Quarkus - Contexts and Dependency Injection

1. Bean Discovery
2. Private Members
3. Supported Features
4. Limitations
5. Build Time Extension Points
6. Removing Unused Beans
7. Default Beans

Quarkus DI solution is based on the Contexts and Dependency Injection for Java 2.0 specification. However, it is not a full CDI implementation verified by the TCK. Only a subset of the CDI features is implemented - see also the list of supported features and the list of limitations.

Most of the existing CDI code should work just fine but there are some small differences which follow from the Quarkus architecture and goals.

1. Bean Discovery

Bean discovery in CDI is a complex process which involves legacy deployment structures and accessibility requirements of the underlying module architecture. Quarkus is using a simplified bean discovery. There is only one bean archive with annotated bean discovery mode and no visibility boundaries.

The bean archive is synthesized from:

the application,
application dependencies that contain a beans.xml descriptor or a generated Jandex index (META-INF/jandex.idx),
and Quarkus integration code.

Bean classes that don’t have a bean defining annotation are not discovered. This behavior is defined by CDI. But producer methods and fields and observer methods are discovered even if the declaring class is not annotated with a bean defining annotation (this behavior is different to what is defined in CDI). In fact, the declaring bean classes are considered annotated with @Dependent.

Quarkus extension may declare additional discovery rules. For example @Scheduled business methods are registered even if the declaring class is not annotated with a bean defining annotation.

2. Private Members

Quarkus is designed with Substrate VM in mind. One of the limitations is the usage of Reflection. Substrate VM does support reflective calls but for a price of a bigger native executable. Quarkus must use reflection fallback to access private members. That’s why Quarkus users are encouraged not to use private members in their beans. This involves injection fields, constructors and initializers, observer methods, producer methods and fields, disposers and interceptor methods.

You can use for example package-private modifiers:

@ApplicationScoped
public class CounterBean {

    @Inject
    CounterService counterService; (1)

    void onMessage(@Observes Event msg) { (2)
    }
}

1	A package-private injection field.
2	A package-private observer method.

Or constructor injection:

@ApplicationScoped
public class CounterBean {

    private CounterService service;

    CounterBean(CounterService service) { (1)
      this.service = service;
    }
}

1	A package-private constructor injection.

3. Supported Features

Programming model
- Managed beans implemented by a Java class
  - @PostConstruct and @PreDestroy lifecycle callbacks
- Producer methods and fields, disposers
- Qualifiers
- Alternatives
- Stereotypes
Dependency injection and lookup
- Field, constructor and initializer/setter injection
- Type-safe resolution
- Programmatic lookup via javax.enterprise.inject.Instance
- Client proxies
- Injection point metadata
Scopes and contexts
- @Dependent, @ApplicationScoped, @Singleton, @RequestScoped and @SessionScoped
- Custom scopes and contexts
Interceptors
- Business method interceptors: @AroundInvoke
- Interceptors for lifecycle event callbacks: @PostConstruct, @PreDestroy, @AroundConstruct
Events and observers, including asynchronous events

4. Limitations

@ConversationScoped is not supported
Decorators are not supported
Portable Extensions are not supported
BeanManager - only the following methods are implemented: getBeans(), createCreationalContext(), getReference(), getInjectableReference() , resolve(), getContext(), fireEvent(), getEvent() and createInstance()
Specialization is not supported
beans.xml descriptor content is ignored
Passivation and passivating scopes are not supported
Interceptor methods on superclasses are not implemented yet

5. Build Time Extension Points

5.1. Portable Extensions

Quarkus incorporates build-time optimizations in order to provide instant startup and low memory footprint. The downside of this approach is that CDI Portable Extensions cannot be supported. Nevertheless, most of the functionality can be achieved using Quarkus extensions.

5.2. Additional Bean Defining Annotations

As described in Bean Discovery bean classes that don’t have a bean defining annotation are not discovered. However, BeanDefiningAnnotationBuildItem can be used to extend the set of default bean defining annotations (@Dependent, @Singleton, @ApplicationScoped, @RequestScoped and @Stereotype annotations):

@BuildStep
BeanDefiningAnnotationBuildItem additionalBeanDefiningAnnotation() {
    return new BeanDefiningAnnotationBuildItem(DotName.createSimple("javax.ws.rs.Path")));
}

Bean registrations that are result of a BeanDefiningAnnotationBuildItem are unremovable by default. See also Removing Unused Beans.

5.3. Resource Annotations

ResourceAnnotationBuildItem is used to specify resource annotations that make it possible to resolve non-CDI injection points, such as Java EE resources.

An integrator must also provide a corresponding io.quarkus.arc.ResourceReferenceProvider implementation.

@BuildStep
void setupResourceInjection(BuildProducer<ResourceAnnotationBuildItem> resourceAnnotations, BuildProducer<GeneratedResourceBuildItem> resources) {
    resources.produce(new GeneratedResourceBuildItem("META-INF/services/io.quarkus.arc.ResourceReferenceProvider",
        JPAResourceReferenceProvider.class.getName().getBytes()));
    resourceAnnotations.produce(new ResourceAnnotationBuildItem(DotName.createSimple(PersistenceContext.class.getName())));
}

5.4. Additional Beans

AdditionalBeanBuildItem is used to specify additional bean classes to be analyzed during discovery. Additional bean classes are transparently added to the application index processed by the container.

@BuildStep
List<AdditionalBeanBuildItem> additionalBeans() {
     return Arrays.asList(
          new AdditionalBeanBuildItem(SmallRyeHealthReporter.class),
          new AdditionalBeanBuildItem(HealthServlet.class));
}

A bean registration that is a result of an AdditionalBeanBuildItem is removable by default. See also Removing Unused Beans.

5.5. Synthetic Beans

Sometimes it is very useful to register a synthetic bean, i.e. a bean that doesn’t need to have a corresponding java class. In CDI this could be achieved using AfterBeanDiscovery.addBean() methods. In Quarkus we produce a BeanRegistrarBuildItem and leverage the io.quarkus.arc.processor.BeanConfigurator API to build a synthetic bean definition.

@BuildStep
BeanRegistrarBuildItem syntheticBean() {
     return new BeanRegistrarBuildItem(new BeanRegistrar() {

            @Override
            public void register(RegistrationContext registrationContext) {
                 registrationContext.configure(String.class).types(String.class).qualifiers(new MyQualifierLiteral()).creator(mc -> mc.returnValue(mc.load("foo"))).done();
            }
        }));
}

The output of a BeanConfigurator is recorded as bytecode. Therefore there are some limitations in how a synthetic bean instance is created. See also BeanConfigurator.creator() methods.

5.6. Annotation Transformations

A very common task is to override the annotations found on the bean classes. For example you might want to add an interceptor binding to a specific bean class. Here is how to do it - use the AnnotationsTransformerBuildItem:

@BuildStep
AnnotationsTransformerBuildItem transform() {
    return new AnnotationsTransformerBuildItem(new AnnotationsTransformer() {

        public boolean appliesTo(org.jboss.jandex.AnnotationTarget.Kind kind) {
            return kind == org.jboss.jandex.AnnotationTarget.Kind.CLASS;
        }

        public void transform(TransformationContext context) {
            if (contex.getTarget().asClass().name().toString().equals("com.foo.Bar")) {
                context.transform().add(MyInterceptorBinding.class).done();
            }
        }
    });
}

5.7. Injection Point Transformation

Every now and then it is handy to be able to change qualifiers of an injection point programmatically. You can do just that with InjectionPointTransformerBuildItem. The following sample shows how to apply transformation to injection points with type Foo that contain qualifier MyQualifier:

@BuildStep
InjectionPointTransformerBuildItem transform() {
    return new InjectionPointTransformerBuildItem(new InjectionPointsTransformer() {

        public boolean appliesTo(Type requiredType) {
            return requiredType.equals(Type.create(DotName.createSimple(Foo.class.getName()), Type.Kind.CLASS));
        }

        public void transform(TransformationContext transformationContext) {
            if (transformationContext.getQualifiers().stream()
                    .anyMatch(a -> a.name().equals(DotName.createSimple(MyQualifier.class.getName())))) {
                transformationContext.transform().removeAll()
                        .add(DotName.createSimple(MyOtherQualifier.class.getName()))
                        .done();
            }
        }
    });
}

5.8. Bean Deployment Validation

Once the bean deployment is ready an extension can perform additional validations and inspect the found beans, observers and injection points. Register a BeanDeploymentValidatorBuildItem:

@BuildStep
BeanDeploymentValidatorBuildItem beanDeploymentValidator() {
    return new BeanDeploymentValidatorBuildItem(new BeanDeploymentValidator() {
         public void validate(ValidationContext validationContext) {
             for (InjectionPointInfo injectionPoint : validationContext.get(Key.INJECTION_POINTS)) {
                 System.out.println("Injection point: " + injectionPoint);
             }
         }
    });
}

See also io.quarkus.arc.processor.BuildExtension.Key to discover the available metadata.

5.9. Custom Contexts

An extension can register a custom InjectableContext implementation by means of a ContextRegistrarBuildItem:

@BuildStep
ContextRegistrarBuildItem customContext() {
    return new ContextRegistrarBuildItem(new ContextRegistrar() {
         public void register(RegistrationContext registrationContext) {
            registrationContext.configure(CustomScoped.class).normal().contextClass(MyCustomContext.class).done();
         }
    });
}

5.10. Available Build Time Metadata

Any of the above extensions that operates with BuildExtension.BuildContext can leverage certain build time metadata that are generated during build. The built-in keys located in io.quarkus.arc.processor.BuildExtension.Key are:

ANNOTATION_STORE
- Contains an AnnotationStore that keeps information about all AnnotationTarget annotations after application of annotation transformers
INJECTION_POINTS
- List<InjectionPointInfo> containing all injection points
BEANS
- List<BeanInfo> containing all beans
OBSERVERS
- List<ObserverInfo> containing all observers
SCOPES
- List<ScopeInfo> containing all scopes, including custom ones
QUALIFIERS
- Map<DotName, ClassInfo> containing all qualifiers
INTERCEPTOR_BINDINGS
- Map<DotName, ClassInfo> containing all interceptor bindings
STEREOTYPES
- Map<DotName, ClassInfo> containing all stereotypes

To get hold of these, simply query the extension context object for given key. Note that these metadata are made available as build proceeds which means that extensions can only leverage metadata that were build before they are invoked. If your extension attempts to retrieve metadata that wasn’t yet produced, null will be returned. Here is a summary of which extensions can access which metadata:

AnnotationsTransformer
- Shouldn’t rely on any metadata as this is one of the first CDI extensions invoked
ContextRegistrar
- Has access to ANNOTATION_STORE
InjectionPointsTransformer
- Has access to ANNOTATION_STORE, QUALIFIERS, INTERCEPTOR_BINDINGS, STEREOTYPES
BeanRegistrar
- Has access to all build metadata
BeanDeploymentValidator
- Has access to all build metadata

6. Removing Unused Beans

The container attempts to remove all unused beans during build by default. This optimization can be disabled by setting quarkus.arc.remove-unused-beans to none or false.

An unused bean:

is not a built-in bean or an interceptor,
is not eligible for injection to any injection point,
is not excluded by any extension,
does not have a name,
does not declare an observer,
does not declare any producer which is eligible for injection to any injection point,
is not directly eligible for injection into any javax.enterprise.inject.Instance or javax.inject.Provider injection point

Users can instruct the container to not remove any of their specific beans (even if they satisfy all the rules specified above) by annotating them with io.quarkus.arc.Unremovable.

Furthermore, extensions can eliminate possible false positives by producing UnremovableBeanBuildItem.

Finally, Quarkus provides a middle ground for the bean removal optimization where application beans are never removed whether or not they are unused, while the optimization proceeds normally for non application classes. To use this mode, set quarkus.arc.remove-unused-beans to fwk or framework.

7. Default Beans

Quarkus adds a capability that CDI currently does not support which is to conditionally declare a bean if no such bean has been declared in the typical ways Quarkus supports. This is done using the @io.quarkus.arc.DefaultBean annotation and is best explained with an example.

Say there is a Quarkus extension that among other things declares a few CDI beans like the following code does:

@Dependent
public class TracerConfiguration {

    @Produces
    public Tracer tracer(Reporter reporter, Configuration configuration) {
        return new Tracer(reporter, configuration);
    }

    @Produces
    @DefaultBean
    public Configuration configuration() {
        // create a Configuration
    }

    @Produces
    @DefaultBean
    public Reporter reporter(){
        // create a Reporter
    }
}

The idea is that the extension auto-configures things for user, eliminating a lot of boilerplate - we can just @Inject a Tracer wherever it is needed. Now imagine that in our application we would like to utilize the configured Tracer, but we need to customize it a little, for example by providing a custom Reporter. The only thing that would be needed in our application would be something like the following:

@Dependent
public class CustomTracerConfiguration {

    @Produces
    public Reporter reporter(){
        // create a custom Reporter
    }
}

@DefaultBean allows extensions (or any other code for that matter) to provide defaults while backing off if beans of that type are supplied in any way Quarkus supports.