST is the usual way Therefore, a servlet should implement

the doPost() method to process a form JSP hides these GET/POST details (see request.getParameter and <jsp:setProperty>)

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Helper classes for servlets To generate forms (+ other HTML) Process the form input

JSP + JavaBeans (more later) JSP + Tag Library (will be covered later)

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Come in two types Simple (this course) Enterprise (EJB: more complex, not covered)

Simple JavaBeans Data bound classes Define properties (fields) Define get/set methods

See following example

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<body><h3 align="center">Welcome to the Hotel

California</h3><form method="POST" action="BookHotel.jsp"><p>Name: <input type="text" name="name"

size="20"></p><p>How many nights: <select size="1" name="nNights"> <option selected="">1</option> <option>2</option> <option>3</option></select></p><p><input type="submit" value="Submit" name="B1"></p></form></body>

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<html><head><title>Bean test</title></head><body><h2> <%=request.getParameter("name") %> to stay for <%= request.getParameter("nNights") %>

nights.</h2> </body></html>

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<jsp:useBean id='roomBooking' scope='page' class='beans.HotelBean' />

<jsp:setProperty name='roomBooking' property='*' />

<html><head><title>Bean test</title></head>

<body><h2> <%=roomBooking.getName()%> to stay for <%= roomBooking.getnNights() %> nights. </h2> </body></html>

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package beans;public class HotelBean { String name; int nNights;

public String getName() { return name; } public void setName(String name) { this.name = name; } public int getnNights() { return nNights; } public void setnNights(int nNights) { this.nNights = nNights; }}

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Note the setting: scope='page' Scope has four possible settings: Page

Bean exists for execution of that page only Request

Like page, but survives any forward or include requests

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Session The Bean exists for multiple requests

within the web application, from a particular web browser instance

Used for shopping baskets etc. Application

The Bean exists for all requests from all users, for all pages that use it.

Survives until the Web Application Server is restarted

Can be used for a database connection (or connection pool)

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For this example, consider the differences: JavaBean JSP page: more complex JavaBean also required an external class

definition Discussion question:

When would JavaBean solutions be better than manual versions?

Answer:

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<%@ page import="java.util.*" %> <HTML> <BODY> <%!    

Date theDate = new Date();     Date getDate()     {        

return theDate;     } %> Hello!  The time is now <%= getDate() %> </BODY> </HTML>

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The JSP can get really intractable with lots of scriplets

A direct example of using scriplet is a for loop to display records from a database in a table format.

Put the code for the view in a function and call it as much as you need

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<%public void printRecord(int Number){

out.print(“<TR>”);out.print(“<TD>Number</TD>”);out.print(“<TD>” +Number+“</TD>”);out.print(“</TR>”);

} %><TABLE BORDER=2><% for ( int i = 0; i < n; i++ ) {

printRecord(i+1); }%></TABLE>

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This doesn’t look anymore like an HTML code.

What is the point of JSP then, is it to get messy code!!

So, sun devised a remedy for that and the final result looked really good, but not the development

The solution is called JSP Standard Tag Library or JSTL.

The last example will be written in one tag as

<mylib: for start=“1” end=“10”/>

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