介绍
在使用Spring的时候，我们只需要一个Enable注解就能实现开启一个模块的功能，非常的方便，那么这个功能是如何实现的？
我们常用的Enable注解如下所示
| 注解 | 作用 |
|---|---|
| @EnableAspectJAutoProxy | 开启AspectJ自动代理 |
| @EnableAsync | 开启异步方法支持 |
| @EnableScheduling | 开启定时任务支持 |
| @EnableWebMVC | 开启web mvc支持 |
| @EnableTransactionManagement | 开启注解式事务支持 |
| @EnableCaching | 开启注解式缓存支持 |
| @EnableAutoConfiguration | 开启自动配置 |
其实Enable注解底层是通过@Import注解实现的，@Import注解会往spring容器中注入所需要的Bean
@Import注解注入Bean的方式有如下三种
- 基于Configuration Class
- 基于ImportSelector接口
- 基于ImportBeanDefinitionRegistrar接口
演示一波
@EnableHelloWorld
public class EnableModuleDemo {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
context.register(EnableModuleDemo.class);
context.refresh();
String helloWorld = context.getBean("helloWorld", String.class);
// hello world
System.out.println(helloWorld);
context.close();
}
}
可以看到启动容器后就能从容器中拿到名字为helloWorld的Bean，注入的过程就是@EnableHelloWorld注解中
基于Configuration Class
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(HelloWorldConfiguration.class)
public @interface EnableHelloWorld {
}
@Configuration
public class HelloWorldConfiguration {
@Bean
public String helloWorld() {
return "hello world";
}
}
基于ImportSelector接口
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(HelloWorldImportSelector.class)
public @interface EnableHelloWorld {
}
public class HelloWorldImportSelector implements ImportSelector {
@Override
public String[] selectImports(AnnotationMetadata importingClassMetadata) {
return new String[]{
"com.javashitang.HelloWorldConfiguration"};
}
}
基于ImportBeanDefinitionRegistrar接口
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(HelloWorldImportBeanDefinitionRegistrar.class)
public @interface EnableHelloWorld {
}
public class HelloWorldImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
AnnotatedGenericBeanDefinition beanDefinition = new AnnotatedGenericBeanDefinition(HelloWorldConfiguration.class);
BeanDefinitionReaderUtils.registerWithGeneratedName(beanDefinition, registry);
}
}
不就简单注入一个Bean吗？为啥还要搞这么多方式？一句话，按需扩展
看看我们之前的Enable注解是如何实现的？
基于Configuration Class
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(SchedulingConfiguration.class)
@Documented
public @interface EnableScheduling {
}
@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();
}
}
通过配置类，注入一个新的BeanPostProcessor，在BeanPostProcessor增加对@Scheduled注解的支持
基于ImportSelector接口
@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;
}
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;
}
}
}
按照用户配置的不同的代码方式，选择不同的配置类，返回要加载的类的名字
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
// 要排除的bean的类型
Class<?>[] exclude() default {
};
// 要排除的bean的名字
String[] excludeName() default {
};
}
基于ImportBeanDefinitionRegistrar接口
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {
boolean proxyTargetClass() default false;
boolean exposeProxy() default false;
}
class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
@Override
public void registerBeanDefinitions(
AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
// 直接往registry中注册BeanDefinition
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);
}
}
}
}
直接把BeanDefinitionRegistry给你，自己往里面注入BeanDefinition，适用于比较底层的实现
参考博客
[1]https://blog.csdn.net/andy_zhang2007/article/details/96612243
[2]https://blog.csdn.net/qq_26525215/article/details/53524844?utm_medium=distribute.pc_relevant.none-task-blog-baidujs_title-1&spm=1001.2101.3001.4242
ImportAware接口的作用
[3]https://javazhiyin.blog.csdn.net/article/details/101186808