Spring - Qualify injection candidates by designated environment

Question

Edit: Perhaps a more concise way to ask this question is: Does Spring provide a way for me to resolve ambiguous candidates at injection time by providing my own listener/factory/decision logic?

In fact, arguably the @Environmental qualifier on the member field below is unnecessary: if an @Inject-ion is ambiguous... let me help? In fact, @ResolveWith(EnvironmentalResolver.class) would be alright too..

When Spring attempts to inject a dependency (using annotations) I understand that I need to @Qualifier an @Inject point if I am to have multiple components that implement that interface.

What I'd like to do is something like this:

class MyFoo implements Foo {
    @Inject
    @Environmental
    private Bar bar;
}

@Environmental(Environment.Production)
class ProductionBar implements Bar {
}

@Environmental({Environment.Dev, Environment.Test})
class DevAndTestBar implements Bar {
}

I would expect that I need to create some kind of ambiguity resolver which would look something (vaguely) like this:

class EnvironmentalBeanAmbiguityResolver {

    // set from configuration, read as a system environment variable, etc.
    private Environment currentEnvironment;

    public boolean canResolve(Object beanDefinition) {
        // true if definition has the @Environmental annotation on it
    }

    public Object resolve(Collection<Object> beans) {
        for (Object bean : beans) {
            // return bean if bean @Environmental.values[] contains currentEnvironment
        }
        throw new RuntimeException(...);
    }
}

One example of where this would be useful is we have a service that contacts end-users. Right now I just have a hacked together AOP aspect that before the method call to the "MailSender', checks for a "Production" environment flag and if it is not set, it sends the email to us instead of the users email. I'd like to instead of wrapping this in an AOP aspect specific to mail sending, instead be able to differentiate services based on the current environment. Sometime's it is just a matter of "production" or "not production" as I've demonstrated above, but a per-environment definition works too.

I think this can be reused for region too... e.g. @Regional and @Regional(Region.UnitedStates) and so on and so forth.

I'd imagine @Environmental would actually be a @Qualifier that way if you wanted to depend directly on something environmental you could (an @Environmental(Production) bean would likely depend directly on an @Environmental(Production) collaborator - so no ambiguity for lower level items --- same a @Regional(US) item would depend on other @Regional(US) items expiclitly and would bypass my yet-to-be-understood BeanAmbiguityResolver)

Thanks.

score 0 · Accepted Answer · answered Sep 22 '11 at 22:31

I think I solved this!

Consider the following:

public interface Ambiguity {
    public boolean isSatisfiedBy(BeanDefinitionHolder holder);
}

@Target({ METHOD, CONSTRUCTOR, FIELD })
@Retention(RUNTIME)
public @interface Ambiguous {
    Class<? extends Ambiguity> value();
}

@Target(TYPE)
@Retention(RUNTIME)
public @interface Environmental {
    public static enum Environment {
        Development, Testing, Production
    };
    Environment[] value() default {};
}

@Named
public class EnvironmentalAmbiguity implements Ambiguity {

    /* This can be set via a property in applicationContext.xml, which Spring
       can use place holder, environment variable, etc. */
    Environment env = Environment.Development;

    @Override
    public boolean isSatisfiedBy(BeanDefinitionHolder holder) {
        BeanDefinition bd = holder.getBeanDefinition();
        RootBeanDefinition rbd = (RootBeanDefinition) bd;

        Class<?> bc = rbd.getBeanClass();

        Environmental env = bc.getAnnotation(Environmental.class);

        return (env == null) ? false : hasCorrectValue(env);
    }

    private boolean hasCorrectValue(Environmental e) {
        for (Environment env : e.value()) {
            if (env.equals(this.env)) {
                return true;
            }
        }
        return false;
    }

}

@Named
public class MySuperDuperBeanFactoryPostProcessor implements
        BeanFactoryPostProcessor, AutowireCandidateResolver {

    private DefaultListableBeanFactory beanFactory;
    private AutowireCandidateResolver defaultResolver;

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory arg)
            throws BeansException {
        if (arg instanceof DefaultListableBeanFactory) {
            beanFactory = (DefaultListableBeanFactory) arg;

            defaultResolver = beanFactory.getAutowireCandidateResolver();
            beanFactory.setAutowireCandidateResolver(this);

            return;
        }

        throw new FatalBeanException(
                "BeanFactory was not a DefaultListableBeanFactory");
    }

    @Override
    public Object getSuggestedValue(DependencyDescriptor descriptor) {
        return defaultResolver.getSuggestedValue(descriptor);
    }

    @Override
    public boolean isAutowireCandidate(BeanDefinitionHolder holder,
            DependencyDescriptor descriptor) {
        Ambiguity ambiguity = getAmbiguity(descriptor);

        if (ambiguity == null) {
            return defaultResolver.isAutowireCandidate(holder, descriptor);
        }
        return ambiguity.isSatisfiedBy(holder);
    }

    private Ambiguity getAmbiguity(DependencyDescriptor descriptor) {
        Ambiguous ambiguous = getAmbiguousAnnotation(descriptor);

        if (ambiguous == null) {
            return null;
        }

        Class<? extends Ambiguity> ambiguityClass = ambiguous.value();
        return beanFactory.getBean(ambiguityClass);
    }

    private Ambiguous getAmbiguousAnnotation(DependencyDescriptor descriptor) {
        Field field = descriptor.getField();

        if (field == null) {
            MethodParameter methodParameter = descriptor.getMethodParameter();

            if (methodParameter == null) {
                return null;
            }
            return methodParameter.getParameterAnnotation(Ambiguous.class);
        }
        return field.getAnnotation(Ambiguous.class);
    }

}

Now if I have an interface MyInterface and two classes that implement it MyFooInterface and MyBarInterface like this:

public interface MyInterface {
    public String getMessage();
}

@Named
@Environmental({ Environment.Testing, Environment.Production })
public class MyTestProdInterface implements MyInterface {
    @Override
    public String getMessage() {
        return "I don't always test my code, but when I do, I do it in production!";
    }
}

@Named
@Environmental(Environment.Development)
public class DevelopmentMyInterface implements MyInterface {
    @Override
    public String getMessage() {
        return "Developers, developers, developers, developers!";
    }
}

If I want to @Inject MyInterface I would get the same multiple bean definition error that one would expect. But I can add @Ambiguous(EnvironmentalAmbiguity.class) and then the EnvironmentalAmbiguity will tell which bean definition it is satisfied by.

Another approach would have been to use a List and go through them all seeing if they are satisfied by a given bean definition, this would mean that the dependnecy wouldn't need the @Ambiguous annotation. That might be more "IoC-ish" but I also thought it might perform poorly. I have not tested that.

Spring - Qualify injection candidates by designated environment

1 Answers1