通过自定义注解来实现注入功能有两种方法
第一种通过元注解的方式，在自定义注解上面用@Autowired标注既可以实现
@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Autowired
public @interface MyAutowried {
}
第二种是通过向Spring中注册一个AutowiredAnnotationBeanPostProcessor实现，重点介绍第二种方式
public class DependencyResolutionDemo {
@MyAutowried
private User user;
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
context.register(DependencyResolutionDemo.class);
context.refresh();
DependencyResolutionDemo bean = context.getBean(DependencyResolutionDemo.class);
System.out.println(bean.user);
context.close();
}
@Bean
public User user() {
User user = new User();
user.setId(2L);
return user;
}
@Bean
public static AutowiredAnnotationBeanPostProcessor beanPostProcessor() {
AutowiredAnnotationBeanPostProcessor postProcessor = new AutowiredAnnotationBeanPostProcessor();
postProcessor.setAutowiredAnnotationType(MyAutowried.class);
return postProcessor;
}
}
上述代码可以在不影响原有实现的同时实现对自定义注解的扩展
原理
在Spring启动时，会获取类型为BeanPostProcessor.class的bean名称，然后获取对应Bean的实例，并加入beanFactory中的beanPostProcessors列表当中，代码流程如下
public static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));
List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
// 获取bean实例
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
// 将bean添加进集合
internalPostProcessors.add(pp);
}
}
}
// 对internalPostProcessors根据Order进行排序
sortPostProcessors(internalPostProcessors, beanFactory);
// 将processors添加进beanFactory的beanPostProcessors中
registerBeanPostProcessors(beanFactory, internalPostProcessors);
}
// BeanFactory中的processors
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
private final List<BeanPostProcessor> beanPostProcessors = new CopyOnWriteArrayList<>();
}
对自定义注解的处理
public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
// 获取beanFactory中所有的processor遍历处理
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof MergedBeanDefinitionPostProcessor) {
MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
// 处理自定义注解相关依赖
bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
}
}
}
}
注
在创建 AutowiredAnnotationBeanPostProcessor 对象时，方法最好声明成静态方法，这样在Spring初始化该对象时就可以不用依赖它的外部对象
在该例中，如果beanPostProcessor() 方法不用static修饰，会造成User对象无法注入问题，
原因就在于Spring要先获取所有的AutowiredAnnotationBeanPostProcessor 对象，在执行beanPostProcessor()方法时需要先初始化其外部类 DependencyResolutionDemo，当初始化此对象时，就会对User对象开始注入，但是此时处理@MyAutowoired的注解处理器还没有初始化完成，就会造成@MyAutowoired 注解不能被处理
解决方法就是将beanPostProcessor()方法标注为static方法，或者将beanPostProcessor()单独写入另一个配置类当中
