title: How to configure defaults for a parameter with multiple values for a Maven plugin
body: <p>I'm writing a Maven plugin and I am using default values for all parameters such as this:</p>
  
  <pre><code>/**
   * The file with the site structure.
   * 
   * @parameter expression="${generateSite.siteFile}" default-value="${basedir}/src/oda/site.xml"
   */
  private File siteFile;
  </code></pre>
  
  <p>Now I am adding a new parameter which is a collection. Is there a way to set default values for a parameter like the following one?</p>
  
  <pre><code>/**
   * A list of file/directory names to exclude in the processing.
   * 
   * @parameter ????
   */
  private Set&lt;String&gt; excludes;
  </code></pre>