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Interceptor functionality is defined in the Java Interceptors specification.
The Interceptors specification defines three kinds of interception points:
A business method interceptor applies to invocations of methods of the bean by clients of the bean:
public class TransactionInterceptor {
@AroundInvoke
public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}
A lifecycle callback interceptor applies to invocations of lifecycle callbacks by the container:
public class DependencyInjectionInterceptor {
@PostConstruct
public void injectDependencies(InvocationContext ctx) { ... }
}
An interceptor class may intercept both lifecycle callbacks and business methods.
A timeout method interceptor applies to invocations of EJB timeout methods by the container:
public class TimeoutInterceptor {
@AroundTimeout
public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}
Suppose we want to declare that some of our beans are transactional. The first thing we need is an interceptor binding type to specify exactly which beans we’re interested in:
@InterceptorBinding
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface Transactional {}
Now we can easily specify that our ShoppingCart is a transactional
object:
@Transactional
public class ShoppingCart { ... }
Or, if we prefer, we can specify that just one method is transactional:
public class ShoppingCart {
@Transactional public void checkout() { ... }
}
That’s great, but somewhere along the line we’re going to have to
actually implement the interceptor that provides this transaction
management aspect. All we need to do is create a standard interceptor,
and annotate it @Interceptor and @Transactional.
@Transactional @Interceptor
public class TransactionInterceptor {
@AroundInvoke
public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}
Interceptors can take advantage of dependency injection:
@Transactional @Interceptor
public class TransactionInterceptor {
@Resource UserTransaction transaction;
@AroundInvoke
public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}
Multiple interceptors may use the same interceptor binding type.
By default, all interceptors are disabled. We need to enable our
interceptor. We can do it using beans.xml descriptor of a bean
archive. However, this activation only applies to the beans in that
archive. From CDI 1.1 onwards the interceptor can be enabled for the
whole application using @Priority annotation.
<beans
xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/beans_1_1.xsd">
<interceptors>
<class>org.mycompany.myapp.TransactionInterceptor</class>
</interceptors>
</beans>
Whoah! Why the angle bracket stew?
Well, having the XML declaration is actually a good thing. It solves two problems:
Having two interceptors without @Priority, we could specify that our
security interceptor runs before our transaction interceptor.
<beans
xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/beans_1_1.xsd">
<interceptors>
<class>org.mycompany.myapp.SecurityInterceptor</class>
<class>org.mycompany.myapp.TransactionInterceptor</class>
</interceptors>
</beans>
Or we could turn them both off in our test environment by simply not
mentioning them in beans.xml! Ah, so simple.
It gets quite tricky when used along with interceptors annotated with
@Priority. Interceptors enabled using @Priority are called before
interceptors enabled using beans.xml, the lower priority values are
called first.
An interceptor enabled by @Priority and in the same time listed in beans.xml is only invoked once in the @Priority part of the
invocation chain. E.g. the enablement via beans.xml will be ignored.
Suppose we want to add some extra information to our @Transactional
annotation:
@InterceptorBinding
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface Transactional {
boolean requiresNew() default false;
}
CDI will use the value of requiresNew to choose between two different
interceptors, TransactionInterceptor and
RequiresNewTransactionInterceptor.
@Transactional(requiresNew = true) @Interceptor
public class RequiresNewTransactionInterceptor {
@AroundInvoke
public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}
Now we can use RequiresNewTransactionInterceptor like this:
@Transactional(requiresNew = true)
public class ShoppingCart { ... }
But what if we only have one interceptor and we want the container to
ignore the value of requiresNew when binding interceptors? Perhaps
this information is only useful for the interceptor implementation. We
can use the @Nonbinding annotation:
@InterceptorBinding
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface Secure {
@Nonbinding String[] rolesAllowed() default {};
}
Usually we use combinations of interceptor bindings types to bind
multiple interceptors to a bean. For example, the following declaration
would be used to bind TransactionInterceptor and SecurityInterceptor
to the same bean:
@Secure(rolesAllowed="admin") @Transactional
public class ShoppingCart { ... }
However, in very complex cases, an interceptor itself may specify some combination of interceptor binding types:
@Transactional @Secure @Interceptor
public class TransactionalSecureInterceptor { ... }
Then this interceptor could be bound to the checkout() method using
any one of the following combinations:
public class ShoppingCart {
@Transactional @Secure public void checkout() { ... }
}
@Secure
public class ShoppingCart {
@Transactional public void checkout() { ... }
}
@Transactional
public class ShoppingCart {
@Secure public void checkout() { ... }
}
@Transactional @Secure
public class ShoppingCart {
public void checkout() { ... }
}
One limitation of the Java language support for annotations is the lack of annotation inheritance. Really, annotations should have reuse built in, to allow this kind of thing to work:
public @interface Action extends Transactional, Secure { ... }
Well, fortunately, CDI works around this missing feature of Java. We may annotate one interceptor binding type with other interceptor binding types (termed a meta-annotation). The interceptor bindings are transitive — any bean with the first interceptor binding inherits the interceptor bindings declared as meta-annotations.
@Transactional @Secure
@InterceptorBinding
@Target(TYPE)
@Retention(RUNTIME)
public @interface Action { ... }
Now, any bean annotated @Action will be bound to both
TransactionInterceptor and SecurityInterceptor. (And even
TransactionalSecureInterceptor, if it exists.)
The @Interceptors annotation defined by the Interceptors specification
(and used by the Managed Beans and EJB specifications) is still
supported in CDI.
@Interceptors({TransactionInterceptor.class, SecurityInterceptor.class})
public class ShoppingCart {
public void checkout() { ... }
}
However, this approach suffers the following drawbacks:
Therefore, we recommend the use of CDI-style interceptor bindings.
For even more control over interceptors, Weld offers enhanced version of javax.interceptor.InvocationContext - org.jboss.weld.interceptor.WeldInvocationContext.
It comes with two additional methods - getInterceptorBindings and getInterceptorBindingsByType(Class<T> annotationType).
You shouldn’t need this in most cases, but it comes handy when working with @Nonbinding values in interceptor binding annotations.
Assume you have the following interceptor binding:
@InterceptorBinding
@Inherited
@Target({ TYPE, METHOD, CONSTRUCTOR})
@Retention(RUNTIME)
public @interface FooBinding {
@Nonbinding
String secret() default "";
}
Then, in the interceptor class, you can retrieve the secret String in the following way:
@Priority(value = Interceptor.Priority.APPLICATION)
@Interceptor
@FooBinding(secret = "nonbinding")
public class AroundConstructInterceptor {
@SuppressWarnings("unchecked")
@AroundConstruct
void intercept(InvocationContext ctx) throws Exception {
if(ctx instanceof WeldInvocationContext) {
Set<Annotation> bindings = ((WeldInvocationContext)ctx).getInterceptorBindings();
for (Annotation annotation : bindings) {
if (annotation.annotationType().equals(FooBinding.class)) {
FooBinding fooBinding = (FooBinding) annotation;
String secret = fooBinding.secret();
}
}
}
ctx.proceed();
}
}
WeldInvocationContext can be used with the following interceptor types:
@AroundConstruct@PostConstruct@AroundInvokeAlternatively, you can gain access to these binding directly from InvocationContext as we store them there using a key org.jboss.weld.interceptor.bindings.
This key is easily accessible from WeldInvocationContext.INTERCEPTOR_BINDINGS_KEY.
Let’s alter the previous example to demonstrate this:
@Priority(value = Interceptor.Priority.APPLICATION)
@Interceptor
@FooBinding(secret = "nonbinding")
public class AroundConstructInterceptor {
@SuppressWarnings("unchecked")
@AroundConstruct
void intercept(InvocationContext ctx) throws Exception {
// retrieve data directly from InvocationContext
Set<Annotation> bindings = (Set<Annotation>) ctx.getContextData().get(WeldInvocationContext.INTERCEPTOR_BINDINGS_KEY);
if (bindings != null) {
for (Annotation annotation : bindings) {
if (annotation.annotationType().equals(FooBinding.class)) {
FooBinding fooBinding = (FooBinding) annotation;
String secret = fooBinding.secret();
}
}
}
ctx.proceed();
}
}