Implementations are expected to return implementations of the BeanDefinitionParser interface for custom top-level tags and implementations of the BeanDefinitionDecorator interface for custom nested tags.

The parser will call parse(Element, ParserContext) when it encounters a custom tag directly under the <beans> tags and decorate(Node, BeanDefinitionHolder, ParserContext) when it encounters a custom tag directly under a <bean> tag.

Developers writing their own custom element extensions typically will not implement this interface drectly, but rather make use of the provided NamespaceHandlerSupport class.