@Resource依赖注入原理

spring可以通过@Resource注解注入依赖，和@Autowired存在区别：

@Autowired默认按类型注入，而@Resource默认按名称和类型结合注入
@Autowired支持通过属性required设置为非必须注入，当依赖不存在时不会报错；@Resource注解不支持
@Autowired能够配合@Qualifier、@Primary、@Priority共同使用

那么spring实现@Resource注解注入依赖的原理是什么，现在可以看下，逻辑不复杂，先判断容器中是否包含需要注入的依赖bean实例对象，没有的话就跟@Autowired解析注入的依赖的方式一样，可以看我前一篇**@Autowired依赖注入原理**，存在的话则到容器中根据name和type获取依赖bean实例对象。

java 复制代码

	protected Object autowireResource(BeanFactory factory, LookupElement element, @Nullable String requestingBeanName)
			throws NoSuchBeanDefinitionException {
......
		if (factory instanceof AutowireCapableBeanFactory) {
			AutowireCapableBeanFactory beanFactory = (AutowireCapableBeanFactory) factory;
			DependencyDescriptor descriptor = element.getDependencyDescriptor();
			//容器中没有依赖bean实例对象，则走@Autowired自动注入逻辑
			if (this.fallbackToDefaultTypeMatch && element.isDefaultName && !factory.containsBean(name)) {
				autowiredBeanNames = new LinkedHashSet<>();
				resource = beanFactory.resolveDependency(descriptor, requestingBeanName, autowiredBeanNames, null);
				if (resource == null) {
					throw new NoSuchBeanDefinitionException(element.getLookupType(), "No resolvable resource object");
				}
			}
			//到容器中获取根据name和type获取bean实例对象
			else {
				resource = beanFactory.resolveBeanByName(name, descriptor);
				autowiredBeanNames = Collections.singleton(name);
			}
		}
		else {
			resource = factory.getBean(name, element.lookupType);
			autowiredBeanNames = Collections.singleton(name);
		}
......
		return resource;
	}
	//调用BeanFactory的getBean方法根据name和type获取bean实例对象
	public Object resolveBeanByName(String name, DependencyDescriptor descriptor) {
		InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
		try {
			return getBean(name, descriptor.getDependencyType());
		}
		finally {
			ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
		}
	}

获取到依赖bean实例对象后，就会以反射方式set到字段中，进而完成依赖注入。这里可以看出@Resource获取依赖再注入到字段的逻辑相对简单，虽然使用起来没有@Autowired灵活，但是效率会高很多。

java 复制代码

		protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)
				throws Throwable {

			if (this.isField) {
				Field field = (Field) this.member;
				ReflectionUtils.makeAccessible(field);
				//反射注入依赖到字段
				field.set(target, getResourceToInject(target, requestingBeanName));
			}
			else {
				if (checkPropertySkipping(pvs)) {
					return;
				}
				try {
					Method method = (Method) this.member;
					ReflectionUtils.makeAccessible(method);
					method.invoke(target, getResourceToInject(target, requestingBeanName));
				}
				catch (InvocationTargetException ex) {
					throw ex.getTargetException();
				}
			}
		}

总结下，spring处理通过@Resource注解注入依赖时，会到容器中根据name和type获取依赖bean实例对象，再反射set注入，更多请关注微信公众号葡萄开源