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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"; 
        } 
    } 
}
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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 {}
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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() { 
            ... 
        } 
}
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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 { 
    ... 
}
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You can even have a client-side interceptor, for built-in EJB3 functions: 
@Interceptor(type=CLIENT) 
public class LoggedInInterceptor { 
    ... 
}
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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).
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