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JSP Tag Libraries : Chapter 3: Developing your first tags

3. 5 A tag with a body

Remember that tags can have a body or be bodyless. Our HelloWorldTag was an example of a tag without a body, so let's see an example of a tag with one. We create them whenever we want to take a block of content (typically HTML) and modify it or include it in the server's response. Think back to the HTML <font> tag. The body of the <font> is where you put text to which you wish to apply a particular font. Tags with bodies are great for translating content (from, say, HTML to WML), applying formatting, or indicating that a grouping of content should be treated in a special way, as is the case with the HTML <form> tag.

Here is an extremely simplified example that illustrates how a tag with a body works. Suppose we need to create a tag that will change a block of text from capital letters to lower case. We'll be creative and call this tag LowerCaseTag. Our new tag will have a lot in common with HelloWorldTag, but there are a few differences. The first is that LowerCaseTag doesn't extend from TagSupport, rather from BodyTag-Support. The formula is elementary: if your custom tag doesn't have a body or will include just its body verbatim, it should either implement the Tag interface or extend its utility class, TagSupport. If, however, your tag will modify or control its body, it needs to implement BodyTag or extend its utility class called BodyTagSupport. We'll cover several additional examples of both types in the next chapters.

3.5.1 LowerCaseTag handler

Here is the code for our LowerCaseTag handler class:

The method doAfterBody() is executed by the JSP runtime, once it has read in the tag's body. Retrieves the body that was just read in by the JSP runtime.

Gets JspWriter to output the lowercase content.

Writes the body out to the user in lowercase.

Returns SKIP_ BODY is returned to tell the JSP runtime to continue processing the rest of the page.

With the tag handler class written, the next step is, once again, to create a TLD. This time our tag entry looks like this:

<tag> 
	<name> lowercase</ name> 
	<tagclass> book. simpletasks. LowerCaseTag</ tagclass> 
	<bodycontent> JSP</ bodycontent> 
	<info> 
		Put body in lowercase. 
	</ info> 

</ tag> 

The only difference in this listing is that the <bodycontent> field is no longer empty but now must be JSP. This is the way to indicate to the runtime that Lower-CaseTag will have a body, unlike our HelloWorldTag that did not. There will be much more about bodycontent and other TLD fields in chapters 5 and 6.

We have returned to the stage where we need to use this new tag in a JSP file. Our JSP looks like this:

Declares that the JSP file uses the library referenced by the URI and that the library's tags are referenced by jspx.

Uses the lowercase tag to change its body to lowercase.

Now we add our tag to our deployment directory, pull up the JSP in our browser (figure 3. 4), and voila!

This tag doesn't do anything especially useful, however it is always possible to modify it to do something worthwhile with the body. Some examples might include the body as the message of an email, translating the body from one markup lan-guage to another, or parsing the body of XML and outputting certain nodes or attributes. In the next chapters, we'll see how the body of a custom tag can include other custom tags to allow cooperation with very powerful results.

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