String getCategory() {
        return this.category;
    }
Caches Maven-based plugin's loaded classes such as embulk-ruby's Embulk::PluginRegistry.

<p>See {@code /embulk-ruby/lib/embulk/plugin_registry.rb}.
embulk-core/src/main/java/org/embulk/plugin/maven/MavenPluginRegistry.java