JSP taglib 自定义标签 tld web.xml 笔记

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本文介绍了从JSP 2.0开始，如何不再需要在web.xml中指定<taglib>元素，并详细解释了JSP如何查找TLD文件，推荐的JAR包打包方式，以及升级应用时的注意事项。通过移除<taglib>，简化部署并提高模块化。

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http://docs.oracle.com/javaee/5/tutorial/doc/bnalj.html Java EE 5 Tutorial 的说明。（Java EE 7太新，不符合现在所学，故没有引用7的链接）

http://www.cnblogs.com/zhaoyang/archive/2011/12/25/2301108.html

http://wiki.metawerx.net/wiki/Web.xml

==============================================================

http://wiki.metawerx.net/wiki/Web.xml.JspConfig

<jsp-config>

This parent element wraps one or more <taglib> elements and <jsp-property-group> elements.

Note that in Tomcat 6 and earlier, <taglib> did not need to be inside <jsp-config>. However on Tomcat 7, context will fail to load with the following error if you specify <taglib> elements outside of a <jsp-config> element. This occurs even if you specify an earlier web-app version in your web.xml, so can be confusing when migrating to Tomcat 7.

IllegalArgumentException: taglib definition not consistent with specification version

Child-elements:

<taglib> - contains examples
<jsp-property-group> - contains examples

Removing <taglib> from web.xml

Author: Neale Rudd, Metawerx

Date: 16-Nov-2006

JSP 1.2 required the <taglib> directive to be used in web.xml for every JSP Custom Tag Library used by the application. In JSP 2.0 this is no longer required. This guide shows how to get rid of your <taglib> directives and how JSP 2.0 finds TLD files.

The <taglib> directive looks like this. You will find one in web.xml for older versions of Struts, and any other software which uses tag libraries in JSP 1.2.

<taglib>
	<taglib-uri>mytags</taglib-uri>
	<taglib-location>/WEB-INF/jsp/mytaglib.tld</taglib-location>
</taglib>

Since JSP 2.0, this is now optional.

The TLD is discovered automatically when the taglib is first referenced in a JSP file.

The actual JSP 1.2 TLD files themselves are upwardly compatible with JSP 2.0, so do not need to be modified.

Where does JSP look for the TLD files?

JSP 2.0 containers such as Tomcat 5.5 search for TLD files in the following places:

The following locations are searched:

<appName>/WEB-INF folder
All subfolders of <appName>/WEB-INF
Inside each JAR file in <appName>/WEB-INF/lib, in the META-INF folder of the JAR. This method is prefered for easy deployment of a JAR containing a taglib.

How should I package my JAR file?

Place TLD files in a folder named META-INF, at the root level of the JAR file . eg:

META-INF/
    mytags.tld
com/
    yourCompanyName/
        yourTag1.class
        yourTag2.class
        yourTag3.class

ANT build.xml example

This example copies TLD files from src dir into your compiled-classes dir, then includes them in the <jar> elements. In this case, we are making two JAR files, containing all files from the com.yourCompanyName.jsputil and waptools folders, and including the relevant TLD file in each JAR. You could also combine to make a single jar file, with both TLDs included.

<!-- Copy TLD files into the out.dir, so all files for the JAR are in the same place -->
<copy todir="${out.dir}/META-INF">
	<fileset dir="${src.dir}/META-INF">
		<include name="**.tld" />
	</fileset>
</copy>

<!-- Create the jsputil jar file -->
<jar jarfile="${dist.dir}/jsputil.jar"
     basedir="${out.dir}"
     includes="com/yourCompanyName/jsputil/** META-INF/jsputil.tld" />

<!-- Create the waptools jar file -->
<jar jarfile="${dist.dir}/waptools.jar"
     basedir="${out.dir}"
     includes="com/yourCompanyName/waptools/** META-INF/waptools.tld" />

Why is this better?

For library developers, distribution is now simpler as packaging is as simple as including the TLD file in the JAR file .

For end-users, deployment and re-use is simpler, as a tag library JAR file can simply be dropped into the WEB-INF/lib folder with no web.xml configuration required.

TLD Tips in JSP 2.0

The TLD file does not need to have the same name as the library. For example, your mytags library definition can be in a file called mytaglist.tld. However, for easier identification later, it is good practise to use the same name as you intend to use in the JSP files. For example, if you plan to reference your library as mytags, with <mytags:testtag> in JSP, call the file mytags.tld.
The <short-name> element in the TLD file does not need the same name as the library. Once again, for easier maintenance and to avoid potential clashes, it is good practise to use the same name. For example <short-name>mytags</short-name>.
The <uri> in the TLD file is the main reference for matching JSP declarations with the correct TLD. When automatically finding TLD files, the container matches the uri attribute in your JSP taglib declaration with the <uri> tag in the TLD file. Ensure this is always unique to avoid clashes. For example:

This declaration in your JSP ...
<%@ taglib prefix="mytags" uri="http://metawerx.net/taglibs/mytags" %>

Will match this element in your TLD ...
<uri>http://metawerx.net/taglibs/mytags</uri>

Upgrading your application

Many older applications still include the <taglib> directive in web.xml. If your application falls into this category, try removing it and check if everything still works. Don't forget to move your TLD file into WEB-INF, or preferably into your JAR file. Your web.xml will be simpler, and the Tag Library will be more modular (easier to remove from the app, replace, or reuse).
In older applications, the TLD file is sometimes in a separate folder, such as in the example above (/WEB-INF/jsp/mytaglib.tld). In this case, move the file into the JAR file if possible. TLD files will still work from subfolders of WEB-INF, but since the location of the TLD files is no longer recorded in web.xml, it can make them harder to find when debugging your system later - especially after a few months of not looking at it, or if someone else is trying to debug your code. It therefore makes sense to either include them in the JAR file which contains the classes for the tag, or at least place them in an easy to notice location such as a subfolder called WEB-INF/tld or WEB-INF/taglibs.