7.11. Seam interceptors
EJB3 introduced a standard interceptor model for session bean components. To add an interceptor to a bean, you need to write a class with a method annotated
@AroundInvoke and annotate the bean with an @Interceptors annotation that specifies the name of the interceptor class. For example, the following interceptor checks that the user is logged in before allowing invoking an action listener method:
public class LoggedInInterceptor {
@AroundInvoke
public Object checkLoggedIn(InvocationContext invocation)
throws Exception {
boolean isLoggedIn = Contexts.getSessionContext()
.get("loggedIn")!=null;
if (isLoggedIn) {
//the user is already logged in return invocation.proceed();
} else {
//the user is not logged in, fwd to login page return "login";
}
}
}
To apply this interceptor to a session bean acting as an action listener, we must annotate the session bean
@Interceptors(LoggedInInterceptor.class). However, Seam builds upon the interceptor framework in EJB3 by allowing you to use @Interceptors as a meta-annotation for class level interceptors (those annotated @Target(TYPE)). In this example, we would create an @LoggedIn annotation, as follows:
@Target(TYPE)
@Retention(RUNTIME)
@Interceptors(LoggedInInterceptor.class)
public @interface LoggedIn {}
We can now annotate our action listener bean with
@LoggedIn to apply the interceptor.
@Stateless
@Name("changePasswordAction")
@LoggedIn
@Interceptors(SeamInterceptor.class)
public class ChangePasswordAction implements ChangePassword {
...
public String changePassword() {
...
}
}
Where interceptor order is important, add
@Interceptor annotations to your interceptor classes to specify a particular order of interceptors.
@Interceptor(around={BijectionInterceptor.class,
ValidationInterceptor.class,
ConversationInterceptor.class},
within=RemoveInterceptor.class)
public class LoggedInInterceptor {
...
}
You can even have a client-side interceptor, for built-in EJB3 functions:
@Interceptor(type=CLIENT)
public class LoggedInInterceptor {
...
}
EJB interceptors are stateful, and their lifecycles match that of the component they intercept. For interceptors that do not need to maintain state, Seam allows performance optimization where
@Interceptor(stateless=true) is specified.
Much of Seam's functionality is implemented as a set of built-in Seam interceptors, including the interceptors named in the previous example. These interceptors exist for all interceptable Seam components; you need not specify them explicitly through annotation.
Seam interceptors can also be used with JavaBean components.
EJB defines interception not only for business methods (using
@AroundInvoke), but also for the lifecycle methods @PostConstruct, @PreDestroy, @PrePassivate and @PostActive. Seam supports these lifecycle methods on both component and interceptor, not only for EJB3 beans, but also for JavaBean components (except @PreDestroy, which is not meaningful for JavaBean components).
