@Autowired 源码分析

缘起

@Autowired 注解可以说是几乎每个使用spring的童鞋都要使用的. 那么他的原理是什么呢?

分析

以demo【1】为例，最后起作用的关键类就是

org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(PropertyValues, PropertyDescriptor[], Object, String)

它的源码如下

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@Override
	public PropertyValues postProcessPropertyValues(
			PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {

		InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
		try {
			metadata.inject(bean, beanName, pvs);
		}
		catch (BeanCreationException ex) {
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
		}
		return pvs;
	}

其中第七行源码就是注入bean中去. metadata 包含了要注入的类的信息，比如beanName为bookController的时候，metadata就是AutowiredFieldElement for private com.yfs.service.BookService com.yfs.controller.BookController.bookService.

在这里发生注入的. 其实 不只是 Spring的注入注解 @Autowired, 对于JSR 的 @Inject、@Resource 注解也是在这个AutowiredAnnotationBeanPostProcessor中发生的. 何以为证? 下面的AutowiredAnnotationBeanPostProcessor的构造函数就是证据

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@SuppressWarnings("unchecked")
	public AutowiredAnnotationBeanPostProcessor() {
		this.autowiredAnnotationTypes.add(Autowired.class);
		this.autowiredAnnotationTypes.add(Value.class);
		try {
			this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
					ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
			logger.info("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
		}
		catch (ClassNotFoundException ex) {
			// JSR-330 API not available - simply skip.
		}
	}

DEMO

【1】https://github.com/yfsyfs/backend/tree/master/spring-annotation-autowired-qualified-primary