Creating New Tag Libraries in Java

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13 Creating New Tag Libraries
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Since 4, we have seen that custom tag libraries are an invaluable asset The time has now come to learn how to create new ones Fundamentally, tags are not much more complicated than servlets; in fact, servlets could be used to construct a very limited form of custom tag If it rendered the current time to a page, a servlet could be used almost as a "tag":
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<jsp:include page="/dateServlet"> <jsp:param name="format" value="HH:MM:SS"/> </jsp:include>
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Alternatively, if tags were implemented as servlets and held to the bean naming conventions, the page compiler could take a simple JSP:
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Here is the date: <awl:date format="HH:MM:SS"/> <p>
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and turn it into the following code:
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public void service(HttpServletRequest request, HttpServletResponse response) { responsesetStatus(resSC_OK); responsesetContentType("text/html");
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PrintWriter out = responsegetWriter(); outprintln("Here is the date:");
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DateServlet tag = new DateServlet(); tagsetFormat("HH:MM:SS"); tagservice(request,response);
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outprintln("<p>");
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Including the contents of a servlet within a page using either of these approaches is not quite enough to do everything that a tag does However, this concept will serve as a convenient jumping-off point in exploring how tag libraries are constructed
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131 The Tag Life Cycle
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The first step in being able to write new tags is to understand how pages will use them Consider a standard usage of a tag, such as the awl:date tag from 4
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<awl:date format="HH:MM:SS"/>
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Clearly, this request must be handled by a class The name of this class will be associated with the name awl:date through a configuration file that will be described shortly For now, the class is called comawljspbookch04DateTag and must implement an interface called javaxservletjsptagextTag A logical question at this point is whether the lookup of this class should happen at request time or translation time[1] Doing it at request time would be more dynamic and might allow for some additional functionality, such as changing tag definitions on the fly However, the introspection mechanisms that allow for this kind of dynamic behavior can be slow, and as tags are so ubiquitous, it is worth doing everything possible to make them fast
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If the terms translation time and request time are unclear, refer to 2
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Therefore, the resolution from tag names to class names happens at translation time, and code to build the tag class will be placed in the resulting servlet Likewise, the tag configuration file can specify all the parameters the tag will accept, so there is no need to look them up dynamically as is done to obtain bean properties However, if tag classes stick to the bean naming conventions, the page translator will, when it sees a tag attribute called format, know to construct a call to setFormat() in the DateTag class In addition to any parameters that the tag accepts, it will need some other information in order to do its job At the very least, the DateTag will need access to out, the output stream to which it should send the formatted date It is reasonable to expect that in general, tags will need access to the full HttpServeltRequest and HttpServletResponse objects Both of these objects, as well as a great deal of
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additional information, is handily contained in the class introduced in 12 The tag class must therefore provide a setPageContext() method to receive this information Some tags may also need to know whether they have been nested within another tag The c:when and c:otherwise tags need a way to access the c:choose tag that surrounds them The c:choose tag can keep track of whether a matching condition has been found yet, and each c:when tag can then ask the c:choose tag whether it should bother to check its test condition The outer tag is called the parent, and so the tag class must have a setParent() method Next, the tag will need to provide something akin to the servlet service() method to do the work Unlike a servlet, however, a tag consists of two parts: the opening and closing tags The preceding example has only an opening tag, and a /> is used to indicate the absence of a closing tag, but this is really just shorthand for <awl:time ></awl:time> In general, there may also be body content between these open and close tags Therefore, rather than having a single service() method, tags must provide doStartTag() and doEndTag() methods Finally, once it has completed its task, a tag may need to clean up some resources, as a servlet does in its destroy() method The equivalent for tags is called release() A few modifications to this basic scheme need to be considered before it will be possible to write DateTag To allow maximum flexibility, a tag may wish to specify whether its body content should be evaluated, an obvious example of which is the c:if tag This is accomplished by allowing doStartTag() to return a code indicating how the tag's body should be treated Possible values are EVAL_BODY_INCLUDE and SKIP_BODY Similarly, doEndTag() may decide that the rest of the page should not be evaluated, such as in a custom security tag that wishes to hide the contents of a page from unauthorized users Therefore, the doEndTag() will also return a status code, which may be EVAL_PAGE or SKIP_PAGE Given all this, the page translator will, when it encounters the awl:date tag, inject something like Listing 131 into the servlet Listing 131 Tag code generated by the page translator
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comawljspbookch13DateTag t = new comawljspbookch13DateTag(); tsetFormat("HH:MM:SS"); tsetPageContext( the page context ); tsetParent( the tag's parent );
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if(tdoStartTag() == EVAL_BODY) { code built for the contents of the tag body }
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