Recipe14.2.Extending Tile Definitions

Recipe 14.2. Extending Tile Definitions

Problem

You want to create a new Tiles definition that reuses a similar one.

Solution

When you define a new definition in your Tiles definition file (tiles-def.xml), you can specify a definition to extend using the extends="definitionName" attribute. The new definition inherits attributes of the extended definition and can override attribute values. (See Example 14-5.)

Example 14-5. Extending a new definition from a base definition

<?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE tiles-definitions PUBLIC        "-//Apache Software Foundation//DTD Tiles Configuration 1.1//EN"        "http://jakarta.apache.org/struts/dtds/tiles-config_1_1.dtd"> <tiles-definitions>     <!-- A base Tile -->     <definition name="mainLayoutDef2" path="/layouts/mainLayout.jsp">         <put name="title" value="Struts Cookbook - Chapter 14 : Tiles"/>         <put name="header" value="/common/header.jsp"/>         <put name="navbar" value="/common/navbar2.jsp"/>         <put name="body"   type="string"/>         <put name="news"   value="/common/news.html"/>         <put name="footer" value="/common/footer.jsp"/>             </definition>     <!-- Extensions of the base mainLayoutDef2 tile -->     <definition name=".start" extends="mainLayoutDef2">         <put name="title" value="Start Page"/>         <put name="body"  value="/pages/pageStart.jsp"/>     </definition>     <definition name=".pageOne" extends="mainLayoutDef2">         <put name="title" value=" Page One"/>         <put name="body"  value="/pages/pageOne.jsp"/>         <put name="news"  value="/pages/pageOneNews.jsp"/>     </definition> <tiles-definitions>

Discussion

With Tiles, you can base a Tile definition on the definition of another Tile. The new definition, which extends the base definition, inherits the attributes of the base definition and overrides attributes as needed. When using Tiles, developers commonly use the approach shown in the Solution. The mainLayoutDef2 definition defines the basic layout to be used across the application. In object-oriented terms, you can think of the base definition as an abstract class. The .start and .pageOne definitions extend mainLayoutDef2. These definitions are like concrete subclasses of the abstract base class.

The .start definition overrides two attributes: title and body. The overridden title specifies title text appropriate for the Start page. The body attribute in the base mainLayoutDef2 does not specify a value; think of it as an abstract property. The .start definition overrides the body attribute by specifying a concrete value, a JSP page. The .pageOne definition, which extends the mainLayoutDef2 definition, overrides the news attribute and sets the title and body.

To create a page that uses the new definition, you insert the definition using the tiles:insert JSP tag as shown by Example 14-6 (startPage.jsp).

Example 14-6. JSP that references a definition extension

<%@ taglib uri="http://struts.apache.org/tags-tiles" prefix="tiles" %> <tiles:insert definition=".start"/>