Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Using Interceptors in CDI Applications

An interceptor is a class used to interpose in method invocations or lifecycle events that occur in an associated target class. The interceptor performs tasks, such as logging or auditing, that are separate from the business logic of the application and are repeated often within an application. Such tasks are often called cross-cutting tasks. Interceptors allow you to specify the code for these tasks in one place for easy maintenance. When interceptors were first introduced to the Java EE platform, they were specific to enterprise beans. On the Java EE platform, you can use them with Java EE managed objects of all kinds, including managed beans.

For information on Java EE interceptors, see Chapter 57, "Using Java EE Interceptors".

An interceptor class often contains a method annotated `@AroundInvoke`, which specifies the tasks the interceptor will perform when intercepted methods are invoked. It can also contain a method annotated `@PostConstruct`, `@PreDestroy`, `@PrePassivate`, or `@PostActivate`, to specify lifecycle callback interceptors, and a method annotated `@AroundTimeout`, to specify EJB timeout interceptors. An interceptor class can contain more than one interceptor method, but it must have no more than one method of each type.

Along with an interceptor, an application defines one or more interceptor binding types, which are annotations that associate an interceptor with target beans or methods. For example, the `billpayment` example contains an interceptor binding type named `@Logged` and an interceptor named `LoggedInterceptor`. The interceptor binding type declaration looks something like a qualifier declaration, but it is annotated with `javax.interceptor.InterceptorBinding`:

    @Inherited
    @InterceptorBinding
    @Retention(RUNTIME)
    @Target({METHOD, TYPE})
    public @interface Logged {
    }

An interceptor binding also has the `java.lang.annotation.Inherited` annotation, to specify that the annotation can be inherited from superclasses. The `@Inherited` annotation also applies to custom scopes (not discussed in this tutorial) but does not apply to qualifiers.

An interceptor binding type may declare other interceptor bindings.

The interceptor class is annotated with the interceptor binding as well as with the `@Interceptor` annotation. For an example, see The LoggedInterceptor Interceptor Class.

Every `@AroundInvoke` method takes a `javax.interceptor.InvocationContext` argument, returns a `java.lang.Object`, and throws an `Exception`. It can call `InvocationContext` methods. The `@AroundInvoke` method must call the `proceed` method, which causes the target class method to be invoked.

Once an interceptor and binding type are defined, you can annotate beans and individual methods with the binding type to specify that the interceptor is to be invoked either on all methods of the bean or on specific methods. For example, in the `billpayment` example, the `PaymentHandler` bean is annotated `@Logged`, which means that any invocation of its business methods will cause the interceptor’s `@AroundInvoke` method to be invoked:

    @Logged
    @SessionScoped
    public class PaymentHandler implements Serializable {...}

However, in the `PaymentBean` bean, only the `pay` and `reset` methods have the `@Logged` annotation, so the interceptor is invoked only when these methods are invoked:

    @Logged
    public String pay() {...}

    @Logged
    public void reset() {...}

In order for an interceptor to be invoked in a CDI application, it must, like an alternative, be specified in the `beans.xml` file. For example, the `LoggedInterceptor` class is specified as follows:

    <interceptors>
        <class>javaeetutorial.billpayment.interceptors.LoggedInterceptor</class>
    </interceptors>

If an application uses more than one interceptor, the interceptors are invoked in the order specified in the `beans.xml` file.

The interceptors that you specify in the `beans.xml` file apply only to classes in the same archive. Use the `@Priority` annotation to specify interceptors globally for an application that consists of multiple modules, as in the following example:

    @Logged
    @Interceptor
    @Priority(Interceptor.Priority.APPLICATION)
    public class LoggedInterceptor implements Serializable { ... }

Interceptors with lower priority values are called first. You do not need to specify the interceptor in the `beans.xml` file when you use the `@Priority` annotation.

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