Tags with Bodies in Java

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42 Tags with Bodies
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Custom tags can do more than output data controlled by parameters A custom tag can have a body, which it can control in arbitrary ways Recall a similar tag, jsp:useBean, which renders its body only when the bean it is accessing is created Listing 42 shows such a custom tag that can be used to display its body, hide it, or even reverse it The result is shown in Figure 41 Figure 41 The result of a custom tag
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Listing 42 A custom tag with a body
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<%@ taglib prefix="awl" uri="http://jspbookawlcom/samples" %> <awl:maybeShow show="no"> You can't see me! </awl:maybeShow><br>
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<awl:maybeShow show="yes"> The time is: <awl:date format="hh:mm:ss MM/dd/yy"/> </awl:maybeShow><br>
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<awl:maybeShow show="reverse"> The time is: <awl:date format="hh:mm:ss MM/dd/yy"/> </awl:maybeShow><br>
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This example loads the same tag library used in Listing 41 and again specifies that it will be using the awl prefix to access the tags The tag used this time is called awl:maybeShow, and it has a parameter, show, that controls what the tag should do with
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its body This parameter may be set to no, in which case the body is hidden from the page; yes, in which case the body is displayed; or reverse, in which case the body is shown backward Note that the body of the awl:maybeShow tag may include anything, including other JSP tags This was also true of the jsp:useBean tag and in fact is true of any custom tag that has been properly programmed This property is described by saying that JSP tags can be nested From here on, it will simply be assumed, unless otherwise noted, that the body of any tag can contain any other tag
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43 Dynamic Attributes in Tags
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For the standard tag library to be able to do all the wonderful things it claims to do, the tags will need to take parameters that are more complicated than such simple instructions as "yes" and "no" In fact, the parameters to the standard tag library comprise a full language, although one that is significantly simpler than Java itself and much better suited for building pages This language is built into the very core of JSPs in the latest version of the JSP specification This means that programmers creating new tags may use this language for their own purposes; this will also be illustrated in 13 Expressions in this language are surrounded by braces and preceded by a dollar sign The simplest kinds of expressions in the language are constants, such as strings or numbers:
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${23} ${986} ${'hello'}
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These expressions don't mean anything on their own, but when used as the value of a parameter, they are evaluated by the expression language before they are sent to the tag Because numbers and strings evaluate to themselves, this means that the following two expressions mean the same thing:
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<awl:maybeShow show="${'yes'}">
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<awl:maybeShow show="yes">
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Note that within an expressions, literals are surrounded by single quotes and that the whole expression is surrounded by double quotes
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Errors to Watch For
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If an expression is written incorrectly, such as leaving off a closing quote or a brace, a JSP page error will report something like
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An error occurred while parsing custom action
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Now for the fun part: The scripting language can also refer to beans and properties of beans Listing 31 used a bean to display some static properties, including the seventh prime number Suppose that bean were loaded into a page with this tag:
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<jsp:useBean id="bean1" class="comawljspbookch03Bean1"/>
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In that case, then the scripting language would refer to the seventh prime number property as
${bean1seventhPrimeNumber}
Note the pattern: first, the name of the bean as defined in the jsp:useBean tag, then a dot, then the name of the property This is not exactly equivalent to the jsp:getProperty tag, as dropping this script fragment into a page will not display the value In fact, it will not do anything at all However, this would serve perfectly as a way to send the seventh prime number to a custom tag Admittedly, there would probably never be any need to do such a thing, but often it will be necessary to send a value from a form to a tag We now have the means to do this: Send the form inputs into a bean with the jsp:setProperty tag and then send the value from the bean to a tag with a scripted parameter