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| <h1 class="content-header header-section1" id="pgui_datamodel_basics" itemprop="headline">Basics</h1> |
| </div></div><p>You have seen how to build a data-model in the <a href="pgui_quickstart.html">Getting Started</a> using the standard |
| Java classes (<code class="inline-code">Map</code>, <code class="inline-code">String</code>, |
| etc.). Internally, the variables available in the template are java |
| objects that implement the |
| <code class="inline-code">freemarker.template.TemplateModel</code> interface. But |
| you could use standard java collections as variables in your |
| data-model, because these were replaced with the appropriate |
| <code class="inline-code">TemplateModel</code> instances behind the scenes. This |
| facility is called <strong>object wrapping</strong>. |
| The object wrapping facility can convert <em>any</em> kind |
| of object transparently to the instances of classes that implement |
| <code class="inline-code">TemplateModel</code> interface. This makes it possible, |
| for example, to access <code class="inline-code">java.sql.ResultSet</code> as |
| sequence variable in templates, or to access |
| <code class="inline-code">javax.servlet.ServletRequest</code> objects as a hash |
| variable that contains the request attributes, or even to traverse XML |
| documents as FTL variables (<a href="xgui.html">see here</a>). To |
| wrap (convert) these objects, however, you need to plug the proper, so |
| called, object wrapper implementation (possibly your custom |
| implementation); this will be discussed <a href="pgui_datamodel_objectWrapper.html">later</a>. The meat for now |
| is that any object that you want to access from the templates, sooner |
| or later must be converted to an object that implements |
| <code class="inline-code">TemplateModel</code> interface. So first you should |
| familiarize yourself with writing of <code class="inline-code">TemplateModel</code> |
| implementations.</p><p>There is roughly one |
| <code class="inline-code">freemarker.template.TemplateModel</code> descendant |
| interface corresponding to each basic type of variable |
| (<code class="inline-code">TemplateHashModel</code> for hashes, |
| <code class="inline-code">TemplateSequenceModel</code> sequences, |
| <code class="inline-code">TemplateNumberModel</code> for numbers, etc.). For |
| example, if you want to expose a <code class="inline-code">java.sql.ResultSet</code> |
| as a sequence for the templates, then you have to write a |
| <code class="inline-code">TemplateSequenceModel</code> implementation that can read |
| <code class="inline-code">java.sql.ResultSet</code>-s. We used to say on this, that |
| you <em>wrap</em> the |
| <code class="inline-code">java.sql.ResultSet</code> with your |
| <code class="inline-code">TemplateModel</code> implementation, as basically you just |
| encapsulate the <code class="inline-code">java.sql.ResultSet</code> to provide |
| access to it with the common <code class="inline-code">TemplateSequenceModel</code> |
| interface. Note that a class can implement multiple |
| <code class="inline-code">TemplateModel</code> interfaces; this is why FTL variables |
| can have multiple types (see: <a href="dgui_datamodel_basics.html">Template Author's Guide/Values, Types/Basics</a>)</p><p>Note that a trivial implementation of these interfaces is |
| provided with the <code class="inline-code">freemarker.template</code> package. For |
| example, to convert a <code class="inline-code">String</code> to FTL string |
| variable, you can use <code class="inline-code">SimpleScalar</code>, to convert a |
| <code class="inline-code">java.util.Map</code> to FTL hash variable, you can use |
| <code class="inline-code">SimpleHash</code>, etc.</p><p>An easy way to try your own <code class="inline-code">TemplateModel</code> |
| implementation, is to create an instance of that, and drop it directly |
| into the data-model (as <code class="inline-code">put</code> it into the root hash). |
| The object wrapper will expose it untouched for the template, as it |
| already implements <code class="inline-code">TemplateModel</code>, so no conversion |
| (wrapping) needed. (This trick is also useful in cases when you do not |
| want the object wrapper to try to wrap (convert) a certain |
| object.)</p><div class="bottom-pagers-wrapper"><div class="pagers bottom"><a class="paging-arrow previous" href="pgui_datamodel.html"><span>Previous</span></a><a class="paging-arrow next" href="pgui_datamodel_scalar.html"><span>Next</span></a></div></div></div></div> </div> |
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