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jsp : JSP directive Tags

JSP directives

 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

 <%@ directive attribute="value" %> 

 SP page directive

 The page directive defines attributes that apply to an entire JSP page.

 Syntax of JSP page directive

 <%@ page attribute="value" %>  

 Attributes of JSP page directive

 import
contentType
extends
info
buffer
language
isELIgnored
isThreadSafe
autoFlush
session
pageEncoding
errorPage
isErrorPage

 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

 <html>  
<body>  
  
<%@ page import="java.util.Date" %>  
Today is: <%= new Date() %>  
  
</body>  
</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

 <html>  
<body>  
  
<%@ page contentType="application/msword" %>  
Today is: <%= new java.util.Date() %>  
  
</body>  
</html> 

 3)extends

 The extends attribute defines the parent class that will be inherited by the generated servlet.It is rarely used.

 4)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

 <html>  
<body>  
  
<%@ page buffer="16kb" %>  
Today is: <%= new java.util.Date() %>  
  
</body>  
</html>  

 6)language

 The language attribute specifies the scripting language used in the JSP page. The default value is "java".

 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.
<%@ 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:

 public class SimplePage_jsp extends HttpJspBase   
  implements SingleThreadModel{  
.......  
} 

 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

 //index.jsp  
<html>  
<body>  
  
<%@ page errorPage="myerrorpage.jsp" %>  
  
 <%= 100/0 %>  
  
</body>  
</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

 //myerrorpage.jsp  
<html>  
<body>  
  
<%@ page isErrorPage="true" %>  
  
 Sorry an exception occured!<br/>  
The exception is: <%= exception %>  
  
</body>  
</html>  

 Jsp 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).

 Advantage of Include directive

 Code Reusability

 Syntax of include directive

 <%@ 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.

 <html>  
<body>  
  
<%@ include file="header.html" %>  
  
Today is: <%= java.util.Calendar.getInstance().getTime() %>  
  
</body>  
</html>  

 Note: The include directive includes the original content, so the actual page size grows at runtime.

 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

 <%@ 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.

 <html>  
<body>  
  
<%@ taglib uri="WEB-INF/mytags.tld" prefix="mytags" %>  
  
<mytags:today> 
  
</body>  
</html>  

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