SpringBoot 的自動配置如此強大,好比咱們常常使用的@Enable* 註解來開啓對某方面的支持。那麼@Enable* 註解的原理是什麼呢?html
@Enable* 舉例:java
等等spring
咱們觀察這些@Enable* 的源碼能夠看出,全部@Enable* 註解都是有@Import的組合註解,@Enable* 自動開啓的實現其實就是導入了一些自動配置的Bean緩存
看下 Spring Boot Reference Guide原文bash
You need not put all your @Configuration into a single class. The @Import annotation
can be used to import additional configuration classes.
您不須要把全部的 @Configuration 放到一個類中。@Import 註解能夠導入額外的配置類。
複製代碼
@Import 註解的最主要功能就是導入額外的配置信息app
官方介紹:異步
* <p>Provides functionality equivalent to the {@code <import/>} element in Spring XML.
* Allows for importing {@code @Configuration} classes, {@link ImportSelector} and
* {@link ImportBeanDefinitionRegistrar} implementations, as well as regular component
* classes (as of 4.2; analogous to {@link AnnotationConfigApplicationContext#register}).
複製代碼
有如下三種使用方式ide
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(SchedulingConfiguration.class)
@Documented
public @interface EnableScheduling {
}
複製代碼
能夠看到EnableScheduling註解直接導入配置類SchedulingConfiguration,這個類註解了@Configuration,且註冊了一個scheduledAnnotationProcessor的Bean,SchedulingConfiguration的源碼以下:spring-boot
@Configuration
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public class SchedulingConfiguration {
@Bean(name = TaskManagementConfigUtils.SCHEDULED_ANNOTATION_PROCESSOR_BEAN_NAME)
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public ScheduledAnnotationBeanPostProcessor scheduledAnnotationProcessor() {
return new ScheduledAnnotationBeanPostProcessor();
}
}
複製代碼
若是並不肯定引入哪一個配置類,須要根據@Import註解所標識的類或者另外一個註解(一般是註解)裏的定義信息選擇配置類的話,用這種方式。ui
ImportSelector接口只有一個方法
String[] selectImports(AnnotationMetadata importingClassMetadata);
複製代碼
AnnotationMetadata:用來得到當前配置類上的註解
例:
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AsyncConfigurationSelector.class)
public @interface EnableAsync {
Class<? extends Annotation> annotation() default Annotation.class;
boolean proxyTargetClass() default false;
AdviceMode mode() default AdviceMode.PROXY;
int order() default Ordered.LOWEST_PRECEDENCE;
}
複製代碼
AsyncConfigurationSelector繼承AdviceModeImportSelector,AdviceModeImportSelector類實現ImportSelector接口 根據AdviceMode的不一樣來選擇生明不一樣的Bean
public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {
private static final String ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME =
"org.springframework.scheduling.aspectj.AspectJAsyncConfiguration";
@Override
@Nullable
public String[] selectImports(AdviceMode adviceMode) {
switch (adviceMode) {
case PROXY:
return new String[] {ProxyAsyncConfiguration.class.getName()};
case ASPECTJ:
return new String[] {ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME};
default:
return null;
}
}
}
複製代碼
通常只要用戶確切知道哪些Bean須要放入容器的話,本身能夠經過spring 提供的註解來標識就能夠了,好比@Component,@Service,@Repository,@Bean等。 若是是不肯定的類,或者不是spring專用的,因此並不想用spring的註解進行侵入式標識,那麼就能夠經過@Import註解,實現ImportBeanDefinitionRegistrar接口來動態註冊Bean。 好比:
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {
boolean proxyTargetClass() default false;
boolean exposeProxy() default false;
}
複製代碼
AspectJAutoProxyRegistrar實現了ImportBeanDefinitionRegistrar接口,ImportBeanDefinitionRegistrar的做用是在運行時自動添加Bean到已有的配置類,經過重寫方法:
public void registerBeanDefinitions( AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry);
複製代碼
源碼:
@Override
public void registerBeanDefinitions( AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);
AnnotationAttributes enableAspectJAutoProxy =
AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);
if (enableAspectJAutoProxy != null) {
if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {
AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
}
if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {
AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);
}
}
}
複製代碼
Mybatis 中大名鼎鼎的@MapperScan 也是如此