code: def create_jar_builder(self, jar):
      
      builder = self.JarBuilder(self.context, jar)
      yield builder
      builder.commit_manifest(jar)
docs: Creates a ``JarTask.JarBuilder`` ready for use.
  
      This method should be called during in `execute` context and only after ensuring
      `JarTask.JarBuilder.prepare` has already been called in `prepare` context.
  
      :param jar: An opened ``pants.backend.jvm.tasks.jar_task.Jar`.