- JSP directives
1. page directive
2. Attributes of page directive
The jsp directives are messages that tells the web
container how to translate a JSP page into the corresponding servlet.
There are three types of
directives:
- page
directive
- include
directive
- taglib
directive
Syntax of JSP Directive
1. <%@ directive attribute="value" %>
JSP
page directive
The page directive defines attributes that apply to an entire
JSP page.
Syntax
of JSP page directive
1.
<%@ page attribute="value" %>
Attributes
of JSP page directive
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·
import
·
contentType
·
extends
·
info
·
buffer
·
language
·
isELIgnored
·
isThreadSafe
·
autoFlush
·
session
·
pageEncoding
·
errorPage
·
isErrorPage
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1)import
|
The import attribute is used to import class,interface or all
the members of a package.It is similar to import keyword in java class or
interface.
|
Example of import attribute
1.
<html>
2.
<body>
3.
4.
<%@ page import="java.util.Date" %>
5.
Today is: <%= new Date() %>
6.
7.
</body>
8.
</html>
2)contentType
|
The contentType attribute defines the MIME(Multipurpose Internet
Mail Extension) type of the HTTP response.The default value is
"text/html;charset=ISO-8859-1".
|
Example of contentType
attribute
1.
<html>
2.
<body>
3.
4.
<%@ page contentType=application/msword %>
5.
Today is: <%= new java.util.Date() %>
6.
7.
</body>
8.
</html>
3)extends
|
The extends attribute defines the parent class that will be
inherited by the generated servlet.It is rarely used.
|
4)info
|
This attribute simply sets the information of the JSP page which
is retrieved later by using getServletInfo() method of Servlet interface.
|
Example of info attribute
1. <html>
2. <body>
3.
4. <%@ page info="composed by Sonoo Jaiswal" %>
5. Today is: <%= new java.util.Date() %>
6.
7. </body>
8. </html>
|
The web
container will create a method getServletInfo() in the resulting servlet.For
example:
|
1. public String getServletInfo() {
2. return "composed by Sonoo Jaiswal";
3. }
5)buffer
|
The buffer attribute sets the buffer size in kilobytes to handle
output generated by the JSP page.The default size of the buffer is 8Kb.
|
Example of buffer attribute
1.
<html>
2.
<body>
3.
4.
<%@ page buffer="16kb" %>
5.
Today is: <%= new java.util.Date() %>
6.
7.
</body>
8.
</html>
6)language
|
The language attribute specifies the scripting language used in
the JSP page. The default value is "java".
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7)isELIgnored
|
We can ignore the Expression Language (EL) in jsp by the
isELIgnored attribute. By default its value is false i.e. Expression Language
is enabled by default. We see Expression Language later.
|
1.
<%@ page isELIgnored="true" %>//Now EL will be ignored
8)isThreadSafe
|
Servlet and JSP both are multithreaded.If you want to control
this behaviour of JSP page, you can use isThreadSafe attribute of page
directive.The value of isThreadSafe value is true.If you make it false, the
web container will serialize the multiple requests, i.e. it will wait until
the JSP finishes responding to a request before passing another request to
it.If you make the value of isThreadSafe attribute like:
|
<%@ page isThreadSafe="false" %>
|
The web container in such a case, will generate the servlet as:
|
1.
public class SimplePage_jsp extends HttpJspBase
2.
implements SingleThreadModel{
3.
.......
4.
}
9)errorPage
|
The errorPage attribute is used to define the error page, if
exception occurs in the current page, it will be redirected to the error
page.
|
Example of errorPage
attribute
1.
//index.jsp
2.
<html>
3.
<body>
4.
5.
<%@ page errorPage="myerrorpage.jsp" %>
6.
7.
<%= 100/0 %>
8.
9.
</body>
10. </html>
10)isErrorPage
|
The isErrorPage attribute is used to declare that the current
page is the error page.
|
Note: The exception object can only
be used in the error page.
Example of isErrorPage
attribute
1.
//myerrorpage.jsp
2.
<html>
3.
<body>
4.
5.
<%@ page isErrorPage="true" %>
6.
7.
Sorry an exception occured!<br>
8.
The exception is: <%= exception %>
9.
10. </body>
11. </html>
Include directive
|
The include
directive is used to include the contents of any resource it may be jsp file,
html file or text file. The include directive includes the original content
of the included resource at page translation time (the jsp page is translated
only once so it will be better to include static resource).
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Advantage of Include directive
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Code Reusability
|
Syntax
of include directive
1.
<%@ include file="resourceName" %>
Example of include directive
|
In this example, we are including the content of the header.html
file. To run this example you must create an header.html file.
|
1.
<html>
2.
<body>
3.
4.
<%@ include file="header.html" %>
5.
6.
Today is: <%= java.util.Calendar.getInstance().getTime() %>
7.
8.
</body>
9.
</html>
JSP Taglib directive
|
The JSP
taglib directive is used to define a tag library that defines many tags. We
use the TLD (Tag Library Descriptor) file to define the tags. In the custom
tag section we will use this tag so it will be better to learn it in custom
tag.
|
Syntax JSP Taglib directive
1.
<%@ taglib uri="uriofthetaglibrary" prefix="prefixoftaglibrary" %>
Example
of JSP Taglib directive
|
In this example, we are using our tag named currentDate. To use
this tag we must specify the taglib directive so the container may get
information about the tag.
|
1.
<html>
2.
<body>
3.
4.
<%@ taglib uri="http://www.javatpoint.com/tags" prefix="mytag" %>
5.
6.
<mytag:currentdate>
7.
8.
</body>
9.
</html>
10.
11. </mytag:currentdate>
Hidden Fields:
Hidden field
is part of HTML, but hidden field can be used with JSP. Hidden field can carry
hidden data from one page to another action page. Value of hidden field can be
get from request.getParameter method. Hidden field can be set from java data
types e.g. String, int, float.
Page1.jsp
<%@ page language="java" %>
<%
String hiddenFieldValue="MyHiddenValue";
%>
<html>
<head>
<title>Hidden fields in JSP</title>
</head>
<body>
<form name="frm" action="page2.jsp">
<input type="hidden" name="hdField" value="<%=hiddenFieldValue%>" />
<input type="text" name="txtField" />
<input type="submit" name="Submit" value="Submit">
</form>
</body>
</html>
page2.jsp
<%@ page language="java" %>
<%
String getHiddenValue=request.getParameter("hdField");
System.out.println("Hidden field Value :"+getHiddenValue);
%>
Displaying applet in JSP (jsp:plugin action
tag)
- Displaying applet in JSP
- Syntax of jsp:plugin action tag
- Example of displaying applet in JSP
|
The
jsp:plugin action tag is used to embed applet in the jsp file. The jsp:plugin
action tag downloads plugin at client side to execute an applet or bean.
|
Syntax of jsp:plugin action tag
<jsp:plugin type= "applet | bean" code= "nameOfClassFile"
codebase= "directoryNameOfClassFile"
</jsp:plugin>
Example of displaying applet in JSP
|
In this example, we are simply displaying applet in jsp using
the jsp:plugin tag. You must have MouseDrag.class file (an applet class file)
in the current folder where jsp file resides. You may simply download this
program that contains index.jsp, MouseDrag.java and MouseDrag.class files to
run this application.
|
index.jsp
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Mouse Drag</title>
</head>
<body bgcolor="khaki">
<h1>Mouse Drag Example</h1>
<jsp:plugin align="middle" height="500" width="500"
type="applet" code="MouseDrag.class" name="clock" codebase="."/>
</body>
</html>
Retrieving a Query String !
An include action executes the included JSP
page and appends the generated output onto its own output stream. Request
parameters parsed from the URL's query string are available not only to the
main JSP page but to all included JSP pages as well. It is possible to
temporarily override a request parameter or to temporarily introduce a new
request parameter when calling a JSP page. This is done by using the jsp:param
action.
In this example, param1 is specified in the
query string and is automatically made available to the callee JSP page. param2
is also specified in the query string but is overridden by the caller. Notice
that param2 reverts to its original value after the call. param3 is a new
request parameter created by the caller. Notice that param3 is only available
to the callee and when the callee returns, param3 no longer exists. Here is the
caller JSP page:
If the example is called with the URL:
http://hostname.com?param1=a¶m2=b
the output would be:
Troubleshooting !
Troubleshooting is a form of problem
solving most often applied to repair of failed products or processes. It is a
logical, systematic search for the source of a problem so that it can be
solved, and so the product or process can be made operational again.
Troubleshooting is needed to develop and maintain complex systems where the
symptoms of a problem can have many possible causes. Troubleshooting is used in
many fields such as engineering, system administration, electronics, automotive
repair, and diagnostic medicine. Troubleshooting requires identification of the
malfunction(s) or symptoms within a system. Then, experience is commonly used
to generate possible causes of the symptoms. Determining which cause is most
likely is often a process of elimination - eliminating potential causes of a
problem. Finally, troubleshooting requires confirmation that the solution
restores the product or process to its working state.
In general, troubleshooting is the
identification of, or diagnosis of "trouble" in a [system] caused by
a failure of some kind. The problem is initially described as symptoms of
malfunction, and troubleshooting is the process of determining the causes of
these symptoms.
A system can be described in terms of its
expected, desired or intended behavior (usually, for artificial systems, its
purpose). Events or inputs to the system are expected to generate specific
results or outputs. (For example selecting the "print" option from
various computer applications is intended to result in a hardcopy emerging from
some specific device). Any unexpected or undesirable behavior is a symptom.
Troubleshooting is the process of isolating the specific cause or causes of the
symptom. Frequently the symptom is a failure of the product or process to
produce any results. (Nothing was printed, for example).
Every web page resides
inside a browser window which can be considered as an object.
A Document object
represents the HTML document that is displayed in that window. The Document
object has various properties that refer to other objects which allow access to
and modification of document content.
The way that document
content is accessed and modified is called the Document
Object Model, or DOM. The
Objects are organized in a hierarchy. This hierarchical structure applies to
the organization of objects in a Web document.
- Window object: Top of the
hierarchy. It is the outmost element of the object hierarchy.
- Document object: Each HTML
document that gets loaded into a window becomes a document object. The
document contains the content of the page.
- Form object: Everything
enclosed in the <form>...</form> tags sets the form object.
- Form control elements: The form object
contains all the elements defined for that object such as text fields,
buttons, radio buttons, and checkboxes.
There are several DOMs in
existence. The following sections explain each of these DOMs in detail and
describe how you can use them to access and modify document content.
- The Legacy DOM: This is the
model which was introduced in early versions of JavaScript language. It is
well supported by all browsers, but allows access only to certain key
portions of documents, such as forms, form elements, and images.
- The W3C DOM: This document
object model allows access and modification of all document content and is
standardized by the World Wide Web Consortium (W3C). This model is
supported by almost all the modern browsers.
- The IE4 DOM: This document
object model was introduced in Version 4 of Microsoft's Internet Explorer
browser. IE 5 and later versions include support for most basic W3C DOM
features.
DOM compatibility
If you want to write a
script that uses the W3C DOM when it is available, and otherwise uses the IE 4
DOM if it is available, you can use a capability-testing approach that first
checks for the existence of a method or property to determine whether the
browser has the capability you desire. For example:
|
if (document.getElementById) {
// If
the W3C method exists, use it
}
else if (document.all) {
// If
the all[] array exists, use it
}
else {
//
Otherwise use the legacy DOM
}
|
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