title: Maven failed to execute goal org.apache.maven.plugins:maven-jar-plugin
body: <p>Like below, I had an error when running <code>mvn package</code></p>
  
  <pre><code>[ERROR] Failed to execute goal org.apache.maven.plugins:maven-jar-plugin:2.3.1:jar (default-jar) on project hello-world: Execution default-jar of goal org.apache.maven.plugins:maven-jar-plugin:2.3.1:jar failed: Unable to load the mojo 'jar' in the plugin 'org.apache.maven.plugins:maven-jar-plugin:2.3.1'. A required class is missing: org/codehaus/plexus/components/io/resources/PlexusIoResourceCollection
  [ERROR] -----------------------------------------------------
  [ERROR] realm =    plugin&gt;org.apache.maven.plugins:maven-jar-plugin:2.3.1
  [ERROR] strategy = org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy
  [ERROR] urls[0] = file:/C:/maven/repository/org/apache/maven/plugins/maven-jar-plugin/2.3.1/maven-jar-plugin-2.3.1.jar
  [ERROR] urls[1] = file:/C:/maven/repository/junit/junit/3.8.1/junit-3.8.1.jar
  [ERROR] urls[2] = file:/C:/maven/repository/org/apache/maven/maven-archiver/2.4.1/maven-archiver-2.4.1.jar
  [ERROR] urls[3] = file:/C:/maven/repository/org/codehaus/plexus/plexus-archiver/1.0/plexus-archiver-1.0.jar
  [ERROR] urls[4] = file:/C:/maven/repository/commons-lang/commons-lang/2.1/commons-lang-2.1.jar
  [ERROR] urls[5] = file:/C:/maven/repository/org/codehaus/plexus/plexus-utils/2.0.5/plexus-utils-2.0.5.jar
  [ERROR] Number of foreign imports: 1
  [ERROR] import: Entry[import  from realm ClassRealm[maven.api, parent: null]]
  [ERROR] 
  [ERROR] -----------------------------------------------------: org.codehaus.plexus.components.io.resources.PlexusIoResourceCollection
  [ERROR] -&gt; [Help 1]
  [ERROR] 
  [ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
  [ERROR] Re-run Maven using the -X switch to enable full debug logging.
  [ERROR] 
  [ERROR] For more information about the errors and possible solutions, please read the following articles:
  [ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginContainerException
  </code></pre>
  
  <p>This is my pom.xml. I got rid of unnecessary things, but still it didn't work..</p>
  
  <pre><code>&lt;project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"&gt;
  
  &lt;modelVersion&gt;4.0.0&lt;/modelVersion&gt;
  &lt;groupId&gt;sds.jetty.simple&lt;/groupId&gt;
  &lt;artifactId&gt;hello-world&lt;/artifactId&gt;
  &lt;version&gt;1.0&lt;/version&gt;
  &lt;packaging&gt;jar&lt;/packaging&gt;
  &lt;name&gt;Jetty HelloWorld&lt;/name&gt;
  
  &lt;properties&gt;
      &lt;jettyVersion&gt;8.1.2.v20120308&lt;/jettyVersion&gt;
  &lt;/properties&gt;
  
  &lt;dependencies&gt;
      &lt;dependency&gt;
          &lt;groupId&gt;org.eclipse.jetty&lt;/groupId&gt;
          &lt;artifactId&gt;jetty-server&lt;/artifactId&gt;
          &lt;version&gt;${jettyVersion}&lt;/version&gt;
      &lt;/dependency&gt;
  &lt;/dependencies&gt;
  
  &lt;build&gt;
  &lt;/build&gt;
  &lt;/project&gt;
  </code></pre>