JSP – Java Server Page
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Transcript JSP – Java Server Page
JSP – Java Server Page
DBI – Representation and
Management of Data on the Internet
What is JSP
• http://java.sun.com/products/jsp
• A way to create dynamic web pages
• Based on Java Technology
– Large library base
– Platform independence
• Server side processing
• Separates the graphical design from the
dynamic content
Relationships
• In servlets,
– HTML code is printed from java code
• In JSP pages
– Java code is embadded in HTML code
Java
HTML
HTML
Java
In General Lines
• JSP-enabled server
–
–
–
–
picks up .jsp page
parses it
converts it to runnable form
runs it
• Converts page to a Java servlet (JspPage), with
your code inside the _jspService() method
– compiles it
– runs servlet protocol
Separating Graphical Design and
Dynamic Content
• Not a new idea (e.g. PHP, mod_perl, shtml, ASP)
• Graphical Design and System Design are two
separate and distinct specialities:
– Different languages (HTML vs. Java)
– Different goals
– Different Training
• Should be separated for optimal project
management
• JSP does this by allowing special Java tags in
HTML
An Example
<html>
<body>
<b>I’m HTML code.</b><br>
<% out.println(“I’m Java code.”) %>
</body>
</html>
Sending a Request
Translating and Executing JSP Pages
• A JSP page is executed in a JSP container,
generally installed in a Web server
– Think of a “JSP container” as a JVM with suitable
software installed
• The underlying semantic model is that of a servlet
• A typical JSP container will translate the JSP page
to a Java servlet
• By default translation and compilation of a JSP
page is likely to occur the first time it is accessed
– With Tomcat 3.1, you can find the generated Java and the class
files in a subdirectory under jakarta-tomcat/work.
Compilation
Translation
<H1>A Random Number</H1>
<%= Math.random() %>
public void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
request.setContentType("text/html");
HttpSession session = request.getSession(true);
JspWriter out = response.getWriter();
out.println("<H1>A Random Number</H1>");
out.println(Math.random());
...
}
JSP Features
• Standard directives guiding translation of a JSP
page to a servlet
• Standard actions in the form of predefined JSP
tags
• Script language declarations, scriptlets, and
expressions for including Java (or other language)
fragments that embed dynamic content
• A portable tag extension mechanism, for
building tag libraries—effectively extending the
JSP language
JSP vs. Javascript
• Javascript
–
–
–
–
Client side
Less secure
Browser Dependent
Unstable
JSP vs. ASP
• Active Server Pages (ASP)
– Similarities:
•
•
•
•
Server side dynamic web page generators
Share similar syntax <% %>
Modular programming (e.g. ActiveX, JavaBeans)
Focus on database connectivity
– Differences:
• ASP is a product while JSP is a specification
JSP vs. ASP (cont.)
• JSP
– Based on Java Technology
– Platform independence
• Unix, AS400, VMS, Windows
– More vendors choice
• ASP
– Microsoft programming languages
– Near Monopoly
JSP vs. Servlets
• But JSP pages are converted to Servlets?
Aren’t they the same?
• Similarities
– Provide identical results to the end user
– JSP is an additional module to the Servlet
Engine
JSP versus Servlets (cont.)
• Differences
– Servlets: “HTML in Java code”
• HTML code inaccessible to Graphics Designer
• Everything very accessible to Programmer
– JSP: “Java Code Scriptlets in HTML”
• HTML code very accessible to Graphics Designer
• Java code very accessible to Programmer
(Seperating content from appearance)
More Details
Writing JSP Pages
Template HTML
• The HTML code that wraps the code is like
a template
• Created as an ordinary HTML
• The dynamic parts are created on runtime
and are inserted into the template
JSP Scripting Elements
• JSP scripting elements let you insert Java code
into the servlet that will be generated from the JSP
page.
• There are three forms:
1. Expressions of the form <%= expression %> that are
evaluated and inserted into the output,
2. Scriptlets of the form <% code %> that are inserted
into the servlet's service method, and
3. Declarations of the form <%! code %> that are
inserted into the body of the servlet class, outside of
any existing methods
JSP Expressions
• A JSP expression is used to insert Java values
directly into the output
• It has the following form:
<%= Java Expression %>
• The Java expression is
– evaluated,
– converted to a string, and
– inserted in the page
• This evaluation is performed at run-time (when
the page is requested), and thus has full access to
information about the request
Predefined Variables
• The following predefined variables can
be used:
– request, the HttpServletRequest
– response, the HttpServletResponse
– session, the HttpSession associated with the
request (if any)
– out, the PrintWriter (a buffered version of
type JspWriter) used to send output to the
client
• (more will be said later)
Examples
• For example, the following shows the
date/time that the page was requested:
Current time: <%= new java.util.Date() %>
• For example, the following shows the
hostname:
Your hostname:
<%= request.getRemoteHost() %>
<BODY>
<H2>JSP Expressions</H2>
<UL>
<LI>Current time: <%= new java.util.Date() %>
<LI>Your hostname: <%= request.getRemoteHost()
<LI>Your session ID: <%= session.getId() %>
<LI>The <CODE>testParam</CODE>
form parameter:
<%= request.getParameter("testParam") %>
</UL>
</BODY>
JSP Scriplets
• JSP scriptlets let you insert arbitrary code
into the servlet method that will be built to
generate the page
• Scriptlets have the following form:
<% Java Code %>
• Scriptlets have access to the same
automatically defined variables as
expressions
Producing Code
• Scriplets produce output HTML by printing
into the out variable
• Example:
<% String queryData = request.getQueryString();
out.println("Attached GET data: " + queryData);
%>
HTML Code in Scriptlets
• HTML code before and after the scriplets is
converted to print methods:
•<% if (Math.random() < 0.5) { %>
Have
<% }
Have
<% }
a <B>nice</B> day!
else { %>
a <B>lousy</B> day!
%>
if (Math.random() < 0.5) {
out.println("Have a <B>nice</B> day!");
}
else {
out.println("Have a <B>lousy</B> day!");
}
<%= foo() %>
<% bar(); %>
public void _jspService(HttpServletRequest requ
HttpSe
throws ServletException, IOException {
request.setContentType("text/html");
HttpSession session = request.getSession(t
JspWriter out = response.getWriter();
out.println(foo());
bar();
...
}
Example of Using Scriplets (1)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Color Testing</TITLE>
</HEAD>
<%
String bgColor = request.getParameter("bgColor");
boolean hasExplicitColor;
if (bgColor != null) {
hasExplicitColor = true;
} else {
hasExplicitColor = false;
bgColor = "WHITE";
}
%>
Example of Using Scriplets (2)
<BODY BGCOLOR="<%= bgColor %>">
<H2 ALIGN="CENTER">Color Testing</H2>
<%
if (hasExplicitColor) {
out.println("You supplied an explicit background co
bgColor + ".");
} else {
out.println("Using default background color of WHIT
"Supply the bgColor request attribute to try " +
"a standard color, an RRGGBB value, or to see "+
"if your browser supports X11 color names.");
}
%>
</BODY>
</HTML>
JSP Declaration
• A JSP declaration lets you define methods or
fields that get inserted into the main body of the
servlet class (outside of the service method
processing the request)
• It has the following form:
<%! Java Code %>
• Declarations do not produce output
• They are used, for example, to define variables
Example
• We want to print out the number of times
the current page has been requested since
the server booted (or the servlet class was
changed and reloaded):
<%! private int accessCount = 0; %>
Accesses to page since server reboot:
<%= ++accessCount %>
JSP Directives
• A JSP directive affects the overall structure of the
servlet class
• It usually has the following form:
<%@ directive attribute="value" %>
• Multiple attribute settings for a single directive
can be combined:
<%@ directive
attribute1="value1"
attribute2="value2"
...
attributeN="valueN" %>
Directives
• There are three main types of directive:
• page, which lets you do things like
– import classes
– customize the servlet superclass
• include, which lets you
– insert a file into the servlet class at the time the JSP file
is translated into a servlet
• taglib directive
– indicates a library of custom tags that the page can
include
The page Directive
• The page directive lets you define the
following attributes:
– import="package.class“
<%@ page import="java.util.*" %>
– contentType="MIME-Type"
<%@ page contentType="text/plain" %>
(it is the same as
<%response.setContentType("text/plain");
%>)
More Page Directives
• isThreadSafe=“true|false”
– Normal servlet processing or implementing
SingleThreadModel
• session=“true|false”
– Allowing/disallowing sessions
• buffer=“sizekb|none”
– specifies the buffer size for the JspWriter out
• autoflush=“true|false”
– Flush buffer when full or throws an exception when
buffer isfull
And More Directives
• extends=“package.class”
• info=“message”
– A message for the getServletInfo method
• errorPage=“url”
– Define a JSP page that handles uncaught
exceptions
• isErrorPage=“true|false”
• language=“java”
The include Directive
• This directive lets you include files at the
time the JSP page is translated into a servlet
• The directive looks like this:
<%@ include file="relative url" %>
Predefined Variables
• As we have seen before, there are variables
that can be used in the code
• There are eight automatically defined
variables, sometimes called implicit objects
• The available variables are request,
response, out, session, application,
config, pageContext, and page
request
• This is the HttpServletRequest associated
with the request
• It lets you
– look at the request parameters (via
getParameter),
– the request type (GET, POST, HEAD, etc.),
and
– the incoming HTTP headers (cookies, etc.)
response
• This is the HttpServletResponse
associated with the response to the client
• Since the output stream (see out below) is buffered, it is
legal to set HTTP status codes and response headers, even
though this is not permitted in regular servlets once any
output has been sent to the client
out
• This is the PrintWriter used to send output to the
client
• However, in order to make the response object
useful, this is a buffered version of PrintWriter
called JspWriter
• Note that you can adjust the buffer size, or even turn buffering off,
through use of the buffer attribute of the page directive
• Usually out is used in scriptlets, since JSP expressions automatically
get placed in the output stream, and thus rarely need to refer to out
explicitly
session
• This is the HttpSession object associated
with the request
• Sessions are created automatically, so this
variable is bound even if there was no
incoming session reference (unless session was
turned off using the session attribute of the page
directive)
application
• This is the ServletContext as obtained via
getServletConfig().getContext()
config
• This is the ServletContext as obtained via
getServletConfig().getContext()
pageContext
• JSP introduced a new class called
PageContext
• It encapsulate use of server-specific features
like higher performance JspWriters
• The idea is that, if you access the serverspecific features through this class rather
than directly, your code will still run on
"regular" servlet/JSP engines
page
• This is simply a synonym for this
• It is not very useful in Java
• It was created as a placeholder for the time
when the scripting language could be
something other than Java
Actions
• JSP actions use constructs in XML syntax
to control the behavior of the servlet engine
• You can
–
–
–
–
dynamically insert a file,
reuse JavaBeans components,
forward the user to another page, or
generate HTML for the Java plugin
Available Actions
• Available actions include:
– jsp:include - Include a file at the time the page is
requested
– jsp:useBean - Find or instantiate a JavaBean
– jsp:setProperty - Set the property of a JavaBean
– jsp:getProperty - Insert the property of a JavaBean
into the output
– jsp:forward - Forward the requester to a new page
– jsp:plugin - Generate browser-specific code that makes
an OBJECT or EMBED tag for the Java plugin
The jsp:include Action
• This action lets you insert files into the page
being generated
• The file inserted when page is requested
• The syntax looks like this:
<jsp:include page="relative URL"
flush="true" />
The jsp:useBean Action
• This action lets you load in a JavaBean to
be used in the JSP page
• The simplest syntax for specifying that a
bean should be used is:
<jsp:useBean id="name“
class="package.class" />
Example
package dbiTests;
public class SimpleBean {
private String message="No message specified";
public String getMessage() {
return(message);
}
public void setMessage(String message) {
this.message = message;
}
}
<HTML>
<HEAD> <TITLE>Reusing JavaBeans in JSP</TITLE>
</HEAD>
<BODY>
<CENTER>
<TABLE BORDER=5>
<TR><TH CLASS="TITLE"> Reusing JavaBeans in JSP
</TABLE>
</CENTER>
<P>
<jsp:useBean id="test" class=“dbiTest.SimpleBean" />
<jsp:setProperty name="test" property="message"
value="Hello WWW" />
<H1>Message: <I>
<jsp:getProperty name="test" property="message" />
</I></H1>
</BODY>
</HTML>
The jsp:forward Action
• Forwards request to another page
• Syntax:
<jsp:forward page="relative URL"/>
• It has a single attribute, page, which should consist of a
relative URL
• This could be a static value, or could be computed at
request time
• Examples:
<jsp:forward page="/utils/errorReporter.jsp" />
<jsp:forward page="<%= someJavaExpression %>" />
Comments
• <%-- comment --%>
– A JSP comment
– Ignored by JSP-to-scriptlet translator
– Any embedded JSP scripting elements, directives,
or actions are ignored
• <!-- comment -->
– An HTML comment
– Passed through to resultant HTML
– Any embedded JSP scripting elements, directives,
or actions are executed normally
• /* comment */ or // comment
– Java comment
– Part of the Java code
Quoting Conventions
• <\%
– Used in template text (static HTML) and in
attributes where you really want "<%"
• %\>
– Used in scripting elements and in attributes
where you really want "%>“
• \‘ \”
– For using quotes in attributes
Init and Destroy
• JSP pages, like regular servlets, sometimes want to use
init and destroy
• Problem: the servlet that gets built from the JSP page
might already use init and destroy
– Overriding them would cause problems
– Thus, it is illegal to use JSP declarations to declare init or destroy
• Solution: use jspInit and jspDestroy
– The auto-generated servlet is guaranteed to call these methods
from init and destroy, but the standard versions of jspInit and
jspDestroy are empty (placeholders for you to override)
Writing JSP in XML
• We replace
– <% Java Expration %> with
<jsp:expression> Java Expression
</jsp:expression>
– <% Code %> with
<jsp:scriptlet> Code </jsp:scriptlet>
– <%! Code %> with
<jsp:declaration> Code </jsp:declaration>
– For directives we use
<jsp:directive.directiveType attribute=value />