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 <article class="container">
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     <a class="edit-on-gh" href="https://github.com/apache/struts-site/edit/master/source/tag-developers/ognl-basics.md" title="Edit this page on GitHub">Edit on GitHub</a>

     <a href="index" title="back to Tag Developers Guide"><< back to Tag Developers Guide</a>

     <h1 class="no_toc" id="ognl-basics">OGNL Basics</h1>

 <ul id="markdown-toc">
   <li><a href="#struts-specific-language-features" id="markdown-toc-struts-specific-language-features">Struts-specific language features</a>    <ul>
       <li><a href="#accessing-static-properties" id="markdown-toc-accessing-static-properties">Accessing static properties</a></li>
       <li><a href="#differences-from-the-webwork-1x-el" id="markdown-toc-differences-from-the-webwork-1x-el">Differences from the WebWork 1.x EL</a></li>
       <li><a href="#struts-2-named-objects" id="markdown-toc-struts-2-named-objects">Struts 2 Named Objects</a></li>
     </ul>
   </li>
 </ul>

 <h2 id="struts-specific-language-features">Struts-specific language features</h2>

 <p>The biggest addition that Struts provides on top of OGNL is the support for the ValueStack. While OGNL operates under
 the assumption there is only one “root”, Struts’s ValueStack concept requires there be many “roots”.</p>

 <p>For example, suppose we are using standard OGNL (not using Struts) and there are two objects in the OgnlContext map:
 “foo” -&gt; foo and “bar” -&gt; bar and that the foo object is also configured to be the single <strong>root</strong> object.
 The following code illustrates how OGNL deals with these three situations:</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>#foo.blah // returns foo.getBlah()
 #bar.blah // returns bar.getBlah()
 blah      // returns foo.getBlah() because foo is the root
 </code></pre></div></div>

 <p>What this means is that OGNL allows many objects in the context, but unless the object you are trying to access is the root,
 it must be prepended with a namespaces such as @bar. Now let’s talk about how Struts is a little different…</p>

 <blockquote>
   <p>In Struts, the entire ValueStack is the root object in the context. Rather than having your expressions get the object
 you want from the stack and then get properties from that (ie: peek().blah), Struts has a special OGNL PropertyAccessor
 that will automatically look at the all entries in the stack (from the top down) until it finds an object with the property
 you are looking for.</p>
 </blockquote>

 <p>For example, suppose the stack contains two objects: Animal and Person. Both objects have a “name” property, Animal has
 a “species” property, and Person has a “salary” property. Animal is on the top of the stack, and Person is below it.
 The follow code fragments help you get an idea of what is going on here:</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>species    // call to animal.getSpecies()
 salary     // call to person.getSalary()
 name       // call to animal.getName() because animal is on the top
 </code></pre></div></div>

 <p>In the last example, there was a tie and so the animal’s name was returned. Usually this is the desired effect, but
 sometimes you want the property of a lower-level object. To do this, XWork has added support for indexes on the ValueStack.
 All you have to do is:</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>[0].name   // call to animal.getName()
 [1].name   // call to person.getName()
 </code></pre></div></div>

 <p>With expression like <code class="highlighter-rouge">[0] ... [3]</code> etc. Struts will cut the stack and still return back a CompoundRoot object. To get
 the top of that particular stack cut, use <code class="highlighter-rouge">[0].top</code></p>

 <table>
   <thead>
     <tr>
       <th>ognl expression</th>
       <th>description</th>
     </tr>
   </thead>
   <tbody>
     <tr>
       <td><code class="highlighter-rouge">[0].top</code></td>
       <td>would get the top of the stack cut starting from element 0 in the stack (similar to top in this case)</td>
     </tr>
     <tr>
       <td><code class="highlighter-rouge">[1].top</code></td>
       <td>would get the top of the stack cut starting from element 1 in the stack</td>
     </tr>
   </tbody>
 </table>

 <h3 id="accessing-static-properties">Accessing static properties</h3>

 <p>OGNL supports accessing static properties as well as static methods.</p>

 <p>By default, Struts 2 is configured to disallow this - to enable OGNL’s static member support you must set the
 <code class="highlighter-rouge">struts.ognl.allowStaticMethodAccess</code> constant to <code class="highlighter-rouge">true</code> via any of the <a href="../core-developers/constant-configuration.html">Constant Configuration</a> methods.</p>

 <p>OGNL’s static access looks like this:</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>@some.package.ClassName@FOO_PROPERTY
 @some.package.ClassName@someMethod()
 </code></pre></div></div>

 <p>However, Struts allows you to avoid having to specify the full package name and call static properties and methods of your
 action classes using the “vs” prefix:</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>&lt;at:var at:name="vs" /&gt;FOO_PROPERTY
 &lt;at:var at:name="vs" /&gt;someMethod()

 &lt;at:var at:name="vs1" /&gt;FOO_PROPERTY
 &lt;at:var at:name="vs1" /&gt;someMethod()

 &lt;at:var at:name="vs2" /&gt;BAR_PROPERTY
 &lt;at:var at:name="vs2" /&gt;someOtherMethod()
 </code></pre></div></div>

 <p>“vs” stands for “value stack”. The important thing to note here is that if the class name you specify is just “vs”,
 the class for the object on the top of the stack is used. If you specify a number after the “vs” string, an object’s
 class deeper in the stack is used instead.</p>

 <h3 id="differences-from-the-webwork-1x-el">Differences from the WebWork 1.x EL</h3>

 <p>Besides the examples and descriptions given above, there are a few major changes in the EL since WebWork 1.x. The biggest
 one is that properties are no longer accessed with a forward slash (/) but with a dot (.). Also, rather than using “..”
 to traverse down the stack, we now use “[n]” where n is some positive number. Lastly, in WebWork 1.x one could access
 special named objects (the request scope attributes to be exact) by using “@foo”, but now special variables are accessed
 using “#foo”. However, it is important to note that “#foo” does NOT access the request attributes. Because XWork is not
 built only for the web, there is no concept of “request attributes”, and thus “#foo” is merely a request to another
 object in the OgnlContext other than the root.</p>

 <table>
   <thead>
     <tr>
       <th>Old Expression</th>
       <th>New Expression</th>
     </tr>
   </thead>
   <tbody>
     <tr>
       <td>foo/blah</td>
       <td>foo.blah</td>
     </tr>
     <tr>
       <td>foo/someMethod()</td>
       <td>foo.someMethod()</td>
     </tr>
     <tr>
       <td>../bar/blah</td>
       <td>[1].bar.blah</td>
     </tr>
     <tr>
       <td>@baz</td>
       <td>not directly supported, but #baz is similar</td>
     </tr>
     <tr>
       <td>.</td>
       <td>‘top’ or [0]</td>
     </tr>
   </tbody>
 </table>

 <h3 id="struts-2-named-objects">Struts 2 Named Objects</h3>

 <p>Struts 2 places request parameters and request, session, and application attributes on the OGNL stack. They may be accessed
 as shown below.</p>

 <table>
   <thead>
     <tr>
       <th>name</th>
       <th>value</th>
     </tr>
   </thead>
   <tbody>
     <tr>
       <td>#action[‘foo’] or #action.foo</td>
       <td>current action getter (getFoo())</td>
     </tr>
     <tr>
       <td>#parameters[‘foo’] or #parameters.foo</td>
       <td>request parameter [‘foo’] (request.getParameter())</td>
     </tr>
     <tr>
       <td>#request[‘foo’] or #request.foo</td>
       <td>request attribute [‘foo’] (request.getAttribute())</td>
     </tr>
     <tr>
       <td>#session[‘foo’] or #session.foo</td>
       <td>session attribute ‘foo’</td>
     </tr>
     <tr>
       <td>#application[‘foo’] or #application.foo</td>
       <td>ServletContext attributes ‘foo’</td>
     </tr>
     <tr>
       <td>#attr[‘foo’] or #attr.foo</td>
       <td>Access to PageContext if available, otherwise searches request/session/application respectively</td>
     </tr>
   </tbody>
 </table>

   </section>
 </article>


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	<article class="container">
	<section class="col-md-12">
	<a class="edit-on-gh" href="https://github.com/apache/struts-site/edit/master/source/tag-developers/ognl-basics.md" title="Edit this page on GitHub">Edit on GitHub</a>

	<a href="index" title="back to Tag Developers Guide"><< back to Tag Developers Guide</a>

	<h1 class="no_toc" id="ognl-basics">OGNL Basics</h1>

	<ul id="markdown-toc">
	<li><a href="#struts-specific-language-features" id="markdown-toc-struts-specific-language-features">Struts-specific language features</a> <ul>
	<li><a href="#accessing-static-properties" id="markdown-toc-accessing-static-properties">Accessing static properties</a></li>
	<li><a href="#differences-from-the-webwork-1x-el" id="markdown-toc-differences-from-the-webwork-1x-el">Differences from the WebWork 1.x EL</a></li>
	<li><a href="#struts-2-named-objects" id="markdown-toc-struts-2-named-objects">Struts 2 Named Objects</a></li>
	</ul>
	</li>
	</ul>

	<h2 id="struts-specific-language-features">Struts-specific language features</h2>

	<p>The biggest addition that Struts provides on top of OGNL is the support for the ValueStack. While OGNL operates under
	the assumption there is only one “root”, Struts’s ValueStack concept requires there be many “roots”.</p>

	<p>For example, suppose we are using standard OGNL (not using Struts) and there are two objects in the OgnlContext map:
	“foo” -> foo and “bar” -> bar and that the foo object is also configured to be the single <strong>root</strong> object.
	The following code illustrates how OGNL deals with these three situations:</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>#foo.blah // returns foo.getBlah()
	#bar.blah // returns bar.getBlah()
	blah // returns foo.getBlah() because foo is the root
	</code></pre></div></div>

	<p>What this means is that OGNL allows many objects in the context, but unless the object you are trying to access is the root,
	it must be prepended with a namespaces such as @bar. Now let’s talk about how Struts is a little different…</p>

	<blockquote>
	<p>In Struts, the entire ValueStack is the root object in the context. Rather than having your expressions get the object
	you want from the stack and then get properties from that (ie: peek().blah), Struts has a special OGNL PropertyAccessor
	that will automatically look at the all entries in the stack (from the top down) until it finds an object with the property
	you are looking for.</p>
	</blockquote>

	<p>For example, suppose the stack contains two objects: Animal and Person. Both objects have a “name” property, Animal has
	a “species” property, and Person has a “salary” property. Animal is on the top of the stack, and Person is below it.
	The follow code fragments help you get an idea of what is going on here:</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>species // call to animal.getSpecies()
	salary // call to person.getSalary()
	name // call to animal.getName() because animal is on the top
	</code></pre></div></div>

	<p>In the last example, there was a tie and so the animal’s name was returned. Usually this is the desired effect, but
	sometimes you want the property of a lower-level object. To do this, XWork has added support for indexes on the ValueStack.
	All you have to do is:</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>[0].name // call to animal.getName()
	[1].name // call to person.getName()
	</code></pre></div></div>

	<p>With expression like <code class="highlighter-rouge">[0] ... [3]</code> etc. Struts will cut the stack and still return back a CompoundRoot object. To get
	the top of that particular stack cut, use <code class="highlighter-rouge">[0].top</code></p>

	<table>
	<thead>
	<tr>
	<th>ognl expression</th>
	<th>description</th>
	</tr>
	</thead>
	<tbody>
	<tr>
	<td><code class="highlighter-rouge">[0].top</code></td>
	<td>would get the top of the stack cut starting from element 0 in the stack (similar to top in this case)</td>
	</tr>
	<tr>
	<td><code class="highlighter-rouge">[1].top</code></td>
	<td>would get the top of the stack cut starting from element 1 in the stack</td>
	</tr>
	</tbody>
	</table>

	<h3 id="accessing-static-properties">Accessing static properties</h3>

	<p>OGNL supports accessing static properties as well as static methods.</p>

	<p>By default, Struts 2 is configured to disallow this - to enable OGNL’s static member support you must set the
	<code class="highlighter-rouge">struts.ognl.allowStaticMethodAccess</code> constant to <code class="highlighter-rouge">true</code> via any of the <a href="../core-developers/constant-configuration.html">Constant Configuration</a> methods.</p>

	<p>OGNL’s static access looks like this:</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>@some.package.ClassName@FOO_PROPERTY
	@some.package.ClassName@someMethod()
	</code></pre></div></div>

	<p>However, Struts allows you to avoid having to specify the full package name and call static properties and methods of your
	action classes using the “vs” prefix:</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code><at:var at:name="vs" />FOO_PROPERTY
	<at:var at:name="vs" />someMethod()

	<at:var at:name="vs1" />FOO_PROPERTY
	<at:var at:name="vs1" />someMethod()

	<at:var at:name="vs2" />BAR_PROPERTY
	<at:var at:name="vs2" />someOtherMethod()
	</code></pre></div></div>

	<p>“vs” stands for “value stack”. The important thing to note here is that if the class name you specify is just “vs”,
	the class for the object on the top of the stack is used. If you specify a number after the “vs” string, an object’s
	class deeper in the stack is used instead.</p>

	<h3 id="differences-from-the-webwork-1x-el">Differences from the WebWork 1.x EL</h3>

	<p>Besides the examples and descriptions given above, there are a few major changes in the EL since WebWork 1.x. The biggest
	one is that properties are no longer accessed with a forward slash (/) but with a dot (.). Also, rather than using “..”
	to traverse down the stack, we now use “[n]” where n is some positive number. Lastly, in WebWork 1.x one could access
	special named objects (the request scope attributes to be exact) by using “@foo”, but now special variables are accessed
	using “#foo”. However, it is important to note that “#foo” does NOT access the request attributes. Because XWork is not
	built only for the web, there is no concept of “request attributes”, and thus “#foo” is merely a request to another
	object in the OgnlContext other than the root.</p>

	<table>
	<thead>
	<tr>
	<th>Old Expression</th>
	<th>New Expression</th>
	</tr>
	</thead>
	<tbody>
	<tr>
	<td>foo/blah</td>
	<td>foo.blah</td>
	</tr>
	<tr>
	<td>foo/someMethod()</td>
	<td>foo.someMethod()</td>
	</tr>
	<tr>
	<td>../bar/blah</td>
	<td>[1].bar.blah</td>
	</tr>
	<tr>
	<td>@baz</td>
	<td>not directly supported, but #baz is similar</td>
	</tr>
	<tr>
	<td>.</td>
	<td>‘top’ or [0]</td>
	</tr>
	</tbody>
	</table>

	<h3 id="struts-2-named-objects">Struts 2 Named Objects</h3>

	<p>Struts 2 places request parameters and request, session, and application attributes on the OGNL stack. They may be accessed
	as shown below.</p>

	<table>
	<thead>
	<tr>
	<th>name</th>
	<th>value</th>
	</tr>
	</thead>
	<tbody>
	<tr>
	<td>#action[‘foo’] or #action.foo</td>
	<td>current action getter (getFoo())</td>
	</tr>
	<tr>
	<td>#parameters[‘foo’] or #parameters.foo</td>
	<td>request parameter [‘foo’] (request.getParameter())</td>
	</tr>
	<tr>
	<td>#request[‘foo’] or #request.foo</td>
	<td>request attribute [‘foo’] (request.getAttribute())</td>
	</tr>
	<tr>
	<td>#session[‘foo’] or #session.foo</td>
	<td>session attribute ‘foo’</td>
	</tr>
	<tr>
	<td>#application[‘foo’] or #application.foo</td>
	<td>ServletContext attributes ‘foo’</td>
	</tr>
	<tr>
	<td>#attr[‘foo’] or #attr.foo</td>
	<td>Access to PageContext if available, otherwise searches request/session/application respectively</td>
	</tr>
	</tbody>
	</table>

	</section>
	</article>


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