JSP Directives

JSP directives provide directions and instructions to the container, telling it how to handle certain aspects of JSP processing.

There are three directives

1. page Directive
2. include Directive
3. taglib Directive

1. page Directive

Defines attributes that apply to an entire JSP page.

Syntax 
<%@ page

[ language="java" ]
[extends="package.class" ]
[ import="{package.class|package.*}, ..." ]
[ session="true| false" ]
[ buffer="none |8kb|sizekb" ]
[ autoFlush="true| false" ]
[ isThreadSafe="true| false" ]
[ info="text" ]
[ errorPage="relativeURL" ]
[ contentType="mimeType[ ;charset=characterSet]" | "text/html ; charset=ISO-8859-1" ]
[ isErrorPage="true |false" ]
%>

Example :

<%@ page import="java.util.*,java.lang.*" %>

<%@ page buffer="5kb" autoFlush="false" %>

<%@ page errorPage="error.jsp" %>

<%@ page language="java" contentType="text/html; charset=UTF-8"

pageEncoding="UTF-8" import="java.util.*" isErrorPage="true"%>

Description

The <%@ page %> directive applies to an entire JSP file and any of its static include files, which together are called a translation unit. A static include file is a file whose content becomes part of the calling JSP file. The <%@ page %> directive does not apply to any dynamic include files.

You can use the <%@ page %> directive more than once in a translation unit, but you can only use each attribute, except import, once. Because the import attribute is similar to the import statement in the Java programming language, you can use a <%@ page %> directive with import more than once in a JSP file or translation unit.

No matter where you position the <%@ page %> directive in a JSP file or included files, it applies to the entire translation unit. However, it is often good programming style to place it at the top of the JSP file.

Attributes :

• language="java"

The scripting language used in scriptlets, declarations, and expressions in the JSP file and any included files. In this release, the only allowed value is java.

• extends="package.class"

The fully qualified name of the superclass of the Java class file this JSP file will be compiled to. Use this attribute cautiously, as it can limit the JSP container's ability to provide a specialized superclass that improves the quality of the compiled file.

• import="{package.class|package.*}, ..."

A comma-separated list of Java packages that the JSP file should import. The packages (and their classes) are available to scriptlets, expressions, and declarations within the JSP file. If you want to import more than one package, you can specify a comma-separated list after import or you can use import more than once in a JSP file.

The following packages are implicitly imported, so you don't need to specify them with the import attribute:

java.lang.*
javax.servlet.*
javax.servlet.jsp.*
javax.servlet.http.*

You must place the import attribute before the element that calls the imported class.

• session="true | false"

Whether the client must join an HTTP session in order to use the JSP page. If the value is true, the session object refers to the current or new session.

If the value is false, you cannot use the session object or a <jsp:useBean>element with scope=session in the JSP file. Either of these usages would cause a translation-time error.

The default value is true.

• buffer="none | 8kb | sizekb"

The buffer size in kilobytes used by the out object to handle output sent from the compiled JSP page to the client Web browser. The default value is 8kb. If you specify a buffer size, the output is buffered with at least the size you specified.

• autoFlush="true | false"

Whether the buffered output should be flushed automatically when the buffer is full. If set to true (the default value), the buffer will be flushed. If set to false, an exception will be raised when the buffer overflows. You cannot set autoFlush to false when buffer is set to none.

• isThreadSafe="true | false"

Whether thread safety is implemented in the JSP file. The default value is true, which means that the JSP container can send multiple, concurrent client requests to the JSP page. You must write code in the JSP page to synchronize the multiple client threads. If you use false, the JSP container sends client requests one at a time to the JSP page.

• info="text"

A text string that is incorporated verbatim into the compiled JSP page. You can later retrieve the string with the Servlet.getServletInfo() method.

• errorPage="relativeURL"

A pathname to a JSP file that this JSP file sends exceptions to. If the pathname begins with a /, the path is relative to the JSP application's document root directory and is resolved by the Web server. If not, the pathname is relative to the current JSP file.

• isErrorPage="true |false"

Whether the JSP file displays an error page. If set to true, you can use the exception object in the JSP file. If set to false (the default value), you cannot use the exception object in the JSP file.

• contentType="mimeType [ ;charset=characterSet ]" | "text/html;charset=ISO-8859-1"

The MIME type and character encoding the JSP file uses for the response it sends to the client. You can use any MIME type or character set that are valid for the JSP container. The default MIME type istext/html, and the default character set is ISO-8859-1.

Example :

<%@page import="java.text.NumberFormat"%>

<%@ page language="java" contentType="text/html; charset=UTF-8"

         pageEncoding="UTF-8" import="java.util.*" isErrorPage="false"

         isThreadSafe="true" buffer="10kb" autoFlush="true"

         errorPage="error.jsp" isELIgnored="false"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>Insert title here</title>

</head>

<body>

<%=new java.util.Date()%>

<%

      NumberFormat number = NumberFormat.getNumberInstance();

      number.setMaximumIntegerDigits(5);

      String value = number.format(999999);

%>

        <%=value%>

</body>

</html>

2. include Directive

Includes a static file in a JSP file, parsing the file's JSP elements.

Syntax : <%@ include file="relativeURL" %>

Description

The <%@ include %> directive inserts a file of text or code in a JSP file at translation time, when the JSP file is compiled. When you use the <%@ include %> directive, the include process is static. A static include means that the text of the included file is added to the JSP file. The included file can be a JSP file, HTML file, or text file. If the included file is a JSP file, its JSP elements are translated and included (along with any other text) in the JSP file. Once the included file is translated and included, the translation process resumes with the next line of the including JSP file.

The included file can be an HTML file, a JSP file, a text file, or a code file written in the Java programming language. Be careful that the included file does not contain <html>,</html>, <body>, or </body> tags. Because the entire content of the included file is added to the including JSP file, these tags would conflict with the same tags in the including JSP file, causing an error.

Some of the behaviors of the <%@ include %> directive depend on the particular JSP container you are using, for example:

• The included file might be open and available to all requests, or it might have security restrictions.
• The JSP page might be recompiled if the included file changes.

Example :

dateformat.jsp

<%@page import="java.util.Date"%>

<%@page import="java.text.SimpleDateFormat"%>

<%

           Date d = new Date();

           SimpleDateFormat date = new SimpleDateFormat("dd-MM-yyyy");

           String data = date.format(d);

%>

           Today date is :<%=data%>

           Thanks for visting

index.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"

pageEncoding="UTF-8" isErrorPage="false" isThreadSafe="true"

buffer="10kb" autoFlush="true" errorPage="error.jsp"

isELIgnored="false"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>Insert title here</title>

</head>

<body>

         <h3>Welcome to JSL Tech</h3>

<center>

         <%@include file="dateformat.jsp"%>

</center>

</body>

</html>

3. taglib Directive

Defines a tag library and prefix for the custom tags used in the JSP page.

Syntax : <%@ taglib uri="URIToTagLibrary" prefix="tagPrefix" %>

Examples 

1.   <%@ taglib uri="http://www.jspcentral.com/tags" prefix="public"%>
2. <public:loop>
3.      -
4. </public:loop>
5. <%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
6. <%@taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt"%>

The <%@ taglib %> directive declares that the JSP file uses custom tags, names the tag library that defines them, and specifies their tag prefix.

Exception Handling

We can specify exceptions in JSP using page directive and errorPage attribute.

A jsp page has a default implicit Object named exception and this object is an instance of java.lang.Throwable.

As in servlets, we can handle exceptions in jsp by using try/catch block or by forwarding the request to an error page when an uncaught exception occurs. The exception object is only available to pages that have isErrorPage set to true with the directive <%@ page isErrorPage='true' %>. The exception object refers to the exception triggered by the page that uses this page as an exception handler.

The following code demonstrates the use of the exception implicit object. The first page uses the errorPage directive to set up the JSP page to use when an error occurs, and the second page called ErrorPage.jsp uses the isErrorPage directive to set itself up to catch the error.

Examples

index.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"

         pageEncoding="UTF-8" isErrorPage="false" isThreadSafe="true"

         buffer="10kb" autoFlush="true" errorPage="error.jsp"

         isELIgnored="false"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

         <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

         <title>Insert title here</title>

</head>

<body>

<%

             int a = 100, b = 10;

             int res = a / b;

             String name = "Miani";

%>

             The result is :<%=res%>

             The name is :<%=name.substring(9)%>

</body>

</html>

error.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"

pageEncoding="UTF-8" isErrorPage="true"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

       <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>Insert title here</title>

</head>

<body>

               <%=exception%>

</body>

</html>

web.xml

We can also handle exception in deployment descriptor file by specifying the error page locations for specific exception-types.

Syntax :

<error-page>

      <exception-type> </exception-type>

      <location> </location>

</error-page>