Service References

A service represents a Java object that is managed by the RDI container. In order to define the dependencies between the services, it is necessary to have a way to refer to them in a unique way. In most cases referring to a service by its class is sufficient. In some other cases however, you may want to have more than one (but a finite number of) instances for the same class in your application, possibly with different dependencies. This can be achieved by giving unique identifiers to these instances, which can be a simple name. RDI allows both ways to refer to services.

Referring to a service by its class

The easiest way to define a reference to a service is by using the service class. With RDI it is done via the ServiceReference#ofType(Class) method. In this case, the name of the service will be equal to the name of the class as by Class#getName():

ServiceReference<A> ref = ServiceReference.ofType(A.class);
assertEquals(A.class.getName(), ref.getServiceName());

Referring to a service by a unique name

You may as well give a custom name to the reference. This is done via ServiceReference#of(String, Class):

ServiceReference<A> ref1 = ServiceReference.of("a1", A.class);
ServiceReference<A> ref2 = ServiceReference.of("a2", A.class);

Reference equality

The ServiceReference class implements equals and hashCode on the name field only, meaning that two references with the same name but of different type will be considered equal. As a result the following assertions will all be true:

ServiceReference<A> ref1 = ServiceReference.of("a1", A.class);
ServiceReference<A> ref2 = ServiceReference.of("a2", A.class);
ServiceReference<A> ref3 = ServiceReference.ofType(A.class);
ServiceReference<A> ref4 = ServiceReference.of("foo", A.class);
ServiceReference<B> ref5 = ServiceReference.of("foo", B.class);

assertNotEquals(ref1, ref2);
assertNotEquals(ref1, ref3);
assertNotEquals(ref2, ref3);
assertNotEquals(ref3, ref4);
assertEquals(ref4, ref5);

The best practice to deal with service references in your application is to define your ServiceReference instances as constants. You can declare these constants either in a dedicated utility class, or directly in each service class if you always have one reference per class.