Question

I mean in the extensions parameter in a plugin as follows:

<build><plugins><plugin>
    <groupId>...</groupId>
    <artifactId>...</artifactId>
    <version>...</version>
    <extensions>...</extensions>
    ...
</plugin></plugins></build>

Answer 1

http://maven.apache.org/ref/3.0.3/maven-model/maven.html:

Whether to load Maven extensions (such as packaging and type handlers) from this plugin. For performance reasons, this should only be enabled when necessary. Note: While the type of this field is String for technical reasons, the semantic type is actually Boolean. Default value is false.

This parameter is used when developing a plugin. Further resources (since I have no experience with that):

• Using Extensions
• Maven 2.1 Plugin and Extension Loading Design
• How do I create a new packaging type for Maven?