Wednesday, November 25, 2015

HOW-TO: Register components using @Conditional and Condition in Spring

@Profile annotation in Spring can be used on any Spring components (e.g. @Component, @Service, @Configuration etc.) that are candidates for auto-detection. @Profile annotation accepts a single profile or a set of profiles that must be active in order to make the annotated component eligible for auto-detection. For a given @Profile({"p1", "!p2"}), registration will occur if profile p1 is active or if profile p2 is not active. The or is crucial here.

But how to achieve this with @Profile: we want to activate a given component if profile p1 is active and if profiles p2 and p3 are both inactive?

Let’s assume the following situation: we have aNotificationSender interface that is implemented by:
- SendGridNotificationSender - active only if sendgrid profile is active,
- EmailNotificationSender - active only if email profile is active.
- NoOpNotificationSender - active only if development profile is active and none of sendgrid and email are active.

In addition: only one NotificationSender can be registered at a time and development profile can be used in conjunction with sendgrid and email profiles.

In the above situation using the @Profile annotation seems not enough. Maybe I am complicating things a bit, but actually I really wanted to achieve the above without introducing another profile. And how I did it?

I used Spring’s 4 @Conditional annotation. @Conditional allow registration of component when all specified Condition(s) match:

@Component
@Conditional(value = NoOpNotificationSender.ProfilesCondition.class)
class NoOpNotificationSender extends NotificationSenderAdapter {

}

ProfilesCondition implements org.springframework.context.annotation.Condition interface:

public static class ProfilesCondition implements Condition {
    @Override
    public boolean matches(ConditionContext c, AnnotatedTypeMetadata m) {

    }
}

The whole solution to the problem:

@Component
@Conditional(value = NoOpNotificationSender.ProfilesCondition.class)
class NoOpNotificationSender extends NotificationSenderAdapter {

    static class ProfilesCondition implements Condition {
        @Override
        public boolean matches(ConditionContext c, AnnotatedTypeMetadata m) {
            return accepts(c, Profiles.DEVELOPMENT)
                && !accepts(c, Profiles.MAIL)
                && !accepts(c, Profiles.SEND_GRID);
        }

        private boolean accepts(ConditionContext c, String profile) {
            return c.getEnvironment().acceptsProfiles(profile);
        }
    }
}

The other components will be activated when appropriate profile is active:

@Component
@Profile(value = Profiles.SEND_GRID)
public class SendGridNotificationSender extends NotificationSenderAdapter {

}

@Component
@Profile(value = Profiles.MAIL)
class EmailNotificationSender extends NotificationSenderAdapter {

}

Usage examples:

Active profiles Bean
development NoOpNotificationSender
development,sendgrid SendGridNotificationSender
development,mail EmailNotificationSender
sendgrid SendGridNotificationSender
mail EmailNotificationSender

What do you think? How would you solve this problem?

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