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a minimal working example, a MWE, showing how to enable interceptors in a multi jar JavaEE CDI application
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README.markdown

Maven Multi Jar Webapp with Embedded Interceptor

This is a minimal working example, a MWE, showing how to enable interceptors in a multi jar JavaEE CDI application.

What gave me a big aha! moment was the following tip out of JBoss Weld CDI for Java Platform Learn CDI concepts and develop modern web applications using JBoss Weld by Ken Finnigan:

Activation of an interceptor within beans.xml will activate it for all beans within the same archive only.

This is the root of many "why woun't my interceptor work?" questions on StackOverflow.

This project layout

This project consists of a root project POM (my standard issue POM for J2EE projects. It's a bit heavy so don't spend any time on it.), and three child projects/modules: application, beans and intercept.

intercept contains an interceptor binding, @ResourceModel, and it's interceptor.

@InterceptorBinding
@Target({ ElementType.METHOD, ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface ResourceModel {

}

and

@Interceptor
@ResourceModel
public class ResourceModelInterceptor {
	private static final Logger log = LoggerFactory.getLogger(ResourceModelInterceptor.class);

	@AroundInvoke
	public Object aroundMethod(InvocationContext ctx) throws Exception {
		log.info("Before");
		return ctx.proceed();
	}
}

application contains the JAX-RS application ("api") and one REST endpoint ("test"). The endpoint is marked by the @ResourceModel annotation.

@ApplicationPath("/api")
public class TestApplication extends Application {}

and

@ResourceModel
@Path("test")
public class TestEndpoint {

	private String resource = "Hello World!";

	@GET
	@Produces("test/plain")
	public Response simpleGet() {
		return Response.ok(resource)
				.build();

	}
}

beans lastly, contains another endpoint ("beans"). This is also annotated with the @ResourceModel interceptor binding.

@ResourceModel
@Path("beans")
public class BeansResource {

	private String resource = "Hello Beans!";

	@GET
	@Produces("test/plain")
	public Response simpleGet() {
		return Response.ok(resource)
				.build();

	}
}

There is a beans.xml template file enabling the jollobajano.test.ResourceModelInterceptor from the intercept module.

<beans
	xmlns="http://java.sun.com/xml/ns/javaee"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/beans_1_0.xsd">
	<interceptors>
	<class>jollobajano.test.ResourceModelInterceptor</class>
	</interceptors>
</beans>

This template can be found in the root of this project.

Where to put the beans.xml

Taking the hint from the tip from the book.... we need to enable the interceptor in all the modules where we want it to intercept. This means that for beans we copy the file into the src/main/resources/META-INF directory.

Since application is a war type module, we copy the interceptor enabeling beans.xml into the src/main/webapp/WEB-INF of that module.

I first experimented with presence and absence of the beans.xml file in the application and beans modules and was able to control whether the interceptor would work in each particular module that way.

A reoccuring question on StackOverflow and other forums is "where do I put the beans.xml file?". Experimenting with this project I concluded that

  • in a JAR archive, we put the beans.xml file in a META-INF directory in the root of the archive,

  • in a WAR archive the beans.xml file goes into the WEB-INF folder.

Conclusion

The way I see it, enabeling an interceptor via the beans.xml file tells the CDI framework, e.g. Weld, to enable the interceptor for the archive that contains the enabeling beans.xml.

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