The Hundred-Kilobyte Kernel (HK2) is the lightweight and extensible kernel of GlassFish Server. To interact with GlassFish Server, add-on components plug in to this kernel. In the HK2 component model, the functions of an add-on component are declared through a contract-service implementation paradigm. An HK2 contract identifies and describes the building blocks or the extension points of an application. An HK2 service implements an HK2 contract.
For more information, see Chapter 2, Writing HK2 Components.