在本章的第一部分我们通过:
@EnableAspectJAutoProxy开启对AspectJ自动代理的支持。
@EnableAsync开启异步方法的支持。
@EnableScheduling开启计划任务的支持。
在第二部分我们通过:
@EnableWebMvc开启Web MVC的配置支持。
在第三部分我们通过:
@EnableConfigurationProperties开启对@ConfigurationProperties注解配置Bean的支持。
@EnableJpaRepositories开启对Spring Data JPA Repository的支持。
@EnableTransactionManagement开启注解式事务的支持。
@EnableCaching开启注解式的缓存支持。
通过简单的@Enble来开启一项功能的支持,从而避免自己配置大量的代码,大大降低使用难度。那么这个神奇的功能的实现原理是什么呢?我们一起来研究一下。
通过观察这些@Enable注解的源码,我们发现所有的注解都有一个@Import 注解,@Import是用来导入配置类的,这就意味着这些自动开启的实现其实是导入了一些自动配置的Bean。这些导入的配置方式主要分为以下三种类型。
3.6.1 第一类:直接导入配置类
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(SchedulingConfiguration.class)
@Documented
public @interface EnableScheduling {
}
直接导入配置类SchedulingConfiguration,这个类注解了@Configuration,且注册了一个scheduledAnnotationProcessor的Bean,源码如下:
@Configuration
public class SchedulingConfiguration {
@Bean(name = TaskManagementConfigUtils.SCHEDULED_ANNOTATION_PROCESSOR_BEAN_NAME)
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public ScheduledAnnotationBeanPostProcessor scheduledAnnotationProcessor() {
return new ScheduledAnnotationBeanPostProcessor();
}
}
3.6.2 第二类:依据条件选择配置类
@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;
}
AsyncConfigurationSelector通过条件来选择需要导入的配置类,AsyncConfigurationSelector的根接口为ImportSelector,这个接口需重新写selectImports方法,在此方法内进行事先条件判断。此例中,若 activeMode为PORXY,则返回ProxyAsyncConfiguration这个配置类;若 activeMode 为 ASPECTJ,则返回AspectJAsyncConfiguration配置类,源码如下:
public class AsyncConfigurationSelector extends AdviceModeImportSelector {
private static final String ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME =
"org.springframework.scheduling.aspectj.AspectJAsyncConfiguration";
@Override
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;
}
}
}
3.6.3 第三类:动态注册Bean
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {
}
AutoProxyRegistrar 实现了ImportBeanDefinitionRegistrar接口,ImportBeanDefinitionRegistrar的作用是在运行时自动添加Bean到已有的配置类,通过重写方法:
public void registerBeanDefinitions(
AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry);
其中,AnnotationMetadata参数用来获得当前配置类上的注解;BeanDefinitionRegistry参数用来注册Bean。源码如下:
public class AutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
private final Log logger = LogFactory.getLog(getClass());
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
boolean candidateFound = false;
Set annoTypes = importingClassMetadata.getAnnotationTypes();
for (String annoType : annoTypes) {
AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annoType);
Object mode = candidate.get("mode");
Object proxyTargetClass = candidate.get("proxyTargetClass");
if (mode != null && proxyTargetClass != null && mode.getClass().equals(AdviceMode.class) &&
proxyTargetClass.getClass().equals(Boolean.class)) {
candidateFound = true;
if (mode == AdviceMode.PROXY) {
AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
if ((Boolean) proxyTargetClass) {
AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
return;
}
}
}
}
if (!candidateFound) {
String name = getClass().getSimpleName();
logger.warn(String.format("%s was imported but no annotations were found " +
"having both 'mode' and 'proxyTargetClass' attributes of type " +
"AdviceMode and boolean respectively. This means that auto proxy " +
"creator registration and configuration may not have occured as " +
"intended, and components may not be proxied as expected. Check to " +
"ensure that %s has been @Import'ed on the same class where these " +
"annotations are declared; otherwise remove the import of %s " +
"altogether.", name, name, name));
}
}
}