Components References 

Notify Reference

If component A has notify reference to component B, then A will receive all events that refer to a change in the state of B.

Semantics of Notify References

Throughout the life cycle of a component, a notify reference means:

·        There is no requirement that component B is loaded for component A to be resolved.

·        If component B exists, a reference from the class loader of A is set to the class loader of B.

Notify References Appliance Matrix

Reference to	Interface	Library	Service
Reference from
Interface	No	No	No
Library	No	No	No
Service	Yes	Yes	Yes

Only service components can define notify references, since only services can be registered as listeners.

Weak Reference

Component A sets weak reference to component B if it needs to use classes of B.

Semantics of Weak References

Throughout the life cycle of a component, a weak reference means:

·        If component B exists, a reference from the class loader of A is set to the class loader of B.

·        Component B must be resolved for component A to be resolved.

·        All events that refer to a change of state of component B are sent to component A.

·        If the system has to unload component B, it must first unload component A.

Weak Reference Appliance Matrix

Reference to	Interface	Library	Service
Reference from
Interface	Yes	No	No
Library	Yes	Yes	No
Service	Yes	Yes	Yes

Even though it is possible to set weak reference from a library to an interface, it is not common practice. Therefore, if you encounter such a situation, you can double-check the design of your library before you set the link.

Strong Reference

Component A sets strong reference to component B if it needs to use classes and runtime objects that B provides.

Semantics of Strong References

Throughout the life cycle of a component, a strong reference means:

·        If component B exists, a reference from the class loader of A is set to the class loader of B.

·        Component B must be resolved for component A to be resolved.

·        All events that refer to a change of state of component B are sent to component A.

·        Component B must be started for component A to be started.

·        If the system has to stop component B, it must first stop component A.

·        If the system has to unload component B, it must first unload component A.

·        If component A has strong reference to component B, then B cannot have strong reference to A.

Strong Reference Appliance Matrix

Reference to	Interface	Library	Service
Reference from
Interface	No	No	No
Library	No	No	No
Service	Yes	No	Yes

In the case of a service component with a strong reference to an interface, the referencing service will be started only if there is another service that implements the referenced interface and this service is started.