Spring @Conditional注解

背景

团队所负责的一个项目由于功能的不断累加目前变的十分庞大，打算将其拆分成若干个单独的service，在这之中有一些共用的bean（例如interceptor, datasource等）需要移到一个公共的module中。但是这个公共的module已经被其他service依赖，为了避免对其他service产生未知的影响，需要对bean的加载进行一些限制做到按需加载，为此使用Spring提供的@Conditional注解进行控制。

@ConditionalOnProperty

该注解会根据指定的配置，判断是否加载对应的bean

    @Bean@ConditionalOnProperty(prefix = "spring", name = "example.values",havingValue="true")class ExampleAutoConfiguration {}

在这个例子中，如果存在spring.example.values=true时，会加载ExampleAutoConfiguration。除了prefix,name,havingValue之外，还有一个matchIfMissing属性，该属性表明如果指定的配置不存在，是否加载bean，默认为false。如果有一个bean希望默认情况下都加载，特定值不加载，可以设为true。

@ConditionalOnResource

该注解在指定配置文件存在的情况下加载bean

@Configuration
@ConditionalOnResource(resources = "classpath:conf/apiserver.properties")
public class ApiServerConfigurationFacade {}

需要注意的是仅在classPath下的配置文件才会有效，外部配置文件无法检测

@ConditionalOnClass

只有在classpath中存在指定class是才会加载

@Configuration(proxyBeanMethods = false)
@Import({ NoOpMeterRegistryConfiguration.class, CompositeMeterRegistryConfiguration.class })
@ConditionalOnClass(CompositeMeterRegistry.class)
public class CompositeMeterRegistryAutoConfiguration {}

@ConditionalOnExpression

在满足 SpEL表达式的情况下加载bean

 @ConditionalOnExpression( "'${catalina.home}' != '.' ")public static class Tomcat {}

SpEL表达式在此不做过多介绍，${}中为可以在environment中取到的变量，表达式语法与java类似

@ConditionalOnMissingBean

在指定bean不存的情况下加载bean

 @Bean@ConditionalOnMissingBean(name = "kafkaListenerContainerFactory")ConcurrentKafkaListenerContainerFactory<?, ?> kafkaListenerContainerFactory(ConcurrentKafkaListenerContainerFactoryConfigurer configurer,ObjectProvider<ConsumerFactory<Object, Object>> kafkaConsumerFactory) {ConcurrentKafkaListenerContainerFactory<Object, Object> factory = new ConcurrentKafkaListenerContainerFactory<>();configurer.configure(factory, kafkaConsumerFactory.getIfAvailable(() -> new DefaultKafkaConsumerFactory<>(this.properties.buildConsumerProperties())));return factory;}

该注解主要用在公共模块中，通过该注解可以为其他下游模块提供默认的bean，当下游模块决定自行提供bean时不会导致bean confilct。

@Conditional

如果我们去查看以上注解的源码，可以发现他们都依赖于@Conditional

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {/*** All {@link Condition} classes that must {@linkplain Condition#matches match}* in order for the component to be registered.*/Class<? extends Condition>[] value();}

该注解可以申明一个Condition数组，我们可以通过实现Condition来满足自己的需求。我们有如下两种方式来实现Condition

AbstractNestedCondition
SpringBootCondition

AbstractNestedCondition

AbstractNestedCondition可以用来组合多个condition，spring提供了三个基类

AnyNestedCondition
NoneNestedConditions
AllNestedConditions

AnyNestedCondition

满足其中任意一个条件都会加载bean，看一个例子

static class DefaultCookieSerializerCondition extends AnyNestedCondition {DefaultCookieSerializerCondition() {super(ConfigurationPhase.REGISTER_BEAN);}@ConditionalOnMissingBean({ HttpSessionIdResolver.class, CookieSerializer.class })static class NoComponentsAvailable {}@ConditionalOnBean(CookieHttpSessionIdResolver.class)@ConditionalOnMissingBean(CookieSerializer.class)static class CookieHttpSessionIdResolverAvailable {}}

NoneNestedConditions

满足其中任意一个条件都会不会加载bean

@Configuration(proxyBeanMethods = false)
@Conditional(MultipleNonPrimaryMeterRegistriesCondition.class)
class CompositeMeterRegistryConfiguration {@Bean@PrimaryAutoConfiguredCompositeMeterRegistry compositeMeterRegistry(Clock clock, List<MeterRegistry> registries) {return new AutoConfiguredCompositeMeterRegistry(clock, registries);}static class MultipleNonPrimaryMeterRegistriesCondition extends NoneNestedConditions {MultipleNonPrimaryMeterRegistriesCondition() {super(ConfigurationPhase.REGISTER_BEAN);}@ConditionalOnMissingBean(MeterRegistry.class)static class NoMeterRegistryCondition {}@ConditionalOnSingleCandidate(MeterRegistry.class)static class SingleInjectableMeterRegistry {}}}

AllNestedConditions

满足所有条件才会加载bean.

SpringBootCondition

上述提到的几个@Conditional注解都是基本SpringBootCondition来实现的，我们可以直接参考spring的源码来实现自己的需求。
也可以参考这位的实现https://blog.csdn.net/wtopps/article/details/84110904