Class<?> lookup(final DefaultPluginType pluginType) throws PluginSourceNotMatchException  {
        synchronized (this.cacheMap) {  // Synchronize between this.cacheMap.get() and this.cacheMap.put().
            final Class<?> pluginMainClass = this.cacheMap.get(pluginType);
            if (pluginMainClass != null) {
                return pluginMainClass;
            }

            final Class<?> loadedPluginMainClass = this.search(pluginType);
            this.cacheMap.put(pluginType, loadedPluginMainClass);
            return loadedPluginMainClass;
        }
    }
Caches self-contained 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/SelfContainedPluginRegistry.java