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CDI Interceptors

Let’s write a simple application that would allow us to book tickets for a movie show. As with all applications, logging is one cross-cutting concern that we have. Apart from that, there are some methods in our application, that can be accessed only in the working hours. If accessed at non-working-hours we’ll throw out an AccessDeniedException.

How do we mark which methods are to be intercepted? Wouldn’t it be handy to annotate a method like

@Log
public void aMethod(){...}

or

@TimeRestricted
public void bMethod(){...}

Let’s create these annotations that would "mark" a method for interception.

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

And

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

Sure, you haven’t missed the @InterceptorBinding annotation above! Now that our custom annotations are created, lets attach them (or to use a better term for it, "bind them") to interceptors.

So here’s our logging interceptor. An @AroundInvoke method and we are almost done.

@Interceptor
@Log  //binding the interceptor here. now any method annotated with @Log would be intercepted by logMethodEntry
public class BookForAShowLoggingInterceptor implements Serializable {
    private static final long serialVersionUID = 8139854519874743530L;
    private Logger logger = Logger.getLogger("BookForAShowApplicationLogger");
    @AroundInvoke
    public Object logMethodEntry(InvocationContext ctx) throws Exception {
        logger.info("Before entering method:" + ctx.getMethod().getName());
        InterceptionOrderTracker.getMethodsInterceptedList().add(ctx.getMethod().getName());
        InterceptionOrderTracker.getInterceptedByList().add(this.getClass().getSimpleName());
        return ctx.proceed();
    }
}

Now the @Log annotation we created is bound to this interceptor. (Likewise we bind @TimeRestrict for TimeBasedRestrictingInterceptor. See links below for source)

That done, let’s annotate at class-level or method-level and have fun intercepting!

@Log
@Stateful
public class BookForAShowOneInterceptorApplied implements Serializable {
    private static final long serialVersionUID = 6350400892234496909L;
    public List<String> getMoviesList() {
        List<String> moviesAvailable = new ArrayList<String>();
        moviesAvailable.add("12 Angry Men");
        moviesAvailable.add("Kings speech");
        return moviesAvailable;
    }
    public Integer getDiscountedPrice(int ticketPrice) {
        return ticketPrice - 50;
    }
    // assume more methods are present
}

The @Log annotation applied at class level denotes that all the methods should be intercepted with BookForAShowLoggingInterceptor.

Before we say "all done" there’s one last thing we are left with! To enable the interceptors!

Lets quickly put up a beans.xml file like the following in src/main/resources/META-INF/beans.xml:

<beans>
  <interceptors>
    <class>org.superbiz.cdi.bookshow.interceptors.BookForAShowLoggingInterceptor
    </class>
    <class>org.superbiz.cdi.bookshow.interceptors.TimeBasedRestrictingInterceptor
    </class>
  </interceptors>
</beans>

By default, a bean archive has no enabled interceptors; an interceptor must be explicitly enabled by listing its class in the beans.xml.

The order of the interceptor annotations has no real significance. Eg:

@TimeRestrict
@Log
void cMethod(){}

It is the interceptor elements in beans.xml that not only "enable" the interceptors, but also define the "order of execution" of the interceptors. In this case the BookForAShowLoggingInterceptor would be applied first and then TimeBasedRestrictingInterceptor

So now you know that the order is only determined by the order within the interceptors element within the beans.xml. The rule is simple, interceptors which occur earlier in the list are called first.

Also note that a method can be marked for interception using multiple interceptors by applying multiple annotations as above.

This brings us to another question. In the above case there were two interceptors applied together. What if I would require about 4 such interceptors that would go along…​. Having to annotate so much makes the code a little clumsy?

No worries! Just create a custom annotation which inherits from others

@Inherited
@InterceptorBinding
@Target({ TYPE, METHOD })
@Retention(RUNTIME)
@Log
@TimeRestricted
public @interface TimeRestrictAndLog {
}

This is interceptor binding inheritance.

The code below demonstrates the many cases that we have discussed.

Not to forget, the old style binding with @Interceptors(WhicheverInterceptor.class) is also supported. Have a look at BookForAShowOldStyleInterceptorBinding where the comments explain how the newer way discussed above is better.

The Code

BookForAShowOneInterceptorApplied

BookForAShowOneInterceptorApplied shows a simple @Log interceptor applied.

package org.superbiz.cdi.bookshow.beans;

import org.superbiz.cdi.bookshow.interceptorbinding.Log;

import javax.ejb.Stateful;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

@Log
@Stateful
public class BookForAShowOneInterceptorApplied implements Serializable {
    private static final long serialVersionUID = 6350400892234496909L;

    public List<String> getMoviesList() {
        List<String> moviesAvailable = new ArrayList<String>();
        moviesAvailable.add("12 Angry Men");
        moviesAvailable.add("Kings speech");
        return moviesAvailable;
    }

    public Integer getDiscountedPrice(int ticketPrice) {
        return ticketPrice - 50;
    }
}

BookForAShowTwoInterceptorsApplied

BookForAShowTwoInterceptorsApplied shows both @Log and @TimeRestricted being applied.

package org.superbiz.cdi.bookshow.beans;

import org.superbiz.cdi.bookshow.interceptorbinding.Log;
import org.superbiz.cdi.bookshow.interceptorbinding.TimeRestricted;

import javax.ejb.Stateful;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

@Log
@Stateful
public class BookForAShowTwoInterceptorsApplied implements Serializable {
    private static final long serialVersionUID = 6350400892234496909L;

    public List<String> getMoviesList() {
        List<String> moviesAvailable = new ArrayList<String>();
        moviesAvailable.add("12 Angry Men");
        moviesAvailable.add("Kings speech");
        return moviesAvailable;
    }

    @TimeRestricted
    public Integer getDiscountedPrice(int ticketPrice) {
        return ticketPrice - 50;
    }
}

BookShowInterceptorBindingInheritanceExplored

BookShowInterceptorBindingInheritanceExplored shows how @TimeRestrictAndLog (interceptor-binding-inheritance) can be used as an alternative for annotating a method with multiple annotations explicitly.

package org.superbiz.cdi.bookshow.beans;

import org.superbiz.cdi.bookshow.interceptorbinding.TimeRestrictAndLog;

import javax.ejb.Stateful;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

@Stateful
public class BookShowInterceptorBindingInheritanceExplored implements Serializable {
    private static final long serialVersionUID = 6350400892234496909L;

    public List<String> getMoviesList() {
        List<String> moviesAvailable = new ArrayList<String>();
        moviesAvailable.add("12 Angry Men");
        moviesAvailable.add("Kings speech");
        return moviesAvailable;
    }

    @TimeRestrictAndLog
    public Integer getDiscountedPrice(int ticketPrice) {
        return ticketPrice - 50;
    }
}

APIs Used