Quarkus - Contexts and Dependency Injection

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. However, Quarkus is using a simplified bean discovery. There is only single bean archive with the bean discovery mode annotated and no visibility boundaries.

The bean archive is synthesized from:

  • the application classes,

  • dependencies that contain a beans.xml descriptor (content is ignored),

  • dependencies that contain a Jandex index - META-INF/jandex.idx,

  • dependencies referenced by quarkus.index-dependency in application.properties,

  • 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 extensions 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.

1.1. How to Generate a Jandex Index

A dependency with a Jandex index is automatically scanned for beans. To generate the index just add the following to your pom.xml:

<build>
  <plugins>
    <plugin>
      <groupId>org.jboss.jandex</groupId>
      <artifactId>jandex-maven-plugin</artifactId>
      <version>1.0.7</version>
      <executions>
        <execution>
          <id>make-index</id>
          <goals>
            <goal>jandex</goal>
          </goals>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

If you can’t modify the dependency, you can still index it by adding quarkus.index-dependency entries to your application.properties:

quarkus.index-dependency.<name>.group-id=
quarkus.index-dependency.<name>.artifact-id=
quarkus.index-dependency.<name>.classifier=(this one is optional)

For example, the following entries ensure that the org.acme:acme-api dependency is indexed:

quarkus.index-dependency.acme.group-id=org.acme (1)
quarkus.index-dependency.acme.artifact-id=acme-api (2)
1 Value is a group id for a dependency identified by name acme.
2 Value is an artifact id for a dependency identified by name acme.

2. Native Executables and Private Members

Quarkus is using GraalVM to build a native executable. One of the limitations of GraalVM is the usage of Reflection. Reflective operations are supported but all relevant members must be registered for reflection explicitly. Those registrations result in a bigger native executable.

And if Quarkus DI needs to access a private member it has to use reflection. 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.

How to avoid using private members? You can use 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. @Inject is optional in this particular case.

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

  • BEFORE_COMPLETION, AFTER_COMPLETION, AFTER_FAILURE and AFTER_SUCCESS transactional observers are not implemented yet

5. Non-standard Features

5.1. Eager Instantiation of Beans

5.1.1. Lazy By Default

By default, CDI beans are created lazily, when needed. What exactly "needed" means depends on the scope of a bean.

  • A normal scoped bean (@ApplicationScoped, @RequestScoped, etc.) is needed when a method is invoked upon an injected instance (contextual reference per the specification).

    In other words, injecting a normal scoped bean will not suffice because a client proxy is injected instead of a contextual instance of the bean.

  • A bean with a pseudo-scope (@Dependent and @Singleton ) is created when injected.

Lazy Instantiation Example
@Singleton // => pseudo-scope
class AmazingService {
  String ping() {
    return "amazing";
  }
}

@ApplicationScoped // => normal scope
class CoolService {
  String ping() {
    return "cool";
  }
}

@Path("/ping")
public class PingResource {

  @Inject
  AmazingService s1; (1)

  @Inject
  CoolService s2; (2)

  @GET
  public String ping() {
    return s1.ping() + s2.ping(); (3)
  }
}
1 Injection triggers the instantiation of AmazingService.
2 Injection itself does not result in the instantiation of CoolService. A client proxy is injected.
3 The first invocation upon the injected proxy triggers the instantiation of CoolService.

5.1.2. Startup Event

However, if you really need to instantiate a bean eagerly you can:

  • Declare an observer of the StartupEvent - the scope of the bean does not matter in this case:

    @ApplicationScoped
class CoolService {
  void startup(@Observes StartupEvent event) { (1)
  }
}
    1 A CoolService is created during startup to service the observer method invocation.

  • Use the bean in an observer of the StartupEvent - normal scoped beans must be used as described in Lazy By Default:

    @Dependent
class MyBeanStarter {

  void startup(@Observes StartupEvent event, AmazingService amazing, CoolService cool) { (1)
    cool.toString(); (2)
  }
}
    1 The AmazingService is created during injection.
    2 The CoolService is a normal scoped bean so we have to invoke a method upon the injected proxy to force the instantiation.
Quarkus users are encouraged to always prefer the @Observes StartupEvent to @Initialized(ApplicationScoped.class) as explained in the Application Initialization and Termination guide.

5.2. Qualified Injected Fields

In CDI, if you declare a field injection point you need to use @Inject and optionally a set of qualifiers:

  @Inject
  @ConfigProperty(name = "cool")
  String coolProperty;

In Quarkus, you can skip the @Inject annotation completely if an injected field declares at least one qualifier:

  @ConfigProperty(name = "cool")
  String coolProperty;
With the notable exception of one special case discussed below, @Inject is still required for constructor and method injection.

5.3. Simplified Constructor Injection

In CDI, a normal scoped bean must always declare a no-args constructor (this constructor is normally generated by the compiler unless you declare any other constructor). However, this requirement complicates constructor injection - you need to provide a dummy no-args constructor to make things work in CDI.

@ApplicationScoped
public class MyCoolService {

  private SimpleProcessor processor;

  MyCoolService() { // dummy constructor needed
  }

  @Inject // constructor injection
  MyCoolService(SimpleProcessor processor) {
    this.processor = processor;
  }
}

There is no need to declare dummy constructors for normal scoped bean in Quarkus - they are generated automatically. Also if there’s only one constructor there is no need for @Inject.

@ApplicationScoped
public class MyCoolService {

  private SimpleProcessor processor;

  MyCoolService(SimpleProcessor processor) {
    this.processor = processor;
  }
}
We don’t generate a no-args constructor automatically if a bean class extends a class that does not declare a no-args constructor.

5.4. 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

This optimization applies to all forms of bean declarations: bean class, producer method, producer field.

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. This annotation can be placed on the types, producer methods, and producer fields.

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.

When using the dev mode (running mvn clean compile quarkus:dev), you can see more information about which beans are being removed by enabling additional logging via the following line in your application.properties.

quarkus.log.category."io.quarkus.arc.processor".level=DEBUG

5.5. Default Beans

Quarkus adds a capability that CDI currently does not support which is to conditionally declare a bean if no other bean with equal types and qualifiers was declared by any available means (bean class, producer, synthetic bean, …​) 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 the 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.

6. Build Time Extension Points

6.1. 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.

6.2. 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.

6.3. 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())));
}

6.4. 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.

6.5. 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.
You can easily filter all class-based beans via the convenient BeanStream returned from the RegistrationContext.beans() method.

If an extension needs to produce other build items during the "bean registration" phase it should use the BeanRegistrationPhaseBuildItem instead. The reason is that injected objects are only valid during a @BuildStep method invocation.

@BuildStep
void syntheticBean(BeanRegistrationPhaseBuildItem beanRegistrationPhase,
            BuildProducer<MyBuildItem> myBuildItem,
            BuildProducer<BeanConfiguratorBuildItem> beanConfigurators) {
   beanConfigurators.produce(new BeanConfiguratorBuildItem(beanRegistrationPhase.getContext().configure(String.class).types(String.class).qualifiers(new MyQualifierLiteral()).creator(mc -> mc.returnValue(mc.load("foo")))));
   myBuildItem.produce(new MyBuildItem());
}
See BeanRegistrationPhaseBuildItem javadoc for more information.

6.6. 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();
            }
        }
    });
}

6.7. Additional Interceptor Bindings

In rare cases it might be handy to programmatically register an existing annotation as interceptor binding. This is similar to what pure CDI achieves through BeforeBeanDiscovery#addInterceptorBinding(). Though here we are going to use InterceptorBindingRegistrarBuildItem to get it done. Note that you can register multiple annotations in one go:

@BuildStep
InterceptorBindingRegistrarBuildItem addInterceptorBindings() {
    InterceptorBindingRegistrarBuildItem additionalBindingsRegistrar = new InterceptorBindingRegistrarBuildItem(new InterceptorBindingRegistrar() {
        @Override
        public Collection<DotName> registerAdditionalBindings() {
            Collection<DotName> result = new HashSet<>();
            result.add(DotName.createSimple(MyAnnotation.class.getName()));
            result.add(DotName.createSimple(MyOtherAnnotation.class.getName()));
            return result;
        }
    });
    return additionalBindingsRegistrar;
}

6.8. 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();
            }
        }
    });
}

6.9. 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);
             }
         }
    });
}
You can easily filter all registered beans via the convenient BeanStream returned from the ValidationContext.beans() method.

If an extension needs to produce other build items during the "validation" phase it should use the ValidationPhaseBuildItem instead. The reason is that injected objects are only valid during a @BuildStep method invocation.

@BuildStep
void validate(ValidationPhaseBuildItem validationPhase,
            BuildProducer<MyBuildItem> myBuildItem,
            BuildProducer<ValidationErrorBuildItem> errors) {
   if (someCondition) {
     errors.produce(new ValidationErrorBuildItem(new IllegalStateException()));
     myBuildItem.produce(new MyBuildItem());
   }
}
See ValidationPhaseBuildItem javadoc for more information.

6.10. 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();
         }
    });
}

If an extension needs to produce other build items during the "context registration" phase it should use the ContextRegistrationPhaseBuildItem instead. The reason is that injected objects are only valid during a @BuildStep method invocation.

@BuildStep
void addContext(ContextRegistrationPhaseBuildItem contextRegistrationPhase,
            BuildProducer<MyBuildItem> myBuildItem,
            BuildProducer<ContextConfiguratorBuildItem> contexts) {
   contexts.produce(new ContextConfiguratorBuildItem(contextRegistrationPhase.getContext().configure(CustomScoped.class).normal().contextClass(MyCustomContext.class)));
   myBuildItem.produce(new MyBuildItem());
}
See ContextRegistrationPhaseBuildItem javadoc for more information.

6.11. 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

    • Collection<InjectionPointInfo> containing all injection points

  • BEANS

    • Collection<BeanInfo> containing all beans

  • REMOVED_BEANS

  • OBSERVERS

    • Collection<ObserverInfo> containing all observers

  • SCOPES

    • Collection<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 ANNOTATION_STORE, QUALIFIERS, INTERCEPTOR_BINDINGS, STEREOTYPES, BEANS

  • BeanDeploymentValidator

    • Has access to all build metadata

Quarkus is open. All dependencies of this project are available under the Apache Software License 2.0 or compatible license.

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