A great feature of CDI (supported by Java EE starting with version 6) consist in alternatives. Basically, we want to specify an alternative for an injected object. Let's have a simple scenario commonly followed in JSF applications that uses stateless session beans to interact with a database. Supposing that we write the business logic part to query the database. We can start with an interface as:

public interface IFooService {
    List getAllFoo();
}

Furthermore, we can write a stateless session bean that implements this interface and make use of JPA's capabilities to query the database. Well, things didn't get too far for this part because the database is not ready and all we can provide is a skeleton like below:

@Stateless
public class FooService implements IFooService {
   
    @PersistenceContext(unitName = "fooPU")
    private EntityManager em;
    @Override
    public List getAllFoo() {
        // perform the query
        ...
        return new ArrayList();
    }
}

Finally, we inject the stateless session bean in a CDI managed bean as below:

Named
@RequestScoped
public class FooBean {
    private static final Logger LOG = Logger.getLogger(FooBean.class.getName());
    @Inject
    private IFooService fooService;
    public void loadAllFoo() {
        List allfoo = fooService.getAllFoo();
        LOG.log(Level.INFO, "allfoo:{0}", allfoo);
    }
}

Now, let's suppose that we want to run/test this method, but we have an issue. The data that we expect from the database will not be available, so we think to mock this service and provide a set of dummy data as below:

@Stateless
public class FooServiceMock implements IFooService {
    @Override
    public List getAllFoo() {
        return Arrays.asList("foo 1", "foo 2", "foo 3");
    }
}

@Alternative
@Stateless
public class FooServiceMock implements IFooService {
    @Override
    public List getAllFoo() {
        return Arrays.asList("foo 1", "foo 2", "foo 3");
    }
}

If we run now, there will be no errors, but the application will not use the mock. This is happening because our alternative is not activated. We must accomplish this in the beans.xml, as below:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://xmlns.jcp.org/xml/ns/javaee"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/beans_1_1.xsd"
       bean-discovery-mode="all">
   
    <alternatives>
        <class>beans.FooServiceMock</class>
    </alternatives>
</beans>

Done! Now the FooBean will use the FooServiceMock instead of FooService. When the database will be ready/queryable we will simply deactivate the alternative.

Via CDI-Unit we can write a quick test that also uses our mock:

@RunWith(CdiRunner.class)
@ActivatedAlternatives(FooServiceMock.class)
public class FooBeanTest {
   
    @Inject
    FooBean fooBean;
    @Test
    @InRequestScope
    public void testStart() {
        fooBean.loadAllFoo();
    }