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WEB TECHNOLOGIES
A COMPUTER SCIENCE PERSPECTIVE
JEFFREY C. JACKSON
Separating Programming and Presentation:
JSP Technology
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Get vs. Post Methods
• In GET your entire form submission can be
encapsulated in one URL, like a hyperlink.
query length is limited to 255 characters, not
secure, faster, quick and easy. The data is
submitted as part of URL.
• In POST data is submitted inside body of the
HTTP request. The data is not visible on the
URL and it is more secure.
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
What is JSP
• Java Server Pages (JSP) is a platform
independent presentation layer technology
that comes with SUN s J2EE platform.
• JSPs are normal HTML pages with Java code
pieces embedded in them.
• JSP pages are saved to *.jsp files.
• A JSP compiler is used in the background to
generate a Servlet from the JSP page.
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
What is JSP Technology
• The JavaServer Pages technology enables you to
generate dynamic web content, such as HTML, DHTML,
XHTML, and XML files, to include in a Web application.
• JSP files allow a Web server, such as Apache Tomcat, to
add content dynamically to your HTML pages before
they are sent to a requesting browser.
• When you deploy a JSP file to a Web server that
provides a servlet engine, it is preprocessed into a
servlet that runs on the Web server. This is in contrast
with client-side JavaScript™ (within SCRIPT tags), which
is run in a browser.
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Why JSP?
• Servlet/CGI approach: server-side code is a
program with HTML embedded
• JavaServer Pages (and PHP/ASP/ColdFusion)
approach: server-side “code” is a document
with program embedded
– Supports cleaner separation of program logic from
presentation
– Facilitates division of labor between developers
and designers
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP vs. Servlet
JSP
Servlet
JSP is a webpage scripting language that can generate
dynamic content.
Servlets are Java programs that are already compiled
which also creates dynamic web content
JSP run slower compared to Servlet as it takes
Servlets run faster compared to JSP.
compilation time to convert into Java Servlets.
It’s easier to code in JSP than in Java Servlets.
Its little much code to write here.
In MVC, jsp act as a view.
In MVC, servlet act as a controller.
JSP are generally preferred when there is not much
servlets are best for use when there is more processing
processing of data required.
and manipulation involved.
The advantage of JSP programming over servlets is that
There is no such custom tag facility in servlets.
we can build custom tags which can directly call Java beans.
JSP is used mainly for presentation only. A JSP can only be HttpServlet that means the only supported protocol in JSP is
HTTP. But a servlet can support any protocol like HTTP, FTP, SMTP etc
6
Simple Hello World in JSP
<html>
<head><title>Hello World JSP Page.</title></head>
<body>
<font size="10"><%="Hello World!" %></font>
</body>
</html>
7
Simple Hello World in Servlet
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HelloWorld extends HttpServlet{
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException,IOException{
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.println("<html>");
pw.println("<head><title>Hello World</title></title>");
pw.println("<body>");
pw.println("<h1>Hello World</h1>");
pw.println("</body></html>");
}
}
8
Comments in JSP
Two types
• <%– JSP Comment –%>
• <!– HTML Comment –>
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP by Example
• JSP simply puts Java inside HTML pages. You can take any existing HTML
page and change its extension to ".jsp" instead of ".html".
• Take the HTML file you used in the previous exercise. Change its
extension from ".html" to ".jsp". Now load the new file, with the ".jsp"
extension, in your browser.
• You will see the same output, but it will take longer! But only the first
time. If you reload it again, it will load normally.
• What is happening behind the scenes is that your JSP is being turned into
a Java file, compiled and loaded. This compilation only happens once, so
after the first load, the file doesn't take long to load anymore. (But
everytime you change the JSP file, it will be re-compiled again.)
10
JSP Expressions
• Put the following text in a file with .jsp extension (let us call it
hello.jsp), place it in your JSP directory, and view it in a browser.
<HTML>
<BODY>
Hello! The time is now <%= new java.util.Date() %>
</BODY>
</HTML>
• Notice that each time you reload the page in the browser, it
comes up with the current time.
• The character sequences <%= and %> enclose Java expressions,
which are evaluated at run time.
• This is what makes it possible to use JSP to generate dyamic HTML
pages that change in response to user actions or vary from user to
user.
11
JSP Scriptlets
• We have already seen how to embed Java
expressions in JSP pages by putting them
between the <%= and %> character sequences.
• But it is difficult to do much programming just
by putting Java expressions inside HTML.
• JSP also allows you to write blocks of Java code
inside the JSP. You do this by placing your Java
code between <% and %> characters (just like
expressions, but without the = sign at the start
of the sequence.)
12
Scriptlet
• A scriptlet can contain any number of language statements, variable or method
declarations, or expressions that are valid in the page scripting language.
• Within scriptlet tags, you can
• Declare variables or methods to use later in the file (see also Declaration).
• Write expressions valid in the page scripting language (see also Expression).
• Use any of the JSP implicit objects or any object declared with a
<jsp:useBean> tag.
• You must write plain text, HTML-encoded text, or other JSP tags outside the
scriptlet.
• Scriptlets are executed at request time, when the JSP engine processes the
client request. If the scriptlet produces output, the output is stored in the out
object, from which you can display it.
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Variable inside Scriptlets
• Variable declared inside declaration part is treated
as a global variable. That means after conversion jsp
file into servlet that variable will be in outside of
service method or it will be declared as instance
variable.And the scope is available to complete jsp
and to complete in the converted servlet class.
• If u declare a variable inside a scriplet that variable
will be declared inside a service method and the
scope is with in the service method.
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Scriptlets
• Here is a modified version of our JSP from previous section, adding in
a scriptlet.
<HTML>
<BODY>
<%
// This is a scriptlet. Notice that the "date"
// variable we declare here is available in the
// embedded expression later on.
System.out.println( "Evaluating date now" );
java.util.Date date = new java.util.Date();
%>
Hello! The time is now <%= date %>
</BODY>
</HTML>
15
JSP Scriptlets
• If you run the above example, you will notice
the output from the "System.out.println" on
the server log.
• This is a convenient way to do simple
debugging (some servers also have techniques
of debugging the JSP in the IDE.
16
JSP Scriptlets - out variable
• By itself a scriptlet does not generate HTML. If a scriptlet wants to generate
HTML, it can use a variable called "out". This variable does not need to be
declared. It is already predefined for scriptlets, along with some other
variables. The following example shows how the scriptlet can generate
HTML output.
<HTML>
<BODY>
<%
// This scriptlet declares and initializes "date"
System.out.println( "Evaluating date now" );
java.util.Date date = new java.util.Date();
%>
Hello! The time is now
<%
// This scriptlet generates HTML output
out.println( String.valueOf( date ));
%>
</BODY>
</HTML>
17
JSP Scriptlets - request variable
• Another very useful pre-defined variable is "request". It is of type
javax.servlet.http.HttpServletRequest
• A "request" in server-side processing refers to the transaction between a
browser and the server. When someone clicks or enters a URL, the browser
sends a "request" to the server for that URL, and shows the data returned.
As a part of this "request", various data is available, including the file the
browser wants from the server, and if the request is coming from pressing a
SUBMIT button, the information the user has entered in the form fields.
• The JSP "request" variable is used to obtain information from the request as
sent by the browser. For instance, you can find out the name of the client's
host (if available, otherwise the IP address will be returned.)
18
JSP Scriptlets - request variable
<HTML>
<BODY>
<%
// This scriptlet declares and initializes "date"
System.out.println( "Evaluating date now" );
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>
19
JSP Scriptlets - response variable
• A similar variable is "response".
• This can be used to affect the response being sent to the browser.
• For instance, you can call response.sendRedirect( anotherUrl ); to
send a response to the browser that it should load a different URL.
• This response will actualy go all the way to the browser.
• The browser will then send a different request, to "anotherUrl".
• This is a little different from some other JSP mechanisms we will come
across, for including another page or forwarding the browser to
another page
20
JSP Directives
• We have been fully qualifying the java.util.Date in the examples in the previous sections.
• Perhaps you wondered why we don't just import java.util.*;
• It is possible to use "import" statements in JSPs, but the syntax is a little different from normal
Java. Try the following example:
<%@ page import="java.util.*" %>
<HTML>
<BODY>
<%
System.out.println( "Evaluating date now" );
Date date = new Date();
%>
Hello! The time is now <%= date %>
</BODY>
</HTML>
• The first line in the above example is called a "directive". A JSP "directive" starts with <%@
characters.
21
JSP Directives
• The page directive can contain the list of all imported packages.
• To import more than one item, separate the package names by commas, e.g.
<%@ page import="java.util.*,java.text.*" %>
• There are a number of JSP directives, besides the page directive. Besides the page directives, the
other most useful directives are include and taglib.
• The include directive is used to physically include the contents of another file. The included file
can be HTML or JSP or anything else -- the result is as if the original JSP file actually contained the
included text. To see this directive in action, create a new JSP
<HTML>
<BODY>
Going to include hello.jsp...<BR>
<%@ include file="hello.jsp" %>
</BODY>
</HTML>
• View this JSP in your browser, and you will see your original hello.jsp get included in the new JSP.
22
JSP Declarations
•The JSP you write turns into a class definition.
•All the scriptlets you write are placed inside a single method of this class.
•You can also add variable and method declarations to this class. You can then use these variables
and methods from your scriptlets and expressions.
•To add a declaration, you must use the <%! and %> sequences to enclose your declarations, as
shown below.
<%@ 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>
23
JSP Tags
• Another important syntax element of JSP are tags.
• JSP tags do not use <%, but just the < character.
• A JSP tag is somewhat like an HTML tag. JSP tags can have a "start tag", a
"tag body" and an "end tag".
• The start and end tag both use the tag name, enclosed in < and > characters.
• The tag names have an embedded colon character : in them, the part before
the colon describes the type of the tag. For instance:
<some:tag>
body
</some:tag>
• If the tag does not require a body, the start and end can be conveniently
merged together, as
<some:tag/>
24
JSP Tags
• Tags can be of two types: loaded from an external tag library, or predefined tags.
• Predefined tags start with jsp: characters.
• For instance, jsp:include is a predefined tag that is used to include other pages.
• We have already seen the include directive. jsp:include is similar.
• But instead of loading the text of the included file in the original file, it actually calls the included
target at run-time (the way a browser would call the included target.
• In practice, this is actually a simulated request rather than a full round-trip between the browser
and the server).
• Following is an example of jsp:include usage
<HTML>
<BODY>
Going to include hello.jsp...<BR>
<jsp:include page="hello.jsp"/>
</BODY>
</HTML>
25
JSP Tags
• Try it and see what you get by changing the "jsp:include" to
"jsp:forward" and see what is the difference.
• These two predefined tags are frequently very useful.
26
JSP Sessions
• On a typical web site, a visitor might visit several pages and perform
several interactions.
• If you are programming the site, it is very helpful to be able to
associate some data with each visitor. For this purpose, "session"s
can be used in JSP.
• A session is an object associated with a visitor. Data can be put in the
session and retrieved from it, much like a Hashtable. A different set
of data is kept for each visitor to the site.
• Here is a set of pages that put a user's name in the session, and
display it elsewhere. Try out installing and using these.
• First we have a form, let us call it GetName.html
27
JSP Sessions
<HTML>
<BODY>
<FORM METHOD=POST ACTION="SaveName.jsp">
What's your name? <INPUT TYPE=TEXT NAME=username SIZE=20>
<P><INPUT TYPE=SUBMIT>
</FORM>
</BODY>
</HTML>
• The target of the form is "SaveName.jsp", which saves the user's name in
the session.
• Variable “session" is another variable that is normally made available in
JSPs, just like out and request variables.
28
JSP Sessions
<%
String name = request.getParameter( "username" );
session.setAttribute( "theName", name );
%>
<HTML>
<BODY>
<A HREF="NextPage.jsp">Continue</A>
</BODY>
</HTML>
• The SaveName.jsp saves the user's name in the session, and puts a
link to another page, NextPage.jsp.
29
JSP Sessions
• NextPage.jsp shows how to retrieve the saved name.
<HTML>
<BODY>
Hello, <%= session.getAttribute( "theName" ) %>
</BODY>
</HTML>
• If you bring up two different browsers (not different windows of the same
browser), or run two browsers from two different machines, you can put one
name in one browser and another name in another browser, and both
names will be kept track of.
• The session is kept around until a timeout period. Then it is assumed the
user is no longer visiting the site, and the session is discarded.
30
JavaBeans Classes
• JSTL Core actions are designed to be used for
simple, presentation-oriented programming
tasks
• More sophisticated programming tasks should
still be performed with a language such as Java
• JavaBeans technology allows a JSP document
to call Java methods
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JavaBeans Classes
• Using a JavaBeans class in JSP
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JavaBeans Classes
• Using a JavaBeans class as shown:
– Class must have a default (no-argument)
constructor to be instantiated by useBean
• Automatically supplied by Java in this example
– Class should belong to a package (avoids need for
an import)
• This class would go in WEB-INF/classes/my/
directory
• Instance of a JavaBeans class is a bean
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JavaBeans Classes
• Simple property design patterns
– Two types: getter and setter
• Both require that the method be public
• getter:
– no arguments
– returns a value
– name begins with get (or is, if return type is boolean)
followed by upper case letter
• setter:
– one argument (same type as setter return value)
– void
– name begins with set followed by upper case letter
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JavaBeans Classes
• Simple property design pattern methods
associate bean properties with beans
– Name of bean property obtained by removing
get/is/set method prefix and following the rule:
• If remaining name begins with two or more upper case
letters, bean property name is remaining name:
setAValue() AValue
• If remaining name begins with a single upper case letter,
bean property name is remaining name with this letter
converted to lower case:
getWelcome() welcome
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Beans
• Forms are a very common method of interactions in web sites. JSP makes forms
processing specially easy.
• The standard way of handling forms in JSP is to define a "bean". This is not a full Java
bean.
• You just need to define a class that has a field corresponding to each field in the form.
• The class fields must have "setters" that match the names of the form fields.
• For instance, let us modify our GetName.html to also collect email address and age.
36
JSP Beans
• The new version of GetName.html is
<HTML>
<BODY>
<FORM METHOD=POST ACTION="SaveName.jsp">
What's your name? <INPUT TYPE=TEXT NAME=username SIZE=20><BR>
What's your e-mail address? <INPUT TYPE=TEXT NAME=email SIZE=20><BR>
What's your age? <INPUT TYPE=TEXT NAME=age SIZE=4>
<P><INPUT TYPE=SUBMIT>
</FORM>
</BODY>
</HTML>
37
JSP Beans
• To collect this data, we define a Java class with fields "username", "email"
and "age" and we provide setter methods "setUsername", "setEmail" and
"setAge", as shown.
• A "setter" method is just a method that starts with "set" followed by the
name of the field.
• The first character of the field name is upper-cased.
• So if the field is "email", its "setter" method will be "setEmail".
• Getter methods are defined similarly, with "get" instead of "set". Note that
the setters (and getters) must be public.
38
JSP Beans
package user;
public class UserData {
String username;
String email;
int age;
public void setUsername( String value )
{
username = value;
}
public void setEmail( String value )
{
email = value;
}
public void setAge( int value )
{
age = value;
}
public String getUsername() { return username; }
public String getEmail() { return email; }
public int getAge() { return age; }
}
39
JSP Beans
• Now let us change "SaveName.jsp" to use a bean to collect the data.
<jsp:useBean id="user" class="user.UserData" scope="session"/>
<jsp:setProperty name="user" property="*"/>
<HTML>
<BODY>
<A HREF="NextPage.jsp">Continue</A>
</BODY>
</HTML>
• All we need to do now is to add the jsp:useBean tag and the jsp:setProperty tag!
• The useBean tag will look for an instance of the "user.UserData" in the session. If the instance is
already there, it will update the old instance.
• Otherwise, it will create a new instance of user.UserData (the instance of the user.UserData is
called a bean), and put it in the session.
• The setProperty tag will automatically collect the input data, match names against the bean
method names, and place the data in the bean!
40
JSP Beans
• Let us modify NextPage.jsp to retrieve the data from bean..
<jsp:useBean id="user" class="user.UserData" scope="session"/>
<HTML>
<BODY>
You entered<BR>
Name: <%= user.getUsername() %><BR>
Email: <%= user.getEmail() %><BR>
Age: <%= user.getAge() %><BR>
</BODY>
</HTML>
41
JSP Beans
• Notice that the same useBean tag is repeated.
• The bean is available as the variable named "user" of class "user.UserData".
• The data entered by the user is all collected in the bean.
• We do not actually need the "SaveName.jsp", the target of GetName.html
could have been NextPage.jsp, and the data would still be available the same
way as long as we added a jsp:setProperty tag.
42
JSP and Servlets
• JSP documents are not executed directly
– When a JSP document is first visited, Tomcat
1. Translates the JSP document to a servlet
2. Compiles the servlet
– The servlet is executed
• Exceptions provide traceback information for
the servlet, not the JSP
– The servlets are stored under Tomcat work
directory
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP and Servlets
• A JSP-generated servlet has a
_jspService() method rather than
doGet() or doPost()
– This method begins by automatically creating a
number of implicit object variables that can be
accessed by scriptlets
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Web Applications
• A web application is a collection of resources
that are used together to implement some
web-based functionality
• Resources include
– Components: servlets (including JSP-generated)
– Other resources: HTML documents, style sheets,
JavaScript, images, non-servlet Java classes, etc.
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Web Applications
• Sharing data between components of a web
application
– Tomcat creates one ServletContext object
per web application
– Call to getServletContext() method of a
servlet returns the associated ServletContext
– ServletContext supports
setAttribute()/getAttribute()
methods
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Web Applications
• Within Tomcat, all of the files of a simple web
app are placed in a directory under webapps
– JSP documents can go in the directory itself
– “Hidden” files--such as servlet class files--go under
a WEB-INF subdirectory (more later)
• Once the web app files are all installed, use
Tomcat Manager to deploy the app
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Web Applications
• There are four URL patterns (from high to low
precedence)
• If no URL pattern matches, Tomcat treats path as
a relative file name
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Web Applications
• Methods on request object for obtaining
path information:
– Example:
/HelloCounter/visitor/test.jsp
– getContextPath(): returns
/HelloCounter
– getServletPath(): returns /visitor
– getPathInfo(): returns /test.jsp
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Expression Language (EL)
• ${visits+1} is an example of an EL
expression embedded in a JSP document
– ${…} is the syntax used in JSP documents to mark
the contained string as an EL expression
– An EL expression can occur
• In template data: evaluates to Java String
• As (part of) the value of certain JSP attributes: evaluates
to data type that depends on context
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Expression Language (EL)
• EL literals:
– true, false
– decimal integer, floating point, scientific-notation
numeric literals
– strings (single- or double-quoted)
– null
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Expression Language (EL)
• EL variable names: like Java
– Can contain letters, digits, _ , and $
– Must not begin with a digit
– Must not be reserved:
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Expression Language (EL)
• EL operators:
– Relational: <, >, <=, >=, ==, !=
• Or equivalents: lt, gt, le, ge, eq, ne
– Logical: &&, ||, !
• Or equivalents: and, or, not
– Arithmetic:
• +, - (binary and unary), *
• /, % (or div, mod)
– empty: true if arg is null or empty
string/array/Map/Collection
– Conditional: ? :
– Array access: [ ] (or object notation)
– Parentheses for grouping
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• Three types of markup elements:
– Scripting
• Ex: scriptlet
• Inserts Java code into servlet
– Directive
• Ex: directive.page
• Instructs JSP translator
– Action
• Standard: provided by JSP itself
• Custom: provided by a tag library such as JSTL
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• Two JSPX directives
– directive.page; some attributes:
• contentType
• session: false to turn off use of session object
• errorPage: component that will generate response if
an exception is thrown
• isErrorPage: true to access EL implicit exception
object
– directive.include: import well-formed XML
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• Common variables:
– var
• Name of a scoped variable that is assigned to by the
action
• Must be a string literal, not an EL expression
– scope
• Specifies scope of scoped variable as one of the literals
page, request, session, or application
• page default scope, unless otherwise specified
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• set action
– Setting (and creating) a scoped variable
– Setting/creating an element of Map
Map
Key
• Actually, this fails at run time in JWSDP 1.3 (which treats EL
implicit object Maps as read-only)
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• remove action
– Only attributes are var and scope
– Removes reference to the specified scoped
variable from the scope object
<c:remove var=“visits”
scope=“application” />
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• out action
– Normally used to write a string to the out JSP
implicit object
– Automatically escapes all five XML special
characters
– If value is null output is empty string
• Override by declaring default attribute
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• url action
– value attribute value is a URL to be written to
the out JSP implicit object
– URL’s beginning with / are assumed relative to
context path
– param elements can be used to define
parameters that will be URL encoded
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• Alternative to the value attribute (set and
param elements)
– If element has content, this is processed to
produce a String used for value
– Even out element will produce string, not write to
the out object
Assigns value of variable
messy (XML escaped) to
scoped variable clean
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• if action
– General form includes scoped variable to receive
test value
Assigned Boolean value
of test attribute
• The element can be empty if var is present
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• choose action
<c:choose>
<c:when test=“${visits eq 1}”>
Hi!</c:when>
<c:when test=“${visits eq 2}”>
Welcome back!</c:when>
<c:otherwise>
You’re a regular!</c:otherwise>
</c:choose>
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• forEach action
– Used to increment a variable:
– Used to iterate over a data structure:
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
JSP Markup
• forEach action
– Can iterate over array, Map, Collection,
Iterator, Enumeration
– Elements of Map are Map.Entry, which support
key and value EL properties:
Tag Libraries
• Wouldn’t it be nicer to write the mortgage app
as
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Tag Libraries
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
Tag Libraries
• Place custom tag definition in a tag file having
the name of the custom action
– mortgage.tagx
• Place tag file in a tag library (e.g., directory
containing tag files)
– /WEB-INF/tags
• Add namespace declaration for tag library
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
MVC
• Many web apps are based on the Model-ViewController (MVC) architecture pattern
Model Components
View
Controller
HTTP
request
HTTP
response
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
MVC
• Typical JSP implementation of MVC
Model Components
(beans, DBMS)
Controller
(Java servlet)
HTTP
request
View
(JSP document)
HTTP
response
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
MVC
• Forwarding an HTTP request from a servlet to
another component:
– By URL
Ex: /HelloCounter.jspx
– By name
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
MVC
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
MVC
• How does the controller know which
component to forward to?
– getPathInfo() value of URL’s can be used
– Example:
• servlet mapping pattern in web.xml:
• URL ends with:
• getPathInfo() returns:
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
MVC
• JSP include action
Execute specified
component and
include its output
in place of the
include element
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
MVC
• Adding parameters to the request object
seen by an included component:
request object seen by navbar.jspx will include
parameter named currentPage with value home
Jackson, Web Technologies: A Computer Science Perspective, © 2007 Prentice-Hall, Inc. All rights reserved. 0-13-185603-0
