How to configure defaults for a parameter with multiple values for a Maven plugin

Question

I'm writing a Maven plugin and I am using default values for all parameters such as this:

/**
 * The file with the site structure.
 * 
 * @parameter expression="${generateSite.siteFile}" default-value="${basedir}/src/oda/site.xml"
 */
private File siteFile;

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?

/**
 * A list of file/directory names to exclude in the processing.
 * 
 * @parameter ????
 */
private Set<String> excludes;

Answer 1

To my knowledge, this is actually not possible, there is no real way to specify default values for Parameter Types With Multiple Values (like Arrays, Collections, or Maps), at least not as parameter. I had to do this in the past too and, having read threads like array (or collecton) as a default-value of a mojo configuration parameter or configuring a list as default value for a plugin parameter, I ended up setting defaults in the execute() method, like Chris mentioned in a comment to his answer (see for example the flexmojos:wrapper plugin sources and the parameters parameter).

Answer 2

I don't think that Set is explicitly supported but the following will work:

/**
 * A list of file/directory names to exclude in the processing.
 *
 * @parameter
 */
private String[] myFiles;

You can then configure it using:

<myFiles>
  <param>value1</param>
  <param>value2</param>
</myFiles>

BTW this was taken from the Parameter Types With Multiple Values section on this page which also details other ways to allow parameters with multiple values.