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JSP in Action

JSP in Action. continuation. JSP’s Tag Extension Mechanism. You can define your own actions to replace lengthy scriptlets By “hiding” functions behind custom tags, you can increase modularity and maintainability of your pages .

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JSP in Action

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  1. JSP in Action continuation

  2. JSP’s Tag Extension Mechanism • You can define your own actions to replace lengthy scriptlets • By “hiding” functions behind custom tags, • you can increase modularity and maintainability of your pages.

  3. Define Java classes that provide the functionality of the new actions, including the definition of their attributes (e.g., myAttributeName). These classes are called tag handlers. • Provide a formalized description of your action elements, so that Tomcat knows how to handle them. For example, you need to specify which actions can have a body and which attributes can be omitted. Such a description is called a tag library descriptor (TLD). • In the JSP pages, tell Tomcat that the pages need your tag library and specify the prefix that you want to identify those custom tags with.

  4. BodylessCustom Action • <wow:weekday date=“date”/>

  5. Step 1: Define the Tag Handler • A tag handler for a bodyless custom tag is a class that implements the interfaces java.io.Serializable and javax.servlet.jsp.tagext.Tag. • Remember that to satisfy an interface, you have to implement all the methods it defines. • To satisfy Serializable, you only need to define a unique identifier, like this: static final long serialVersionUID = 1L;

  6. Step 2: Define the TLD

  7. Step 3: Use the Custom Action

  8. Bodied Custom Actions

  9. Step 1: Define the Tag Handler • Bodied actions need to implement an interface, javax.servlet.jsp.tagex.BodyTag instead of Tag. • Implement doEndTagmethod,anddoAfterBody <wow:weekdayBody></wow:weekdayBody>

  10. Step 2: Define the TLD

  11. Step 3: Use the Custom Action

  12. Tag Files • Tag files are special JSP files that replace tag handlers written in Java. • There are three directives: tag, attribute, and variable.

  13. Bodyless Tag

  14. The tag directive of a tag file replaces the page directive of a JSP page, and the attributedirective lets you define an input parameter. • Another simplification is in sending the result to the output, because in the tag file the implicit variable out makes it unnecessary to invoke pageContext.getOut().

  15. Bodied Tag

  16. If you omit the attribute var, doBody sends the body’s result to the output; • If you replace varwith varReader, the result is stored as a java.io.Readerobject instead of a java.lang.String; • If you add the attribute scope, you can specify var/varReaderto be defined as a request, session, or application attribute, instead of in the page scope. • Know that jsp:invoke is very similar to jsp:doBody but operates on a JSP fragment instead of the action body. • For example, by writing the following two lines in a tag file <%@attribute name="fragName" fragment="true"%> <jsp:invoke fragment="fragName"/>

  17. The tag Directive

  18. The attribute Directive

  19. The variable Directive

  20. JSTL and EL Next Meeting

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