Custom annotations in Java allow developers to create their own annotations to enhance code functionality. They are defined using the @interface keyword and can be applied at various levels, such as class, method, or field. To create a custom annotation, you also need to specify its target and retention policy using meta-annotations like @Target and @Retention ¹.

The primary purpose of custom annotations is to provide metadata that can be processed at runtime or compile-time. While annotations themselves do not add functionality, they can be utilized by frameworks or libraries to perform specific actions based on the annotated elements. For instance, they can simplify configuration in libraries by allowing users to annotate classes instead of making multiple function calls ²⁴.

To effectively use custom annotations, developers can leverage the Java Reflection API or Annotation Processors, which can read and process these annotations during runtime or compile-time, respectively ²⁴.

If you have more questions about custom annotations or their applications, feel free to ask!