1.2. What is a ServiceRegistry?

A ServiceRegistry, at its most basic, hosts and manages Services. Its contract is defined by the org.hibernate.service.ServiceRegistry interface.

We already gave a basic overview and definition of services. But services have other interesting characteristics as well:

• Services have a lifecycle.

• They have a scope.

• Services might depend on other services.

• They need to be produced (choose using one implementation over another).

The ServiceRegistry fulfills all these needs.

In a concise definition, the ServiceRegistry acts as a Inversion-of-control (IoC) container.

Why not just use an existing IoC framework? The main reason was that this had to be as light-weight and as small of a footprint as possible. The initial design also had called for Services to be swappable at runtime, which unfortunately had to be removed due to performance problems in the proxy-based swapping-solution; the plan is to investigate alternate ways to achieve swap-ability with better performance at a later date.

A Service is associated with a ServiceRegistry. The ServiceRegistry scopes the Service. The ServiceRegistry manages the lifecycle of the Service. The ServiceRegistry handles injecting dependencies into the Service (actually both a pull and a push/injection approach are supported). ServiceRegistries are also hierarchical, meaning a ServiceRegistry can have a parent ServiceRegistry. Services in one registry can depend on and utilize services in that same registry as well as any parent registries.