CSC 2720 Building 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 1 ="value 1 " attribute 2 ="value 2 "... %> 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   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/classes and 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   Purpose  Specify the MIME type of the content generated by the JSP page  Default value is "text/html;charset=ISO "  e.g.: HTML content encoded in simplified Chinese characters   References  Wiki: MIME:  MIME reference:  Character set reference:

First Last Address Marty Hall Larry Brown Bill Gates Larry Ellison <%-- 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.:  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.:

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.:  buffer  Changes the minimum size of buffer used by JspWriter  The unit of the size is in kilobyte  e.g.: means don't buffer the output  e.g.: 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.:  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  The following is illegal page buffer="16384" session="false" buffer="8192" %>  The following is legal

The include Directive  Static include – to include the contents of other files in the current JSP page at translation time.  Syntax: // Content A // 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 under 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 or 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 or …

jsp:include Action // Content A // Content B // Content X // Java codes to produce content A pageContext.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 Menu Item 1 Menu Item 2 Menu Item 3 JSP Menu Demo Contents goes here... menu.html (note: With include directive, you can give any file extension to this file.) In this example, using produces same output

Reusable JSP Content: ContactSection.jsp <%-- 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 = " No previous access "; %> © 2000 my-company.com. This page has been accessed times since server reboot. It was last accessed from at.

… Some Random Page Information about our products and services. Blah, blah, blah. Yadda, yadda, yadda. 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 or …

Note: For both jsp:include and jsp:forward  The parameters passed to the included/forwarded resources can be obtained as request.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  Free Tutorial (Java, JSP, Java Servlets)  Sample JSP codes