The instance of a class can be used directly as a configuration value if this class implements the singleton pattern. In this case, the configuration property must be annotated with @InstanceFormat. In the configuration, a singleton matching the type of the property can then be selected via the class name.

For example, the following configuration allows the selection of a Renderers:

public interface MyConfig extends ConfigurationItem {

   @InstanceFormat
   Renderer getRenderer();

}

Renderer here is an interface for an implementation that outputs a passed object to HTML, for example. If the application has multiple Renderer implementations, the configuration can select one of the renderers by its class name:

<config
  renderer="my.app.MyRenderer"
>

</config>

For this, the MyRenderer class only needs to implement the singleton pattern:

public class MyRenderer implements Renderer {

  /** Singleton {@link MyRenderer}. */
  public MyRenderer INSTANCE = new MyRenderer();

  private MyRenderer() {}

  @Override
  public void render(...) {
    // Do what a Renderer has to do.
  }

}

The Renderer-singleton MyRenderer must define a public static final constant INSTANCE that contains a Renderer. Then the singleton instance can be referenced in the configuration via the class name of the singleton class.