Spring @Condition注解实例讲解

@Condition是Spring4新增的注解，作用是在满足条件时将bean注入到Spring容器，在Spring Boot中大量用到了@Condition注解，例如@ConditionOnBean等为Spring Boot自动化配置的核心。源码：

@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();

}

注解的value是一个Condition接口的实现，Condition接口源码：

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);

}

我们做一个简单的实例，根据设置的profile不同，在Spring容器中注册的bean也不同，首先定义一个Service接口：

public interface ProfileService {

    void show();

}

再定义两个实现类：

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");
    }

}

这两个类只能在Spring窗口中注册一个，方法就是判断profile，两个判断类：

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");
    }

}

这两个类都实现了Condition接口，在注解@Condition需要用到这两个类：

@Configuration
public class ConditionConfiguration {

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

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

}

在注入bean时加上了@Condition注解，所以能否注册成功，取决于@Condition注解的value类的matchs方法返回值。

启动一个容器：

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();
    }

这里设置active profiles为"prd"，因此控制台打印的是this is prd，将active profiles改为"dev"打印的是this is dev