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| <a href="index.html" title="back to Getting started"><< back to Getting started</a> |
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| <h1 class="no_toc" id="preparable-interface">Preparable Interface</h1> |
| |
| <ul id="markdown-toc"> |
| <li><a href="#introduction" id="markdown-toc-introduction">Introduction</a></li> |
| <li><a href="#preparable-interface-1" id="markdown-toc-preparable-interface-1">Preparable Interface</a></li> |
| <li><a href="#example-application" id="markdown-toc-example-application">Example Application</a></li> |
| <li><a href="#summary" id="markdown-toc-summary">Summary</a></li> |
| </ul> |
| |
| <p>The example code for this tutorial, <strong>preparable-interface</strong>, is available at <a href="https://github.com/apache/struts-examples">struts-examples</a>.</p> |
| |
| <h2 id="introduction">Introduction</h2> |
| |
| <p>Often the data used to populate a form control is dynamically generated, perhaps from a database. When the user submits |
| the form, the Struts 2 validation interceptor attempts to validate the user’s form input. If validation fails the Struts 2 |
| framework returns the value <code class="language-plaintext highlighter-rouge">input</code> but the <code class="language-plaintext highlighter-rouge">input</code> action is not re-executed. Rather the view associated with the <code class="language-plaintext highlighter-rouge">input</code> |
| result is rendered to the user. Usually this view is the page that displayed the original form.</p> |
| |
| <p>This work-flow can cause a problem if one or more of the form fields or some other data displayed depends on a dynamic |
| look-up that that is accomplished in the Action class’s input method. Since the Action class’s input method is not |
| re-executed when validation fails, the view page may no longer have access to the correct information to create the form |
| or other display information.</p> |
| |
| <p>The <a href="http://struts.apache.org/mail">Struts 2 user mailing list</a> is an excellent place to get help. If you are having |
| a problem getting the tutorial example applications to work search the Struts 2 mailing list. If you don’t find an answer |
| to your problem, post a question on the mailing list.</p> |
| |
| <h2 id="preparable-interface-1">Preparable Interface</h2> |
| |
| <p>Struts 2 provides the <a href="../maven/struts2-core/apidocs/com/opensymphony/xwork2/Preparable">Preparable interface</a> |
| to overcome this problem. An Action class that implements this interface must override the prepare method. The <code class="language-plaintext highlighter-rouge">prepare</code> |
| method will always be called by the Struts 2 framework’s <a href="../core-developers/prepare-interceptor">prepare interceptor</a> |
| whenever any method is called for the Action class and also when validation fails before the view is rendered.</p> |
| |
| <p>In the <code class="language-plaintext highlighter-rouge">prepare</code> method you should put any statements that must be executed no matter what other Action class method will |
| be called and also statements that should be executed if validation fails. Usually statements in the prepare method set |
| the value for Action class instance fields that will be used to populate form controls and get the values that will be |
| used to set the initial form field values.</p> |
| |
| <p>In addition to automatically running the prepare method the <a href="../core-developers/prepare-interceptor">prepare interceptor</a> |
| will also call a method named <code class="language-plaintext highlighter-rouge">prepare<ActionMethodName></code>. For example, define a prepare method and a <code class="language-plaintext highlighter-rouge">prepareInput</code> method |
| in the Action class that implements <code class="language-plaintext highlighter-rouge">Preparable</code>. When the Struts 2 framework calls the input method, the prepare interceptor |
| will call the prepareInput and the prepare methods before calling the input method.</p> |
| |
| <h2 id="example-application">Example Application</h2> |
| |
| <p>If you examine class EditAction in the example application (see above) you’ll see that it implements the <code class="language-plaintext highlighter-rouge">Preparable</code> interface. |
| In the prepare method is this code:</p> |
| |
| <p><strong>EditAction.java prepare Method</strong></p> |
| |
| <div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">carModelsAvailable</span> <span class="o">=</span> <span class="n">carModelsService</span><span class="o">.</span><span class="na">getCarModels</span><span class="o">();</span> |
| |
| <span class="n">setPersonBean</span><span class="o">(</span><span class="n">editService</span><span class="o">.</span><span class="na">getPerson</span><span class="o">());</span> |
| </code></pre></div></div> |
| |
| <p>The above statements get the car model values used to populate the car model check boxes displayed in the form and also |
| get the information about the Person object being edited.</p> |
| |
| <p>When you run the example application, look in the log to see when the prepare method is called in relation to the input |
| and execute methods. Running the example application and examining the log should help you understand the impact of implementing |
| the <code class="language-plaintext highlighter-rouge">Preparable</code> interface and the prepare method.</p> |
| |
| <h2 id="summary">Summary</h2> |
| |
| <p>When your application requires specific statements to be executed no matter which method of the Action class is called |
| or when validation fails, you should implement the Preparable interface and override the prepare method.</p> |
| |
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| <td>Return to <a href="http-session">HTTP Session</a></td> |
| <td>or</td> |
| <td>onward to <a href="exclude-parameters">Exclude Parameters</a></td> |
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