Architecturally, JSP may be viewed as a high-level abstraction
of Java servlets. JSPs are translated into servlets at runtime; each JSP's
servlet is cached and re-used until the original JSP is modified.
JSP can be used independently or as the view component of a
server-side model–view–controller design, normally with JavaBeans as the model
and Java servlets (or a framework such as Apache Struts) as the controller.
This is a type of Model 2 architecture.
JSP allows Java code and certain pre-defined actions to be
interleaved with static web markup content, with the resulting page being
compiled and executed on the server to deliver a document. The compiled pages,
as well as any dependent Java libraries, use Java bytecode rather than a native
software format. Like any other Java program, they must be executed within a Java
virtual machine (JVM) that integrates with the server's host operating system
to provide an abstract platform-neutral environment.
JSP pages are usually used to deliver HTML and XML
documents, but through the use of OutputStream, they can deliver other types of
data as well.
Lifecycle
of JSP (Java Server Pages)
1. JSP Page Translation:
A java
servlet file is generated from the JSP source file. This is the first step in
its tedious multiple phase life cycle. In the translation phase, the container
validates the syntactic correctness of the JSP pages and tag files. The
container interprets the standard directives and actions, and the custom
actions referencing tag libraries used in the page.
2. JSP Page Compilation:
The generated java servlet file is compiled into a java
servlet class.
Note: The translation of a JSP source page into its
implementation class can happen at any time between initial deployment of the
JSP page into the JSP container and the receipt and processing of a client
request for the target JSP page.
3. Class Loading:
The java servlet class that was compiled from the JSP
source is loaded into the container.
4. Execution phase:
In the execution phase the container manages one or more
instances of this class in response to requests and other events.
The interface JspPage contains jspInit() and jspDestroy(). The JSP
specification has provided a special interface HttpJspPage for JSP pages
serving HTTP requests and this interface contains _jspService().
5. Initialization:
jspInit() method is called immediately after the instance
was created. It is called only once during JSP life cycle.
6. _jspService() execution:
This method is called for every request of this JSP during
its life cycle. This is where it serves the purpose of creation. Oops! it has
to pass through all the above steps to reach this phase. It passes the request
and the response objects. _jspService() cannot be overridden.
7. jspDestroy() execution:
This method is called when this JSP is destroyed. With this
call the servlet serves its purpose and submits itself to heaven (garbage
collection). This is the end of jsp life cycle.
jspInit(), _jspService() and jspDestroy() are called the
life cycle methods of the JSP.
JSP
basic Syntax
The
Scriptlet
A scriptlet can contain any number of JAVA language
statements , variable or method declarations, or expressions that are valid in
the page scripting language. i.e. Scriplet contains java business logic
<% Business Login in Java
%>
Any text, HTML tags, or JSP elements you write must be
outside the scriptlet. Following is the simple and first example for JSP:
<html>
<head><title>Hello World</title></head>
<body>
Hello World!<br/>
<%
out.println("Your IP address is " + request.getRemoteAddr());
%>
</body>
</html>
This would generate following result:Hello World!
Your IP address is 127.0.0.1
NOTE: Assuming that Apache
Tomcat is installed in C:\apache-tomcat-7.0.2 and your environment is setup as
per environment setup tutorial.
JSP
Declarations:
A declaration declares one or more variables or methods
that you can use in Java code later in the JSP file. You must declare the
variable or method before you use it in the JSP file.
<%! declaration; [ declaration; ]+ ... %>
Following is the simple example for JSP Comments:<%! int i = 0; %>
<%! int a, b, c; %>
<%! Circle a = new Circle(2.0); %>
JSP
Expression:
A JSP expression element contains a scripting language
expression that is evaluated, converted to a String, and inserted where the
expression appears in the JSP file.
Because the value of an expression is converted to a
String, you can use an expression within a line of text, whether or not it is
tagged with HTML, in a JSP file.
The expression element can contain any expression that is
valid according to the Java Language Specification but you cannot use a
semicolon to end an expression.
Following is the syntax of JSP Expression:
<%= expression %>
Following is the simple example for JSP Expression:
<html>
<head><title>A Comment Test</title></head>
<body>
<p>
Today's date: <%= (new java.util.Date()).toLocaleString()%>
</p>
</body>
</html>
This would generate following result:
Today's date: 01-Jan-2013 00:00:00
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