A comment that is
sent to the client in the viewable page source.The JSP engine handles an output
comment as uninterpreted HTML text, returning the comment in the HTML output
sent to the client. You can see the comment by viewing the page source from your
Web browser.
JSP
Syntax <!-- comment [ <%= expression %> ]
--> Example 1 <!-- This is a commnet sent
to client on <%= (new java.util.Date()).toLocaleString()
%> --> Displays in the page
source: <!-- This is a commnet sent to client on January 24,
2004 -->
A comments that
documents the JSP page but is not sent to the client. The JSP engine ignores a
hidden comment, and does not process any code within hidden comment tags. A
hidden comment is not sent to the client, either in the displayed JSP page or
the HTML page source. The hidden comment is useful when you want to hide or
"comment out" part of your JSP page.
You can use any
characters in the body of the comment except the closing --%> combination. If
you need to use --%> in your comment, you can escape it by typing --%\>.
JSP Syntax <%-- comment
--%>
Examples <%@ page language="java"
%> <html> <head><title>A Hidden Comment
</title></head> <body> <%-- This comment will not be
visible to the colent in the page source
--%> </body> </html>
An
expression tag 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 text in a JSP file. Like <%= someexpression
%> <%= (new java.util.Date()).toLocaleString() %> You cannot use a semicolon
to end an expression
A declaration
declares one or more variables or methods for use later in the JSP source file.
A declaration must
contain at least one complete declarative statement. You can declare any number
of variables or methods within one declaration tag, as long as they are
separated by semicolons. The declaration must be valid in the scripting language
used in the JSP file.
<%! somedeclarations %> <%! int i = 0;
%> <%! int a, b, c; %>
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
1.Declare variables
or methods to use later in the file (see also Declaration).
2.Write
expressions valid in the page scripting language (see also Expression).
3.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.
Certain objects
that are available for the use in JSP documents without being declared first.
These objects are parsed by the JSP engine and inserted into the generated
servlet. The implicit objects re listed below
When you invoke a
forward request, the request is sent to another resource on the server, without
the client being informed that a different resource is going to process the
request. This process occurs completly with in the web container. When a
sendRedirtect method is invoked, it causes the web container to return to the
browser indicating that a new URL should be requested. Because the browser
issues a completly new request any object that are stored as request attributes
before the redirect occurs will be lost. This extra round trip a redirect is
slower than forward.
THe generated
servlet class for a JSP page implements the HttpJspPage interface of the
javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage
interface which inturn extends the Servlet interface of the javax.servlet
package. the generated servlet class thus implements all the methods of the
these three interfaces. The JspPage interface declares only two mehtods -
jspInit() and jspDestroy() that must be implemented by all JSP
pages regardless of the client-server protocol. However the JSP specification
has provided the HttpJspPage interfaec specifically for the JSp pages serving
HTTP requests. This interface declares one method _jspService().
The jspInit()- The container calls the jspInit() to initialize te servlet
instance.It is called before any other method, and is called only once for a
servlet instance. The _jspservice()- The container calls the _jspservice()
for each request, passing it the request and the response objects. The
jspDestroy()- The container calls this when it decides take the instance out of
service. It is the last method called n the servlet
instance.
