Weld SiteCommunity Documentation
CDI is intended to be a foundation for frameworks, extensions and integration with other technologies. Therefore, CDI exposes a set of SPIs for the use of developers of portable extensions to CDI. For example, the following kinds of extensions were envisaged by the designers of CDI:
More formally, according to the spec:
A portable extension may integrate with the container by:
- Providing its own beans, interceptors and decorators to the container
- Injecting dependencies into its own objects using the dependency injection service
- Providing a context implementation for a custom scope
- Augmenting or overriding the annotation-based metadata with metadata from some other source
The first step in creating a portable extension is to write a class that
implements Extension. This marker interface does not define any
methods, but it’s needed to satisfy the requirements of Java SE’s
service provider architecture.
import javax.enterprise.inject.spi.Extension;
class MyExtension implements Extension { ... }
Next, we need to register our extension as a service provider by
creating a file named
META-INF/services/javax.enterprise.inject.spi.Extension, which
contains the name of our extension class:
org.mydomain.extension.MyExtension
An extension is not a bean, exactly, since it is instantiated by the container during the initialization process, before any beans or contexts exist. However, it can be injected into other beans once the initialization process is complete.
@Inject
MyBean(MyExtension myExtension) {
myExtension.doSomething();
}
And, like beans, extensions can have observer methods. Usually, the observer methods observe container lifecycle events.
During the initialization process, the container fires a series of events, including:
BeforeBeanDiscoveryProcessAnnotatedType and ProcessSyntheticAnnotatedTypeAfterTypeDiscoveryProcessInjectionTarget and ProcessProducerProcessInjectionPointProcessBeanAttributesProcessBean, ProcessManagedBean, ProcessSessionBean,
ProcessProducerMethod and ProcessProducerFieldProcessObserverMethodAfterBeanDiscoveryAfterDeploymentValidationExtensions may observe these events:
import javax.enterprise.inject.spi.Extension;
class MyExtension implements Extension {
void beforeBeanDiscovery(@Observes BeforeBeanDiscovery bbd) {
Logger.global.debug("beginning the scanning process");
}
<T> void processAnnotatedType(@Observes ProcessAnnotatedType<T> pat) {
Logger.global.debug("scanning type: " + pat.getAnnotatedType().getJavaClass().getName());
}
void afterBeanDiscovery(@Observes AfterBeanDiscovery abd) {
Logger.global.debug("finished the scanning process");
}
}
In fact, the extension can do a lot more than just observe. The extension is permitted to modify the container’s metamodel and more. Here’s a very simple example:
import javax.enterprise.inject.spi.Extension;
class MyExtension implements Extension {
<T> void processAnnotatedType(@Observes @WithAnnotations({Ignore.class}) ProcessAnnotatedType<T> pat) {
/* tell the container to ignore the type if it is annotated @Ignore */
if ( pat.getAnnotatedType().isAnnotationPresent(Ignore.class) ) pat.veto();
}
}
The @WithAnnotations annotation causes the container to deliver the
ProcessAnnotatedType events only for the types which contain the
specified annotation.
The observer method may inject a BeanManager
<T> void processAnnotatedType(@Observes ProcessAnnotatedType<T> pat, BeanManager beanManager) { ... }
An extension observer method is not allowed to inject any other object.
The nerve center for extending CDI is the BeanManager object. The
BeanManager interface lets us obtain beans, interceptors, decorators,
observers and contexts programmatically.
public interface BeanManager {
public Object getReference(Bean<?> bean, Type beanType, CreationalContext<?> ctx);
public Object getInjectableReference(InjectionPoint ij, CreationalContext<?> ctx);
public <T> CreationalContext<T> createCreationalContext(Contextual<T> contextual);
public Set<Bean<?>> getBeans(Type beanType, Annotation... qualifiers);
public Set<Bean<?>> getBeans(String name);
public Bean<?> getPassivationCapableBean(String id);
public <X> Bean<? extends X> resolve(Set<Bean<? extends X>> beans);
public void validate(InjectionPoint injectionPoint);
public void fireEvent(Object event, Annotation... qualifiers);
public <T> Set<ObserverMethod<? super T>> resolveObserverMethods(T event, Annotation... qualifiers);
public List<Decorator<?>> resolveDecorators(Set<Type> types, Annotation... qualifiers);
public List<Interceptor<?>> resolveInterceptors(InterceptionType type, Annotation... interceptorBindings);
public boolean isScope(Class<? extends Annotation> annotationType);
public boolean isNormalScope(Class<? extends Annotation> annotationType);
public boolean isPassivatingScope(Class<? extends Annotation> annotationType);
public boolean isQualifier(Class<? extends Annotation> annotationType);
public boolean isInterceptorBinding(Class<? extends Annotation> annotationType);
public boolean isStereotype(Class<? extends Annotation> annotationType);
public Set<Annotation> getInterceptorBindingDefinition(Class<? extends Annotation> bindingType);
public Set<Annotation> getStereotypeDefinition(Class<? extends Annotation> stereotype);
public boolean areQualifiersEquivalent(Annotation qualifier1, Annotation qualifier2);
public boolean areInterceptorBindingsEquivalent(Annotation interceptorBinding1, Annotation interceptorBinding2);
public int getQualifierHashCode(Annotation qualifier);
public int getInterceptorBindingHashCode(Annotation interceptorBinding);
public Context getContext(Class<? extends Annotation> scopeType);
public ELResolver getELResolver();
public ExpressionFactory wrapExpressionFactory(ExpressionFactory expressionFactory);
public <T> AnnotatedType<T> createAnnotatedType(Class<T> type);
public <T> InjectionTarget<T> createInjectionTarget(AnnotatedType<T> type);
public <T> InjectionTargetFactory<T> getInjectionTargetFactory(AnnotatedType<T> annotatedType);
public <X> ProducerFactory<X> getProducerFactory(AnnotatedField<? super X> field, Bean<X> declaringBean);
public <X> ProducerFactory<X> getProducerFactory(AnnotatedMethod<? super X> method, Bean<X> declaringBean);
public <T> BeanAttributes<T> createBeanAttributes(AnnotatedType<T> type);
public BeanAttributes<?> createBeanAttributes(AnnotatedMember<?> type);
public <T> Bean<T> createBean(BeanAttributes<T> attributes, Class<T> beanClass,
public <T, X> Bean<T> createBean(BeanAttributes<T> attributes, Class<X> beanClass, ProducerFactory<X> producerFactory);
public InjectionPoint createInjectionPoint(AnnotatedField<?> field);
public InjectionPoint createInjectionPoint(AnnotatedParameter<?> parameter);
public <T extends Extension> T getExtension(Class<T> extensionClass);
}
Any bean or other Java EE component which supports injection can obtain
an instance of BeanManager via injection:
@Inject BeanManager beanManager;
Alternatively, a BeanManager reference may be obtained from CDI via a
static method call.
CDI.current().getBeanManager()
Java EE components may obtain an instance of BeanManager from JNDI by
looking up the name java:comp/BeanManager. Any operation of
BeanManager may be called at any time during the execution of the
application.
Let’s study some of the interfaces exposed by the BeanManager.
Application components which cannot obtain a BeanManager reference via
injection nor JNDI lookup can get the reference from the
javax.enterprise.inject.spi.CDI class via a static method call:
BeanManager manager = CDI.current().getBeanManager();
The CDI class can be used directly to programmatically lookup CDI
beans as described in Section 4.10, “Obtaining a contextual instance by programmatic lookup”
CDI.select(Foo.class).get()
The first thing that a framework developer is going to look for in the
portable extension SPI is a way to inject CDI beans into objects which
are not under the control of CDI. The InjectionTarget interface makes
this very easy.
We recommend that frameworks let CDI take over the job of actually instantiating the framework-controlled objects. That way, the framework-controlled objects can take advantage of constructor injection. However, if the framework requires use of a constructor with a special signature, the framework will need to instantiate the object itself, and so only method and field injection will be supported.
import javax.enterprise.inject.spi.CDI;
...
//get the BeanManager
BeanManager beanManager = CDI.current().getBeanManager();
//CDI uses an AnnotatedType object to read the annotations of a class
AnnotatedType<SomeFrameworkComponent> type = beanManager.createAnnotatedType(SomeFrameworkComponent.class);
//The extension uses an InjectionTarget to delegate instantiation, dependency injection
//and lifecycle callbacks to the CDI container
InjectionTarget<SomeFrameworkComponent> it = beanManager.createInjectionTarget(type);
//each instance needs its own CDI CreationalContext
CreationalContext ctx = beanManager.createCreationalContext(null);
//instantiate the framework component and inject its dependencies
SomeFrameworkComponent instance = it.produce(ctx); //call the constructor
it.inject(instance, ctx); //call initializer methods and perform field injection
it.postConstruct(instance); //call the @PostConstruct method
...
//destroy the framework component instance and clean up dependent objects
it.preDestroy(instance); //call the @PreDestroy method
it.dispose(instance); //it is now safe to discard the instance
ctx.release(); //clean up dependent objects
Instances of the interface Bean represent beans. There is an instance
of Bean registered with the BeanManager object for every bean in the
application. There are even Bean objects representing interceptors,
decorators and producer methods.
The BeanAttributes interface exposes all the interesting things we
discussed in Section 2.1, “The anatomy of a bean”.
public interface BeanAttributes<T> {
public Set<Type> getTypes();
public Set<Annotation> getQualifiers();
public Class<? extends Annotation> getScope();
public String getName();
public Set<Class<? extends Annotation>> getStereotypes();
public boolean isAlternative();
}
The Bean interface extends the BeanAttributes interface and defines
everything the container needs to manage instances of a certain bean.
public interface Bean<T> extends Contextual<T>, BeanAttributes<T> {
public Class<?> getBeanClass();
public Set<InjectionPoint> getInjectionPoints();
public boolean isNullable();
}
There’s an easy way to find out what beans exist in the application:
Set<Bean<?>> allBeans = beanManager.getBeans(Object.class, new AnnotationLiteral<Any>() {});
The Bean interface makes it possible for a portable extension to
provide support for new kinds of beans, beyond those defined by the CDI
specification. For example, we could use the Bean interface to allow
objects managed by another framework to be injected into beans.
The most common kind of CDI portable extension registers a bean (or beans) with the container.
In this example, we make a framework class, SecurityManager available
for injection. To make things a bit more interesting, we’re going to
delegate back to the container’s InjectionTarget to perform
instantiation and injection upon the SecurityManager instance.
import javax.enterprise.inject.spi.Extension;
import javax.enterprise.event.Observes;
import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import javax.enterprise.inject.spi.InjectionPoint;
...
public class SecurityManagerExtension implements Extension {
void afterBeanDiscovery(@Observes AfterBeanDiscovery abd, BeanManager bm) {
//use this to read annotations of the class
AnnotatedType<SecurityManager> at = bm.createAnnotatedType(SecurityManager.class);
//use this to instantiate the class and inject dependencies
final InjectionTarget<SecurityManager> it = bm.createInjectionTarget(at);
abd.addBean( new Bean<SecurityManager>() {
@Override
public Class<?> getBeanClass() {
return SecurityManager.class;
}
@Override
public Set<InjectionPoint> getInjectionPoints() {
return it.getInjectionPoints();
}
@Override
public String getName() {
return "securityManager";
}
@Override
public Set<Annotation> getQualifiers() {
Set<Annotation> qualifiers = new HashSet<Annotation>();
qualifiers.add( new AnnotationLiteral<Default>() {} );
qualifiers.add( new AnnotationLiteral<Any>() {} );
return qualifiers;
}
@Override
public Class<? extends Annotation> getScope() {
return ApplicationScoped.class;
}
@Override
public Set<Class<? extends Annotation>> getStereotypes() {
return Collections.emptySet();
}
@Override
public Set<Type> getTypes() {
Set<Type> types = new HashSet<Type>();
types.add(SecurityManager.class);
types.add(Object.class);
return types;
}
@Override
public boolean isAlternative() {
return false;
}
@Override
public boolean isNullable() {
return false;
}
@Override
public SecurityManager create(CreationalContext<SecurityManager> ctx) {
SecurityManager instance = it.produce(ctx);
it.inject(instance, ctx);
it.postConstruct(instance);
return instance;
}
@Override
public void destroy(SecurityManager instance,
CreationalContext<SecurityManager> ctx) {
it.preDestroy(instance);
it.dispose(instance);
ctx.release();
}
} );
}
}
But a portable extension can also mess with beans that are discovered automatically by the container.
One of the most interesting things that an extension class can do is process the annotations of a bean class before the container builds its metamodel.
Let’s start with an example of an extension that provides support for
the use of @Named at the package level. The package-level name is used
to qualify the EL names of all beans defined in that package. The
portable extension uses the ProcessAnnotatedType event to wrap the
AnnotatedType object and override the value() of the @Named
annotation.
import java.lang.reflect.Type;
import javax.enterprise.inject.spi.Extension;
import java.lang.annotation.Annotation;
...
public class QualifiedNameExtension implements Extension {
<X> void processAnnotatedType(@Observes ProcessAnnotatedType<X> pat) {
/* wrap this to override the annotations of the class */
final AnnotatedType<X> at = pat.getAnnotatedType();
/* Only wrap AnnotatedTypes for classes with @Named packages */
Package pkg = at.getJavaClass().getPackage();
if ( !pkg.isAnnotationPresent(Named.class) ) {
return;
}
AnnotatedType<X> wrapped = new AnnotatedType<X>() {
class NamedLiteral extends AnnotationLiteral<Named>
implements Named {
@Override
public String value() {
Package pkg = at.getJavaClass().getPackage();
String unqualifiedName = "";
if (at.isAnnotationPresent(Named.class)) {
unqualifiedName = at.getAnnotation(Named.class).value();
}
if (unqualifiedName.isEmpty()) {
unqualifiedName = Introspector.decapitalize(at.getJavaClass().getSimpleName());
}
final String qualifiedName;
if ( pkg.isAnnotationPresent(Named.class) ) {
qualifiedName = pkg.getAnnotation(Named.class).value()
+ '.' + unqualifiedName;
}
else {
qualifiedName = unqualifiedName;
}
return qualifiedName;
}
}
private final NamedLiteral namedLiteral = new NamedLiteral();
@Override
public Set<AnnotatedConstructor<X>> getConstructors() {
return at.getConstructors();
}
@Override
public Set<AnnotatedField<? super X>> getFields() {
return at.getFields();
}
@Override
public Class<X> getJavaClass() {
return at.getJavaClass();
}
@Override
public Set<AnnotatedMethod<? super X>> getMethods() {
return at.getMethods();
}
@Override
public <T extends Annotation> T getAnnotation(final Class<T> annType) {
if (Named.class.equals(annType)) {
return (T) namedLiteral;
}
else {
return at.getAnnotation(annType);
}
}
@Override
public Set<Annotation> getAnnotations() {
Set<Annotation> original = at.getAnnotations();
Set<Annotation> annotations = new HashSet<Annotation>();
boolean hasNamed = false;
for (Annotation annotation : original) {
if (annotation.annotationType().equals(Named.class)) {
annotations.add(getAnnotation(Named.class));
hasNamed = true;
}
else {
annotations.add(annotation);
}
}
if (!hasNamed) {
Package pkg = at.getJavaClass().getPackage();
if (pkg.isAnnotationPresent(Named.class)) {
annotations.add(getAnnotation(Named.class));
}
}
return annotations;
}
@Override
public Type getBaseType() {
return at.getBaseType();
}
@Override
public Set<Type> getTypeClosure() {
return at.getTypeClosure();
}
@Override
public boolean isAnnotationPresent(Class<? extends Annotation> annType) {
if (Named.class.equals(annType)) {
return true;
}
return at.isAnnotationPresent(annType);
}
};
pat.setAnnotatedType(wrapped);
}
}
Here’s a second example, which adds the @Alternative annotation to any
class which implements a certain Service interface.
import javax.enterprise.inject.spi.Extension;
import java.lang.annotation.Annotation;
...
class ServiceAlternativeExtension implements Extension {
<T extends Service> void processAnnotatedType(@Observes ProcessAnnotatedType<T> pat) {
final AnnotatedType<T> type = pat.getAnnotatedType();
/* if the class implements Service, make it an @Alternative */
AnnotatedType<T> wrapped = new AnnotatedType<T>() {
class AlternativeLiteral extends AnnotationLiteral<Alternative> implements Alternative {}
private final AlternativeLiteral alternativeLiteral = new AlternativeLiteral();
@Override
public <X extends Annotation> X getAnnotation(final Class<X> annType) {
return (X) (annType.equals(Alternative.class) ? alternativeLiteral : type.getAnnotation(annType));
}
@Override
public Set<Annotation> getAnnotations() {
Set<Annotation> annotations = new HashSet<Annotation>(type.getAnnotations());
annotations.add(alternativeLiteral);
return annotations;
}
@Override
public boolean isAnnotationPresent(Class<? extends Annotation> annotationType) {
return annotationType.equals(Alternative.class) ?
true : type.isAnnotationPresent(annotationType);
}
/* remaining methods of AnnotatedType */
...
}
pat.setAnnotatedType(wrapped);
}
}
The AnnotatedType is not the only thing that can be wrapped by an
extension.
Wrapping an AnnotatedType is a low-level approach to overriding CDI
metadata by adding, removing or replacing annotations. Since version
1.1, CDI provides a higher-level facility for overriding attributes of
beans discovered by the CDI container.
public interface BeanAttributes<T> {
public Set<Type> getTypes();
public Set<Annotation> getQualifiers();
public Class<? extends Annotation> getScope();
public String getName();
public Set<Class<? extends Annotation>> getStereotypes();
public boolean isAlternative();
}
The BeanAttributes interface exposes attributes of a bean. The
container fires a ProcessBeanAttributes event for each enabled bean,
interceptor and decorator before this object is registered. Similarly to
the ProcessAnnotatedType, this event allows an extension to modify
attributes of a bean or to veto the bean entirely.
public interface ProcessBeanAttributes<T> {
public Annotated getAnnotated();
public BeanAttributes<T> getBeanAttributes();
public void setBeanAttributes(BeanAttributes<T> beanAttributes);
public void addDefinitionError(Throwable t);
public void veto();
}
The BeanManager provides two utility methods for creating the
BeanAttributes object from scratch:
public <T> BeanAttributes<T> createBeanAttributes(AnnotatedType<T> type);
public BeanAttributes<?> createBeanAttributes(AnnotatedMember<?> type);
The InjectionTarget interface exposes operations for producing and
disposing an instance of a component, injecting its dependencies and
invoking its lifecycle callbacks. A portable extension may wrap the
InjectionTarget for any Java EE component that supports injection,
allowing it to intercept any of these operations when they are invoked
by the container.
Here’s a CDI portable extension that reads values from properties files
and configures fields of Java EE components, including servlets, EJBs,
managed beans, interceptors and more. In this example, properties for a
class such as org.mydomain.blog.Blogger go in a resource named
org/mydomain/blog/Blogger.properties, and the name of a property must
match the name of the field to be configured. So Blogger.properties
could contain:
firstName=Gavin
lastName=King
The portable extension works by wrapping the containers
InjectionTarget and setting field values from the inject() method.
import javax.enterprise.event.Observes;
import javax.enterprise.inject.spi.Extension;
import javax.enterprise.inject.spi.InjectionPoint;
public class ConfigExtension implements Extension {
<X> void processInjectionTarget(@Observes ProcessInjectionTarget<X> pit) {
/* wrap this to intercept the component lifecycle */
final InjectionTarget<X> it = pit.getInjectionTarget();
final Map<Field, Object> configuredValues = new HashMap<Field, Object>();
/* use this to read annotations of the class and its members */
AnnotatedType<X> at = pit.getAnnotatedType();
/* read the properties file */
String propsFileName = at.getJavaClass().getSimpleName() + ".properties";
InputStream stream = at.getJavaClass().getResourceAsStream(propsFileName);
if (stream!=null) {
try {
Properties props = new Properties();
props.load(stream);
for (Map.Entry<Object, Object> property : props.entrySet()) {
String fieldName = property.getKey().toString();
Object value = property.getValue();
try {
Field field = at.getJavaClass().getDeclaredField(fieldName);
field.setAccessible(true);
if ( field.getType().isAssignableFrom( value.getClass() ) ) {
configuredValues.put(field, value);
}
else {
/* TODO: do type conversion automatically */
pit.addDefinitionError( new InjectionException(
"field is not of type String: " + field ) );
}
}
catch (NoSuchFieldException nsfe) {
pit.addDefinitionError(nsfe);
}
finally {
stream.close();
}
}
}
catch (IOException ioe) {
pit.addDefinitionError(ioe);
}
}
InjectionTarget<X> wrapped = new InjectionTarget<X>() {
@Override
public void inject(X instance, CreationalContext<X> ctx) {
it.inject(instance, ctx);
/* set the values onto the new instance of the component */
for (Map.Entry<Field, Object> configuredValue: configuredValues.entrySet()) {
try {
configuredValue.getKey().set(instance, configuredValue.getValue());
}
catch (Exception e) {
throw new InjectionException(e);
}
}
}
@Override
public void postConstruct(X instance) {
it.postConstruct(instance);
}
@Override
public void preDestroy(X instance) {
it.dispose(instance);
}
@Override
public void dispose(X instance) {
it.dispose(instance);
}
@Override
public Set<InjectionPoint> getInjectionPoints() {
return it.getInjectionPoints();
}
@Override
public X produce(CreationalContext<X> ctx) {
return it.produce(ctx);
}
};
pit.setInjectionTarget(wrapped);
}
}
CDI provides a way to override the metadata of an InjectionPoint. This
works similarly to how metadata of a bean may be overridden using
BeanAttributes.
For every injection point of each component supporting injection Weld
fires an event of type
javax.enterprise.inject.spi.ProcessInjectionPoint
public interface ProcessInjectionPoint<T, X> {
public InjectionPoint getInjectionPoint();
public void setInjectionPoint(InjectionPoint injectionPoint);
public void addDefinitionError(Throwable t);
}
An extension may either completely override the injection point metadata
or alter it by wrapping the InjectionPoint object obtained from
ProcessInjectionPoint.getInjectionPoint()
There’s a lot more to the portable extension SPI than what we’ve discussed here. Check out the CDI spec or Javadoc for more information. For now, we’ll just mention one more extension point.
An event of type javax.enterprise.inject.spi.AfterTypeDiscovery is
fired when the container has fully completed the type discovery process
and before it begins the bean discovery process.
public interface AfterTypeDiscovery {
public List<Class<?>> getAlternatives();
public List<Class<?>> getInterceptors();
public List<Class<?>> getDecorators();
public void addAnnotatedType(AnnotatedType<?> type, String id);
}
This event exposes a list of enabled alternatives, interceptors and decorators. Extensions may manipulate these collections directly to add, remove or change the order of the enabled records.
In addition, an AnnotatedType can be added to the types which will be
scanned during bean discovery, with an identifier, which allows multiple
annotated types, based on the same underlying type, to be defined.
The Context and AlterableContext interface support addition of new
scopes to CDI, or extension of the built-in scopes to new environments.
public interface Context {
public Class<? extends Annotation> getScope();
public <T> T get(Contextual<T> contextual, CreationalContext<T> creationalContext);
public <T> T get(Contextual<T> contextual);
boolean isActive();
}
For example, we might implement Context to add a business process
scope to CDI, or to add support for the conversation scope to an
application that uses Wicket.
import javax.enterprise.context.spi.Context;
public interface AlterableContext extends Context {
public void destroy(Contextual<?> contextual);
}
AlterableContext was introduced in CDI 1.1. The destroy method
allows an application to remove instances of contextual objects from a
context.
For more information on implementing a custom context see this blog post.