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| <a href="index.html" title="back to Plugins"><< back to Plugins</a> |
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| <h1 class="no_toc" id="extending-an-application-with-custom-plugins">Extending an Application with Custom Plugins</h1> |
| |
| <ul id="markdown-toc"> |
| <li><a href="#the-interface" id="markdown-toc-the-interface">The Interface</a></li> |
| <li><a href="#the-plugin" id="markdown-toc-the-plugin">The Plugin</a></li> |
| <li><a href="#the-action" id="markdown-toc-the-action">The Action</a></li> |
| <li><a href="#the-jsp" id="markdown-toc-the-jsp">The JSP</a></li> |
| </ul> |
| |
| <p>In this tutorial, we will show how easily our application can be made extensible using the Struts 2 plugin mechanism. |
| To keep the demonstration simple, our plugin will expose a JavaBean that writes a message. Plugins may include any |
| combination of JavaBeans, Actions, Interceptors, Results or other resources we’d like available to an application.</p> |
| |
| <h2 id="the-interface">The Interface</h2> |
| |
| <p>At runtime, plugins are retrieved and referenced via an Interface. So, first, we should define an interface that our |
| plugin will implement. This interface must be available to both our web application and the plugin. To reduce coupling |
| between the web application and the plugins, keep the interface in a separate JAR.</p> |
| |
| <p><strong>IMyPlugin.java</strong></p> |
| |
| <div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">package</span> <span class="nn">example</span><span class="o">;</span> |
| |
| <span class="kd">public</span> <span class="kd">interface</span> <span class="nc">IMyPlugIn</span> <span class="o">{</span> |
| <span class="nc">String</span> <span class="nf">saySomething</span><span class="o">();</span> |
| <span class="o">}</span> |
| </code></pre></div></div> |
| |
| <h2 id="the-plugin">The Plugin</h2> |
| |
| <p>Now that we have an interface to implement we’ll create the plugin. At load time, the framework looks for JARs |
| containing a <code class="language-plaintext highlighter-rouge">struts-plugin.xml</code> file at the root of the archive. To create a plugin, all we need to do is build |
| a JAR and put the expected <code class="language-plaintext highlighter-rouge">struts-plugin.xml</code> at the root.</p> |
| |
| <p>To get started, let’s create a class that implements our IMyPlugin interface.</p> |
| |
| <p><strong>MyPlugin.java</strong></p> |
| |
| <div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">package</span> <span class="nn">example.impl</span><span class="o">;</span> |
| |
| <span class="kn">import</span> <span class="nn">example.IMyPlugin</span><span class="o">;</span> |
| |
| <span class="kd">public</span> <span class="kd">class</span> <span class="nc">MyPlugin</span> <span class="kd">implements</span> <span class="nc">IMyPlugin</span> <span class="o">{</span> |
| <span class="kd">public</span> <span class="nc">String</span> <span class="nf">saySomething</span><span class="o">()</span> <span class="o">{</span> |
| <span class="k">return</span> <span class="s">"We don't need no education"</span><span class="o">;</span> |
| <span class="o">}</span> |
| <span class="o">}</span> |
| </code></pre></div></div> |
| |
| <p>Internally, the framework utilizes a number of JavaBeans. We can use the <code class="language-plaintext highlighter-rouge">bean</code> element to add our own JavaBeans |
| to the set managed by the framework.</p> |
| |
| <p><strong>struts-default.xml</strong></p> |
| |
| <div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp"><?xml version="1.0" encoding="UTF-8" ?></span> |
| |
| <span class="cp"><!DOCTYPE struts PUBLIC |
| "-//Apache Software Foundation//DTD Struts Configuration 2.5//EN" |
| "http://struts.apache.org/dtds/struts-2.5.dtd"></span> |
| |
| <span class="nt"><struts></span> |
| <span class="nt"><bean</span> <span class="na">type=</span><span class="s">"example.IMyInterface"</span> <span class="na">class=</span><span class="s">"example.impl.MyPlugin"</span> <span class="na">name=</span><span class="s">"myPlugin"</span><span class="nt">/></span> |
| <span class="nt"></struts></span> |
| </code></pre></div></div> |
| |
| <p>Finally, to install the plugin, drop the JAR file under WEB-INF/lib.</p> |
| |
| <h2 id="the-action">The Action</h2> |
| |
| <p>The JavaBeans configured by <code class="language-plaintext highlighter-rouge">bean</code> elements can be retrieved via a Container provided by XWork 2. We obtain |
| a reference to the Container by using the <code class="language-plaintext highlighter-rouge">@Inject</code> notation. (This notation is part of the Guice framework that |
| XWork and Struts use under the covers.) The framework predefines a Container object, and the <code class="language-plaintext highlighter-rouge">@Inject</code> annotation tells |
| the framework to set its Container object to the Action property.</p> |
| |
| <p>We might want to supply a number of JavaBeans to the application this way. In the Action, we will obtain a reference |
| to the entire set of JavaBeans that might have been plugged in. Then, we can scroll through the set, displaying each |
| JavaBean’s message.</p> |
| |
| <p><strong>MyAction.java</strong></p> |
| |
| <div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">package</span> <span class="nn">example.actions</span><span class="o">;</span> |
| |
| <span class="kn">import</span> <span class="nn">example.IMyPlugin</span><span class="o">;</span> |
| |
| <span class="kd">public</span> <span class="kd">class</span> <span class="nc">MyAction</span> <span class="kd">extends</span> <span class="nc">ActionSupport</span> <span class="o">{</span> |
| <span class="nc">Set</span><span class="o"><</span><span class="nc">IMyPlugin</span><span class="o">></span> <span class="n">plugins</span><span class="o">;</span> |
| |
| <span class="nd">@Inject</span> |
| <span class="kd">public</span> <span class="kt">void</span> <span class="nf">setContainer</span><span class="o">(</span><span class="nc">Container</span> <span class="n">container</span><span class="o">)</span> <span class="o">{</span> |
| <span class="nc">Set</span><span class="o"><</span><span class="nc">String</span><span class="o">></span> <span class="n">names</span> <span class="o">=</span> <span class="n">container</span><span class="o">.</span><span class="na">getInstanceNames</span><span class="o">(</span><span class="nc">IMyPlugin</span><span class="o">.</span><span class="na">class</span><span class="o">);</span> |
| <span class="n">plugins</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">HashSet</span><span class="o"><</span><span class="nc">IMyPlugin</span><span class="o">>();</span> |
| <span class="k">for</span> <span class="o">(</span><span class="nc">String</span> <span class="n">name</span> <span class="o">:</span> <span class="n">names</span><span class="o">)</span> <span class="o">{</span> |
| <span class="n">plugins</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">container</span><span class="o">.</span><span class="na">getInstance</span><span class="o">(</span><span class="nc">IMyPlugin</span><span class="o">.</span><span class="na">class</span><span class="o">,</span> <span class="n">name</span><span class="o">));</span> |
| <span class="o">}</span> |
| <span class="o">}</span> |
| |
| <span class="kd">public</span> <span class="nc">Set</span><span class="o"><</span><span class="nc">IMyPlugin</span><span class="o">></span> <span class="nf">getPlugins</span><span class="o">()</span> <span class="o">{</span> |
| <span class="k">return</span> <span class="k">this</span><span class="o">.</span><span class="na">plugins</span><span class="o">;</span> |
| <span class="o">}</span> |
| <span class="o">}</span> |
| </code></pre></div></div> |
| |
| <p>As seen by the Action class code, it’s important to define a unique interface for any beans that we plugin, so that |
| we can identify our beans later.</p> |
| |
| <p>In the same way that we plugged in this JavaBean, we could also plugin and configure Action classes, Interceptors, |
| Results, or any other JAR-able resource that an application might utilize.</p> |
| |
| <h2 id="the-jsp">The JSP</h2> |
| |
| <p>Let’s do something with those plugins:</p> |
| |
| <p><strong>Page.jsp</strong></p> |
| |
| <div class="language-jsp highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt"><s:iterator </span><span class="na">id=</span><span class="s">"plugin"</span><span class="na"> value=</span><span class="s">"plugins"</span><span class="nt">></span> |
| <span class="nt"><s:property </span><span class="na">value=</span><span class="s">"#plugin.saySomething()"</span><span class="nt">/></span> |
| <span class="nt"></s:iterator></span> |
| </code></pre></div></div> |
| |
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