JavaSercer Pages The action results in code for locating or creating the CatalogBean, and declaring and assigning a Java variable named catalog. But since we’re talking about custom actions here, let’s focus on the code generated for the action. First, a tag handler instance is created and initialized with the standard properties (pageContext and parent) plus all properties corresponding to the action attributes. Next, the doStartTag( ) and doEndTag( ) methods are called. Then comes the code that makes the object created by the action available as a scripting variable. Note how a variable with the name specified by the id attribute (prod) is declared, using the type specified by the className attribute. Also note that the variable is declared at the top level of the method. This means that it’s available to scripting elements anywhere on the page after the action element. The variable is then assigned the value of the object with same name located in one of the standard JSP scopes, using the findAttribute( ) method. This method searches through the scopes, in the order page, request, session, and application, until it finds the specified object. With the object available in the JSP page scope, the code generated for the action can find it. Since it’s also assigned to a Java variable, the JSP expression works correctly as well. At least two things are required for a tag handler to create a new object and make it accessible for other actions and JSP scripting code: 1. The JSP container must know the name and the Java type for the object, so it can generate the code for the variable declaration. 2. The object must be placed in one of the JSP scopes, so it can be found by findAttribute( ) and assigned to the variable. The first requirement is fulfilled by a class called TagExtraInfo . When you develop a tag handler for an action that introduces an object, you must also create a subclass of the TagExtraInfo class. The JSP container consults an instance of this class when it generates the code. Example 16.6 shows the class associated with the action. Example 16.6. UsePropertyTagExtraInfo Class package com.ora.jsp.tags.generic; import javax.servlet.jsp.tagext.*; public class UsePropertyTagExtraInfo extends TagExtraInfo { public VariableInfo[] getVariableInfo(TagData data) { return new VariableInfo[] { new VariableInfo(data.getAttributeString(”id”), data.getAttributeString(”className”), true, VariableInfo.AT_END) }; } } The method used by the JSP container during code generation is called getVariableInfo( ). It returns an array of VariableInfo objects, one per variable introduced by the tag handler. The VariableInfo class is a simple bean with four properties, all of them initialized to the values passed as arguments to the constructor: varName, className, declare, and scope. The meaning of the first two is not hard to guess: the name of the variable and the name of its class. The declare property is a boolean, in which true means that a new variable is created by the action. In other words, a declaration of the variable must be added to the generated servlet. A value of false means that the variable has already been created by another action or by another occurrence of the same action, so the generated code already contains the declaration. This is all the information the JSP container needs to generate the code for the variable declaration; the first requirement is satisfied. The scope property has nothing to do with the JSP scopes we have seen so far (page, request, session, and application). Instead, it defines where the new variable is available to JSP scripting elements. A value of AT_BEGIN means that it is available from the action’s start tag and stays available after the action’s end tag. AT_END means it is not available until after the action’s end tag. A variable with scope NESTED is available only in the action’s body, between the start and the end tags. The scope therefore controls where the variable declaration and value assignment code is generated, and the tag handler class must make sure the variable is available in one of the JSP scopes at the appropriate time. page 225
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