```ruby
module Ritual
  module Extension
    class JRuby < Base
      def define_tasks
        task 'gem:build' => task_name
        desc "Build the #{name} extension."
        task task_name do
          relative_install_path = relative_path(install_path)
          relative_compiled_paths = compiled_paths.map { |p| relative_path(p) }
          class_path = "#{RbConfig::CONFIG['prefix']}/lib/jruby.jar"

          sh 'javac', '-g', '-classpath', class_path, *source_paths
          Dir.chdir path do
            mkdir_p File.dirname(relative_install_path)
            sh 'jar', 'cf', relative_install_path, *relative_compiled_paths
          end
        end
      end

      def source_paths
        @source_paths ||= FileList["#{path}/**/*.java"]
      end

      protected

      def compiled_path_for(source)
        source.sub(/\.java\z/, '.class')
      end
    end
  end
end
```

Fix jar invocation when building jars in 1.9 mode.
user
```ruby
module Ritual
  module Extension
    class JRuby < Base
      def define_tasks
        task 'gem:build' => task_name
        desc "Build the #{name} extension."
        task task_name do
          relative_install_path = relative_path(install_path)
          relative_compiled_paths = compiled_paths.map { |p| relative_path(p) }
          class_path = "#{RbConfig::CONFIG['prefix']}/lib/jruby.jar"

          sh 'javac', '-g', '-classpath', class_path, *source_paths
          Dir.chdir path do
            mkdir_p File.dirname(relative_install_path)
            sh 'jar', 'cf', relative_install_path.to_s, *relative_compiled_paths.map(&:to_s)
          end
        end
      end

      def source_paths
        @source_paths ||= FileList["#{path}/**/*.java"]
      end

      protected

      def compiled_path_for(source)
        source.sub(/\.java\z/, '.class')
      end
    end
  end
end
```
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