Dependency injection not supported on @FeatureConfiguration classes [SPR-7975]

[spring-projects-issues]

Keith Donald opened SPR-7975 and commented

I'm unable to inject environment properties or ApplicationContext services such as ResourceLoader into @FeatureConfiguration classes. This appears possible with @Configuration classes but not @FeatureConfiguration classes.

An example of a @FeatureConfiguration that fails:

@FeatureConfiguration
public class MvcFeatureConfig {
	
    @Inject
    private ResourceLoader resourceLoader;
	
    @Feature
    public MvcAnnotationDriven annotatedControllers(Validator validator,
        @Value("#{environment['facebook.appId']}") String facebookAppId,
        @Value("#{environment['facebook.appSecret']}") String facebookAppSecret) {
        
        return new MvcAnnotationDriven().validator(validator).
            argumentResolvers(new DeviceWebArgumentResolver(),
            new FacebookWebArgumentResolver(facebookAppId, facebookAppSecret),
            new AccountWebArgumentResolver(),
            new DateTimeZoneWebArgumentResolver(),
            new LocationWebArgumentResolver());
    }
	
    @Feature
    public MvcResources resourceHandler() {
        return new MvcResources("/resources/**", resourceLoader.getResource("/resources/"));
    }
}

This will first fail because Spring tries to subclass the String environment @Values facebookAppId and facebookAppSecret, instead of injecting them. Changing those to member fields doesn't work either, as no injection gets performed on the @Value fields. The ResourceLoader field also remains null, resulting in a NullPointerException when resourceHandler() is invoked.

---

Affects: 3.1 M1

Attachments:

• mylyn-context.zip (948 bytes)

Issue Links:

• Process @ImportResource with current ResourceLoader [SPR-7973] #12628 Process @ImportResource with current ResourceLoader
• @Feature methods do not accept @Value-annotated parameters [SPR-7974] #12629 @Feature methods do not accept @Value-annotated parameters

Referenced from: commits 704b229, 5cfbed8

1 votes, 2 watchers

[spring-projects-issues]

Chris Beams commented

@FeatureConfiguration classes do not support field, setter or constructor injection due to lifecycle issues. Registered @FC classes are instantiated very early -- before any other bean (definitions) have been post processed or instantiated. Allowing for injection into @FC types guarantees 'premature instantiation' problems for the user. This is the primary reason, actually, that @FeatureConfiguration classes must exist, and that @Feature methods cannot be added directly to @Configuration classes.

The one type of dependency injection that @FC types do support is parameter injection into @Feature methods. This approach is both convenient for the author of the @Feature method (no need to declare and reference an @Inject'ed field, and avoids lifecycle issues.

Just as well-known types like ResourceLoader, ApplicationContext, BeanFactory, etc may be @Inject'ed in @Component and @Configuration classes, so too can these types be parameter injected in @Feature methods.

For example:

@Feature
public MyFeatureSpec myFeature(ResourceLoader rl) {
    // work with rl
    return new MyFeatureSpec();
}

A unit test has been added to FeatureConfigurationClassTests to prove this behavior.