This looks like an xbean bug. maven-xbean-plugin has an optional dependency on Ant so the libxbean-java package doesn't have to depend on ant, but the XBeanMojo class does use BuildException from Ant to report errors [1]. It should use a MojoExecutionException from Maven instead, or the Ant dependency should become mandatory.
[1] https://sources.debian.net/src/libxbean-java/4.5-3/maven-xbean-plugin/src/main/java/org/apache/xbean/maven/XBeanMojo.java/#L234
