通過自定義注解來實現注入功能有兩種方法
第一種通過元注解的方式，在自定義注解上面用@Autowired標注既可以實現
@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Autowired
public @interface MyAutowried {
}
第二種是通過向Spring中注冊一個AutowiredAnnotationBeanPostProcessor實現链沼，重點介紹第二種方式
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;
}
}
上述代碼可以在不影響原有實現的同時實現對自定義注解的擴展
原理
在Spring啟動時，會獲取類型為BeanPostProcessor.class的bean名稱鸯旁，然后獲取對應Bean的實例，并加入beanFactory中的beanPostProcessors列表當中量蕊，代碼流程如下
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)) {
// 獲取bean實例
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
// 將bean添加進集合
internalPostProcessors.add(pp);
}
}
}
// 對internalPostProcessors根據Order進行排序
sortPostProcessors(internalPostProcessors, beanFactory);
// 將processors添加進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<>();
}
對自定義注解的處理
public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
// 獲取beanFactory中所有的processor遍歷處理
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof MergedBeanDefinitionPostProcessor) {
MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
// 處理自定義注解相關依賴
bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
}
}
}
}
注
在創(chuàng)建 AutowiredAnnotationBeanPostProcessor 對象時铺罢，方法最好聲明成靜態(tài)方法，這樣在Spring初始化該對象時就可以不用依賴它的外部對象
在該例中残炮，如果beanPostProcessor() 方法不用static修飾韭赘，會造成User對象無法注入問題，
原因就在于Spring要先獲取所有的AutowiredAnnotationBeanPostProcessor 對象势就，在執(zhí)行beanPostProcessor()方法時需要先初始化其外部類 DependencyResolutionDemo泉瞻，當初始化此對象時，就會對User對象開始注入苞冯，但是此時處理@MyAutowoired的注解處理器還沒有初始化完成袖牙，就會造成@MyAutowoired 注解不能被處理
解決方法就是將beanPostProcessor()方法標注為static方法，或者將beanPostProcessor()單獨寫入另一個配置類當中
