JSF is a component-based framework, and JSF custom components are the major proof that sustain JSF flexibility and extensibility. In order to write custom components or extend the existing ones, JSF provides a powerful API that allows us to develop two types of components: custom components, and, from JSF 2.0 onwards, composite components. A custom component implementation is responsible for providing an aspect (optional for non-UI components, such as custom validators, converters, and renderers) and a behavior. Usually the decision to write custom components and the skills for accomplishing it belong to advanced JSF developers.

Before you decide to write a custom component, which can be a time-consuming task, you have to overview the following bullets (especially the first bullet):