JSP - Directives

In this chapter, we will discuss Directives in JSP. These directives provide directions and instructions to the container, telling it how to handle certain aspects of the JSP processing.
A JSP directive affects the overall structure of the servlet class. It usually has the following form −
<%@ directive attribute = "value" %>

Directives can have a number of attributes which you can list down as key-value pairs and separated by commas.
The blanks between the @ symbol and the directive name, and between the last attribute and the closing %>, are optional.
There are three types of directive tag −
<%@ page ... %>
Defines page-dependent attributes, such as scripting language, error page, and buffering requirements.
<%@ include ... %>
Includes a file during the translation phase.
<%@ taglib ... %>
Declares a tag library, containing custom actions, used in the page
The page directive is used to provide instructions to the container. These instructions pertain to the current JSP page. You may code page directives anywhere in your JSP page. By convention, page directives are coded at the top of the JSP page.
Following is the basic syntax of the page directive −
<%@ page attribute = "value" %>

You can write the XML equivalent of the above syntax as follows −
<jsp:directive.page attribute = "value" />

Following table lists out the attributes associated with the page directive −
buffer
Specifies a buffering model for the output stream.
autoFlush
Controls the behavior of the servlet output buffer.
contentType
Defines the character encoding scheme.
errorPage
Defines the URL of another JSP that reports on Java unchecked runtime exceptions.
isErrorPage
Indicates if this JSP page is a URL specified by another JSP page's errorPage attribute.
extends
Specifies a superclass that the generated servlet must extend.
import
Specifies a list of packages or classes for use in the JSP as the Java import statement does for Java classes.
info
Defines a string that can be accessed with the servlet's getServletInfo() method.
isThreadSafe
Defines the threading model for the generated servlet.
language
Defines the programming language used in the JSP page.
session
Specifies whether or not the JSP page participates in HTTP sessions
isELIgnored
Specifies whether or not the EL expression within the JSP page will be ignored.
isScriptingEnabled
Determines if the scripting elements are allowed for use.
Check for more details related to all the above attributes at Page Directive.
The include directive is used to include a file during the translation phase. This directive tells the container to merge the content of other external files with the current JSP during the translation phase. You may code the include directives anywhere in your JSP page.
The general usage form of this directive is as follows −
<%@ include file = "relative url" >

The filename in the include directive is actually a relative URL. If you just specify a filename with no associated path, the JSP compiler assumes that the file is in the same directory as your JSP.
You can write the XML equivalent of the above syntax as follows −
<jsp:directive.include file = "relative url" />

For more details related to include directive, check the Include Directive.
The JavaServer Pages API allow you to define custom JSP tags that look like HTML or XML tags and a tag library is a set of user-defined tags that implement custom behavior.
The taglib directive declares that your JSP page uses a set of custom tags, identifies the location of the library, and provides means for identifying the custom tags in your JSP page.
The taglib directive follows the syntax given below −
<%@ taglib uri="uri" prefix = "prefixOfTag" >

Here, the uri attribute value resolves to a location the container understands and the prefix attribute informs a container what bits of markup are custom actions.
You can write the XML equivalent of the above syntax as follows −
<jsp:directive.taglib uri = "uri" prefix = "prefixOfTag" />

For more details related to the taglib directive, check the Taglib Directive.

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