Improve docs for AnnotatedBeanDefinitionReader, @Configuration, and @ContextConfiguration regarding "annotated classes"

[ttddyy]

Original Description

I find this test interesting:

@ExtendWith(SpringExtension.class)
@ContextConfiguration(classes = RestTemplate.class)
class MyTest {

    @Autowired
    RestTemplate restTemplate;

    @Test
    void check() {
        assertThat(restTemplate).isNotNull();
    }

}

This works because while loading a context, AnnotationConfigContextLoader registers @ContextConfiguration provided classes(RestTemplate in this case) to bean definitions, so that they are available for injections.

So, this is also possible:

@ExtendWith(SpringExtension.class)
@ContextConfiguration(classes = { MyTest2.Foo.class, MyTest2.Bar.class })
class MyTest2 {

    static class Foo {
        public Foo(Bar bar) {  // dependency to Bar
        }
    }

    static class Bar {
        public Bar() {
        }
    }

    @Autowired
    Foo foo;

    @Autowired
    Bar bar;

    @Test
    void check() {
        assertThat(this.foo).isNotNull();
        assertThat(this.bar).isNotNull();
    }

}

I think this is not an intended usage of @ContextConfiguration#classes.

Probably, by default, filter-out or validate those classes to be @Configuration classes.
For the case of allowing non @Configuration classes (for example, lite-mode), probably provide an explicit option(new attribute) on @ContextConfiguration. e.g.: @ContextConfiguration(classes=MyBean.class, liteMode=true)

---

This is what I found in real world code base:

@ExtendWith({MockitoExtension.class, SpringExtension.class})
@ContextConfiguration(classes = {ResourceBundleMessageSource.class, MyService.class,
        MyExceptionHandler.class, MyPropertyConfiguration.class, RestTemplate.class})
public class MyExceptionHandlerTest {
    // ...
}

So, would be nice not seeing such test class :)

---

Deliverables

Improve documentation for the following regarding "annotated classes".

• AnnotationConfigRegistry
• AnnotationConfigApplicationContext
• AnnotationConfigWebApplicationContext
• AnnotatedBeanDefinitionReader
• @Configuration
• @Import
• AnnotationConfigContextLoader
• AnnotationConfigContextLoaderUtils
• @ContextConfiguration
• Reference Manual: testing.adoc

[sbrannen]

The fact that annotated classes (other than @Configuration classes) are supported is a documented feature.

Annotated Classes

The term annotated class can refer to any of the following.

• A class annotated with @Configuration
• A component (i.e., a class annotated with @Component, @Service, @Repository, etc.)
• A JSR-330 compliant class that is annotated with javax.inject annotations
• Any other class that contains @Bean-methods

So we cannot change that, since it would be a breaking change.

In addition, it can be quite useful to test a very limited combination of components.

Having said that, however, the fact that a non-annotated class is automatically registered as a Spring bean is not documented. So I'll investigate that further.

[sbrannen]

Having said that, however, the fact that a non-annotated class is automatically registered as a Spring bean is not documented. So I'll investigate that further.

A valid use case for this behavior of AnnotatedBeanDefinitionReader.register() would be component scanning based on custom annotations or naming conventions and relying on convention based autowiring of non-annotated constructors. @Import is another valid use case.

So, in light of that, I'm changing the focus of this issue to improve the corresponding documentation.