code: public static String getSpringBootMavenPluginClassifier(MavenProject project, Log log) {
          String classifier = null;
          try {
              classifier = MavenProjectUtil.getPluginGoalConfigurationString(project,
                      "org.springframework.boot:spring-boot-maven-plugin", "repackage", "classifier");
          } catch (PluginScenarioException e) {
              log.debug("No classifier found for spring-boot-maven-plugin");
          }
          return classifier;
      }
docs: Read the value of the classifier configuration parameter from the
  spring-boot-maven-plugin
  
  @param project
  @param log
  @return the value if it was found, null otherwise