Spring @Condition annotated examples to explain

@Condition is new Spring4 comment, the role in meeting the conditions for injection into Spring bean container, in Spring Boot a large number of uses @Condition notes, for example @ConditionOnBean as the core Spring Boot automated configuration. Source:

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {

    /**
     * All {@link Condition}s that must {@linkplain Condition#matches match}
     * in order for the component to be registered.
     */
    Class<? extends Condition>[] value();

}

Annotation value is to achieve a Condition interface, Condition Interface Source:

public interface Condition {

    /**
     * Determine if the condition matches.
     * @param context the condition context
     * @param metadata metadata of the {@link org.springframework.core.type.AnnotationMetadata class}
     * or {@link org.springframework.core.type.MethodMetadata method} being checked.
     * @return {@code true} if the condition matches and the component can be registered
     * or {@code false} to veto registration.
     */
    boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);

}

We did a simple example, depending on the profile settings, registered in the Spring bean in the container is different, first define a Service interface:

public interface ProfileService {

    void show();

}

Then define two classes implement:

public class ProfileDevService implements ProfileService {

    @Override
    public void show() {
        System.out.println("this is dev");
    }

}

public class ProfilePrdService implements ProfileService {

    @Override
    public void show() {
        System.out.println("this is prd");
    }

}

Both classes can only register a window in the Spring, the method is to determine the profile, two judging categories:

public class OnDevProfile implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return context.getEnvironment().getActiveProfiles()[0].equals("dev");
    }

}

public class OnPrdProfile implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return context.getEnvironment().getActiveProfiles()[0].equals("prd");
    }

}

Both classes implement the Condition interface in the comment @Condition need to use these two classes:

@Configuration
public class ConditionConfiguration {

    @Bean
    @Conditional(OnDevProfile.class)
    public ProfileService devProfileService() {
        return new ProfileDevService();
    }

    @Bean
    @Conditional(OnPrdProfile.class)
    public ProfileService prdProfileService() {
        return new ProfilePrdService();
    }

}

He added @Condition comment at the time of injection bean, so whether the registration is successful, matchs method value depends on the class notes @Condition return value.

Start a container:

public static void main(String[] args) {

        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
        context.getEnvironment().setActiveProfiles("prd");
        context.register(ConditionConfiguration.class);
        context.refresh();

        ProfileService profileService = context.getBean(ProfileService.class);
        profileService.show();

        context.close();
    }

Setting active profiles here as "prd", therefore the console print is this is prd, the active profiles changed to "dev" print is this is dev