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JavaBeans are reusable software components. It's true that there are JSP tags that let you use JavaBeans within your pages. The only ability those tags allow you, however, is to bind named, scoped instances, and then get/set those instances' properties. If you need to call methods on your JavaBeans, you need to embed the appropriate Java code inside a scriptlet.
Server-side JavaBeans are nonvisual components, and I generally use them for maintaining session state and business information. A good example would be using a bean to maintain the user's current state in a Web-based mail application.
Tags on the other hand can represent actions on a page, and I tend to use those to hide common functionality that generates dynamic HTML or controls the page in some way.
One of the key differences between JavaBeans and JSP tags is that tags are much more aware of the environment in which they are running. That includes the page context (containing the request, response, and so on) and the servlet context of the Web application in which the tag is running. JSP tags can use those contexts to access the HTTP session information and to also make use of JavaBeans that contain session and/or business state.
To keep things simple, I'll use the current reference implementation of the Servlet/JSP specification -- Tomcat from the Apache Group. Tomcat supports the new J2EE Web applications and, to save creating your own, you can use Tomcat's example Web application. That can be found under your Tomcat installation directory (TOMCAT_HOME) in webapps/examples. Further information about Tomcat, including details on where you can download it, can be found in the Resources section.
Obviously, a few options are open to you, one being to use the JavaBean to store the file size. Of course that isn't a good option if the contents of the file change frequently. Instead, you would use a custom tag that, given a file's URI, uses its environment to find out how large the file is.
For the purpose of that example, I'll create a file called taglib.tld and place it under the
web-inf directory. The contents of the file as are follows.
<?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.1//EN" "http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_1.dtd"> <taglib> <tlibversion>1.0</tlibversion> <jspversion>1.1</jspversion> <shortname>examples</shortname> <uri>http://www.mycompany.com/taglib</uri> <info>An example tag library</info> <tag> <name>size</name> <tagclass>examples.SizeTag</tagclass> <info>Works out how large a file, pointed to by a URI, is in bytes</info> <attribute> <name>uri</name> <required>true</required> <rtexprvalue>false</rtexprvalue> </attribute> </tag> </taglib>
I'll skim over the top of the file and concentrate on the tag library that I am defining. A tag library has a short name, a brief description, and a URI. That doesn't need to be a real URI as it's just an identifier for the library. A tag library can have one or more tags, each tag having a name, some brief commentary about its purpose, and a fully qualified class name. That class name is the name of the Java class that contains the functionality provided by your custom tag. I'll write that class shortly.
My tag has one mandatory (required) attribute called
uri, allowing me to specify the URI of the file whose size I want. When using custom tags on a JSP page, you can provide a Java
scriptlet as the value of an attribute. The value between the
<rtexprvalue> tags indicates that such may be the case and that the JSP engine should evaluate the runtime expression beforehand. For this
simple example, I'll just pass a hard-coded URI, so I've set that to false for now.
javax.servlet.jsp.tagext.Taginterface. When a tag is used on a page, the JSP engine instantiates the class and begins by calling the various life cycle methods that the interface defines. That includes telling the tag about its environment by referring to a
The following code shows the
SizeTag class. There are two places where supporting classes can go but, for the purpose of this example, I'll place the tag under
web-inf/classes directory. There should already be an
examples package in which I can place the tag.