Post on 12-Jan-2016
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?)
JSP Document
.java file
.class file
.class file ready to run
Response Document
Translation
Compilation
Reinitialization
Subsequent User Requests
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)
<html><head> <title> Hello JSP </title>
</head><body> <p> Hello World: <%= new java.util.Date() %></p></body></html>
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");
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
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”%>
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
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; } %>
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
Demonstrates much of the above<%! int n = 0; %>
Page accessed: <%= ++n %> times
<% if ( (n % 10) == 0 ) {
n = 0;
}
%>
Application Config Out Request Response Session
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
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
Global object: request, session
<% session.putValue(“username”, request.getParameter(“UserName”)) %>
<html><body>Thanks for giving us your name, <%= session.getValue(“username”) %>
</body></html>
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>)
Helper classes for servlets To generate forms (+ other HTML) Process the form input
JSP + JavaBeans (more later) JSP + Tag Library (will be covered later)
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
<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>
<html><head><title>Bean test</title></head><body><h2> <%=request.getParameter("name") %> to stay for <%= request.getParameter("nNights") %>
nights.</h2> </body></html>
<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>
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; }}
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
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)
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:
<%@ page import="java.util.*" %> <HTML> <BODY> <%!
Date theDate = new Date(); Date getDate() {
return theDate; } %> Hello! The time is now <%= getDate() %> </BODY> </HTML>
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
<%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>
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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