Reviews : Java Books :
Professional Java Server Programming J2EE Edition : Chapter 12

Title: Professional Java Server Programming J2EE Edition ISBN: 1861004656 US Price: $ 64.99 Canadian Price: C$ 97.95 UK Price: £ 46.99 Publication Date: September 2000 Pages: 1633 © Wrox Press Limited, US and UK.

The javax.servlet.jsp.tagext.Tag Interface

The Tag interface defines a simple interaction between the JSP engine and the tag handler, sufficient for tags that don't need to manipulate their body content. Its core methods are the calls implementing classes will receive when the JSP engine encounters the tag's opening and closing tags, doStartTag() and doEndTag(). Before we look at the method contracts in more detail, a sequence diagram helps to visualize the calls made to the tag handler by the compiled servlet. Assume that the container already has a tag handler instance available, and in the default state:

Let's look at the messages in more detail:

• The container initializes the tag handler by setting the tag handler's pageContext property, which the tag handler can use to access information available to the JSP currently using it.
• The container sets the tag handler's parent property. (Parent may be set to null, if the tag is not enclosed in another tag.)
• Any tag attributes defined by the developer will be set. This is a mapping from the XML attributes of the tag to the corresponding properties of the tag handler bean. For example, in the case of a tag invoked like this: <mytags:test name="John" age="43" />, the container will attempt to call the setName() and setAge() methods on the tag handler. The container will attempt to convert each attribute to the type of the corresponding bean property: for example, the String "43" will be converted to an int in this case. If the type conversion fails, an exception will be thrown and must be handled by the calling JSP page. (From a JSP's point of view, there is no difference between an exception thrown by a tag handler and one thrown by an expression of scriptlet in the page.)

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