【Spring Boot相关】@ConditionalOnProperty

@ConditionalOnProperty是Springboot的注解。是spring-boot-autoconfigure包里的注解，从Spring boot v1.1.0就开始就了。

当我们创建bean的时候，有时候需要根据不同的property的值来创建不同的bean，比如property值=dev或者prod。这时候就需要使用@ConditionalOnProperty注解了，它可以在某些环境变量存在的时候并且等于特定值的时候，才去创建某个bean。

@ConditionalOnProperty有5个变量，分别是value, prefix, name, havingValue和matchIfMissing:

ConditionalOnProperty.jpg

1. 简单demo

@Service(value = "simpleCacheService")
@ConditionalOnProperty(prefix = "conditional.cache", name = "type")
public class SimpleCacheServiceImpl implements CacheService {
    public void init() {}
}

在application.properties里配置：

conditional.cache.type=simple

测试SimpleCacheServiceImpl有没有被加载：

@SpringBootTest
public class SimpleCacheServiceTest {
    @Autowired
    private CacheService simpleCacheService;

    @Test
    public void initTest() {
       Assertions.assertTrue(simpleCacheService.getClass().getName().contains("SimpleCacheService"));
    }
}

如果不配置conditional.cache.type，那么simpleCacheService注入不了。
上述@ConditionalOnProperty(prefix = "conditional.cache", name = "type")配置的意思是当有profix.name配置存在的时候，即可加载该Bean。

2. 高级用法

@Service(value = "redisCacheService")
@ConditionalOnProperty(prefix = "conditional.cache", value = "type", havingValue = "redis")
public class RedisCacheServiceImpl implements CacheService {
    public void init() {}
}

测试通过：

@SpringBootTest
public class RedisCacheServiceTest {
    private final ApplicationContextRunner contextRunner = new ApplicationContextRunner();

    @Test
    public void initTest() {
        this.contextRunner.withPropertyValues("conditional.cache.type=redis")
                .withUserConfiguration(RedisCacheServiceImpl.class)
                .run(context -> {
            CacheService redisCacheService = (CacheService) context.getBean("redisCacheService");
            Assertions.assertTrue(redisCacheService.getClass().getName().contains("RedisCacheService"));
        });
    }

}

加了havingValue="redis"，意味着有如下配置并且值等于redis时，才会加载redisCacheService：

conditional.cache.type=redis

3. Spring源码中的@ConditionalOnProperty

@Configuration(proxyBeanMethods = false)
    @ConditionalOnWebApplication(type = Type.SERVLET)
    @ConditionalOnClass(WebMvcConfigurer.class)
    @ConditionalOnMissingBean({ OpenEntityManagerInViewInterceptor.class, OpenEntityManagerInViewFilter.class })
    @ConditionalOnMissingFilterBean(OpenEntityManagerInViewFilter.class)
    @ConditionalOnProperty(prefix = "spring.jpa", name = "open-in-view", havingValue = "true", matchIfMissing = true)
    protected static class JpaWebConfiguration {
        // 略
}

先别管别的Conditional注解，看最后一个：
matchIfMissing=true，意思是，不配置spring.jpa.open-in-view也会加载Bean，但如果有配置，那么需要配置：spring.jpa.open-in-view=true
反过来说，如何不加载这个Bean：spring.jpa.open-in-view=false

4. 最后放下源码：

@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
@Documented
@Conditional(OnPropertyCondition.class)
public @interface ConditionalOnProperty {
    // 同name
    String[] value() default {};
    // 前缀，如：acme.system.feature
    String prefix() default "";
    // 与prefix协同使用：
    String[] name() default {};
    // property里应该配的值，如果相等，则表示激活该bean
    String havingValue() default "";
    // 默认不配置就不激活该bean
    boolean matchIfMissing() default false;
}

---

【参考】
Spring conditionalOnProperty: https://www.baeldung.com/spring-conditionalonproperty
Spring Boot ApplicationContextRunner 测试指南: https://blog.csdn.net/j3T9Z7H/article/details/94365999