CSC 2720Building Web Applications

JavaServer Pages (JSP)JSP Directives and Action Elements

JSP Directives page directives – defines attributes that apply to an entire

JSP page

include directives – includes a resource of text or code when the JSP page is translated

taglib directives – defines a tag library and prefix for the custom tags used in the JSP page

Syntax: <%@ directive attribute="value" %> <%@ directive attribute1="value1" attribute2="value2"

... %>where directive can be page, include, or taglib

page Directives Defines attributes that apply to an entire JSP page.

Which classes are imported

For examples, To specify a different superclass for the current JSP

page To specify the MIME type of the generated content To indicate if the JSP page is thread safe or not To turn on/off automatic session participation To change the size and behavior of the output buffer To indicate which page is to be used to handle

unexpected errors/exceptions

page Directives – import Syntax

<%@ page import="package.class" %> <%@ page import="package1.class1,…,packageM.classN" %>

Note: To import all classes from a package, use package.*

Purpose Generate import statements at for the resulting servlet class

Note: Class files should be placed under the folder …/Web_App_Name/WEB-INF/classesand in the appropriate sub-folders that match the package name.

.jar library files should be placed under the folder…/Web_App_Name/WEB-INF/lib

page Directives – contentType Syntax

<%@ page contentType="MIME-Type" %>

Purpose Specify the MIME type of the content generated by the JSP page Default value is "text/html;charset=ISO-8859-1"

e.g.: HTML content encoded in simplified Chinese characters <%@ page contentType="text/html;charset=GB2312">

References Wiki: MIME: http://en.wikipedia.org/wiki/MIME MIME reference: http://www.w3schools.com/media/media_mimeref.asp Character set reference: http://www.iana.org/assignments/character-sets

<%@ page contentType="application/vnd.ms-excel" %>

First Last Email Address

Marty Hall hall@corewebprogramming.com

Larry Brown brown@corewebprogramming.com

Bill Gates gates@sun.com

Larry Ellison ellison@microsoft.com

<%-- Note: The values are separated by tabs and not by spaces

--%>

Generating Excel Spreadsheets

Other Attributes of the page Directive session

Lets you turn on/off session participation Default value is "true" e.g.: <%@ page session="false" %>

extends Changes parent class of the resulting servlet

isThreadSafe Lets you specify whether the JSP page is thread safe Default value is "true" Setting this attribute to "false" makes the resulting servlet a single-

threaded servlet e.g.: <%@ page isThreadSafe="false" %>

Other Attributes of the page Directive language

Let you specify the scripting language Default value is "java"

info Allows you to insert a string that later can be retrieved using the getServletInfo() method.

e.g.: <%@ page info="Written by XYZ" %>

buffer Changes the minimum size of buffer used by JspWriter The unit of the size is in kilobyte e.g.: <%@ page buffer="none" %> means don't buffer the

output e.g.: <%@ page buffer="12" %> means set buffer size to

12kbytes

autoflush Requires the developer to explicitly flush buffer

Other Attributes of the page Directive errorPage

Designates a page to handle unplanned errors (i.e., when an exception is uncaught in the JSP page)

e.g.: <%@ page errorPage="MyErrorPage.jsp" %>

isErrorPage Indicates if the current page is a page designated for handling error Default value is "false" If "true", the "exception" implicit object is made available to this

page. You can find out from the "exception" object which JSP page is throwing

an exception and what kind of exception it is.

Note: We can also configure error pages in the web.xml (web application deployment file).

A Note On Directive Elements Except for "import", other attributes cannot repeat.

The following is illegal

<%@ page buffer="16384" %>

<%@ page buffer="8192" %>

The following is illegal

<%@ page buffer="16384" session="false"

buffer="8192" %>

The following is legal

<%@ page import="java.io.*" %>

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

The include Directive Static include – to include the contents of other files in the

current JSP page at translation time.

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

// Content A<%@ include file="x.html" %>// Content B

// Content X

// Content A// Content X// Content B

Translated into Servlet codes

original.jsp x.html

Notes: Servers may not detect changes made to the included files. Thus, you need to update the "last modified date" of the JSP files whenever the included files change.

The include Directive

Purposes To reuse JSP content in multiple pages

e.g.: Menus, headers, footers, etc.

When to use If the included file contains static text If the included file is rarely changed To include files (including JSP fragment files) that are placed un

der the /WEB-INF folder.

Shortcoming Lack of locality (The main file and all the included files share the

same scope difficult to debug) Not suitable for files that’s are too big. (A Java method cannot

exceeds 64kbytes in size.)

Standard Action Elements Special Tags that can be embedded in a JSP page

At compile time, these tags are also converted into corresponding Java codes.

7 standard JSP action elements: jsp:include, jsp:forward, jsp:param

More commonly used ones

jsp:useBean, jsp:setProperty, jsp:getProperty For used with JSP Bean (Will discussed later)

jsp:plugin For generating <OBJECT> or <EMBED> tags for Java applet.

jsp:include Action Dynamic include – To incorporate static or dynamic

resources into the current page at request time. Similar to using a RequestDispatcher object to include a resource

Can pass data to the included resources

Syntax<jsp:include page="anotherURL" flush="true" />

or<jsp:include page="anotherURL" flush="true"> <jsp:param name="name1" value="value1" /> …

</jsp:include>

jsp:include Action

// Content A<jsp:include page="x.html" />// Content B

// Content X

// Java codes to produce content ApageContext.include("x.html");// Java codes to produce content B

Translated into servlet codes

original.jsp x.html

Java byte codes

Compiled into Java byte codes

Executed by web container

Included into the output stream at rune time.

If the included resource is a JSP file, the output produced by the JSP file is included in the output stream of original.jsp.

jsp:include Action

Advantages over the include directive Easier to maintain

Changes made to the included files automatically reflected in the files that include them.

Easier to debug (Strong locality)

Shortcoming Slower Cannot include files placed in /WEB-INF folder

When to use If the file is subject to modification very often If the size of the file is huge

Include Example – Menu

<table border="0" cellpadding="0" width="100%"><tr><td>Menu Item 1</td></tr><tr><td>Menu Item 2</td></tr><tr><td>Menu Item 3</td></tr></table>

<%@page contentType="text/html" pageEncoding="UTF-8"%><html><head><title>JSP Menu Demo</title></head><body><table border="0" width="100%" cellpadding="0" cellspacing="0" ><tr><td><%@ include file="menu.html" %></td></tr><tr><td bgcolor="#aacccc">Contents goes here ...</td></tr></table></body></html>

menu.html (note: With include directive, you can give any file extension to this file.)

In this example, using <jsp:include …> produces same output

Reusable JSP Content: ContactSection.jsp<%@ page import="java.util.Date" %><%-- The following become fields in each servlet object that results from a JSP page that includes this file. --%><%! private int accessCount = 0;private Date accessDate = new Date();private String accessHost = "<I>No previous access</I>";%><hr />&copy; 2000 <a href="http://www.my-company.com/">my-company.com</a>.This page has been accessed <%= ++accessCount %>times since server reboot. It was last accessed from <%= accessHost %> at <%= accessDate %>.<% accessHost = request.getRemoteHost(); %><% accessDate = new Date(); %>

…<BODY><TABLE BORDER=5 ALIGN="CENTER"> <TR><TH CLASS="TITLE"> Some Random Page</TABLE><P> Information about our products and services.<P> Blah, blah, blah.<P> Yadda, yadda, yadda.<%@ include file="ContactSection.jsp" %></BODY></HTML>

Using the JSP Content

jsp:forward Action Used to instruct a web server to stop processing

the current page and start another one. Similar to using a RequestDispatcher object to forward

a request

Syntax<jsp:forward page="anotherURL" />

or<jsp:forward page="anotherURL">

<jsp:param name="param1" value="value1">

…

</jsp:forward>

Note: For both jsp:include and jsp:forward

The parameters passed to the included/forwarded resources can be obtained asrequest.getParameter("paramName")

If the value of page attribute begins with '/', then the path is evaluated relative to the application root folder. Otherwise the path is evaluated relative to the current folder.

References Wikipedia: JavaServer Pages

http://en.wikipedia.org/wiki/JavaServer_Pages Free Tutorial (Java, JSP, Java Servlets)

http://www.courses.coreservlets.com/Course-Materials/ Sample JSP codes

http://www.java2s.com/Code/Java/JSP/CatalogJSP.htm