依赖注入

spring有几种依赖注入方式

  从类型角度区分，分两种：手动和自动

手动注入：通过XML中定义Bean时，可手动注入

<!-- 通过set方法注入 -->
<bean name="userService" class="com.luban.service.UserService"><property name="orderService" ref="orderService"/>
</bean><!-- 通过构造器注入 -->
<bean name="userService" class="com.luban.service.UserService"><constructor-arg index="0" ref="orderService"/>
</bean>

自动注入：

// xml的形式配置自动注入
<bean id="userService" class="com.luban.service.UserService" autowire="byType"/>// @Autowired 注解自动注入
@Autowired	// @Value 注解自动注入 
@Value// @Inject 注解自动注入 
@Inject// @Resource 注解自动注入 
@Resource// @Bean 注解中的autowire属性自动注入 （已废弃）
@Bean(autowire = Autowire.BY_NAME)
@Bean(autowire = Autowire.BY_type)

源码解析

  Bean实例化后和初始化之间会进行属性填充，接下来我们将解析属性填充的具体步骤，废话不多说，上代码：

/*** Populate the bean instance in the given BeanWrapper with the property values* from the bean definition.** @param beanName the name of the bean* @param mbd      the bean definition for the bean* @param bw       the BeanWrapper with bean instance*/
@SuppressWarnings("deprecation")  // for postProcessPropertyValues
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {if (bw == null) {if (mbd.hasPropertyValues()) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");} else {// Skip property population phase for null instance.return;}}// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the// state of the bean before properties are set. This can be used, for example,// to support styles of field injection.// 实例化之后，属性设置之前if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {// 执行 InstantiationAwareBeanPostProcessors接口的postProcessAfterInstantiation（）方法 根据返回值判断是否跳过属性填充if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {return;}}}/**** 处理spring自带的依赖注入的功能 @Bean(autowire = Autowire.BY_NAME)** */PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);int resolvedAutowireMode = mbd.getResolvedAutowireMode();if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {// MutablePropertyValues是PropertyValues具体的实现类MutablePropertyValues newPvs = new MutablePropertyValues(pvs);// Add property values based on autowire by name if applicable.if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {autowireByName(beanName, mbd, bw, newPvs);}// Add property values based on autowire by type if applicable.if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {autowireByType(beanName, mbd, bw, newPvs);}pvs = newPvs;}boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);PropertyDescriptor[] filteredPds = null;if (hasInstAwareBpps) {if (pvs == null) {pvs = mbd.getPropertyValues();}for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {// 这里会调用AutowiredAnnotationBeanPostProcessor的postProcessProperties()方法，会直接给对象中的属性赋值// AutowiredAnnotationBeanPostProcessor内部并不会处理pvs，直接返回了PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}}pvs = pvsToUse;}}if (needsDepCheck) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}checkDependencies(beanName, mbd, filteredPds, pvs);}// 如果当前Bean中的BeanDefinition中设置了PropertyValues，那么最终将是PropertyValues中的值，覆盖@Autowiredif (pvs != null) {applyPropertyValues(beanName, mbd, bw, pvs);}
}

  通过上述代码我们可以看到：

1. 首先，第一个判断是检验Bean对象不是null；
2. 第二个判断，当一个BeanPostProcessor实现啦InstantiationAwareBeanPostProcessor接口的postProcessAfterInstantiation()方法，并且返回flash时，就不会走属性填充，返回true时，会继续走属性填充；Spring在这一步通过AutowiredAnnotationBeanPostProcessor类的中postProcessMergedBeanDefinition方法寻找@Value和@Autoword的注入点，CommonAnnotationBeanPostProcessor类中的postProcessMergedBeanDefinition方法寻找@Resource的注入点；
3. 再往下，处理@Bean(autowire = Autowire.BY_NAME)和@Bean(autowire = Autowire.BY_NAME)，详情请移步至《Spring之Bean生命周期~依赖注入（1）》；
4. 再往下，会循环所有实现InstantiationAwareBeanPostProcessor接口的BeanPostProcessor，调用它的postProcessProperties()方法，Spring在这一步通过AutowiredAnnotationBeanPostProcessor类的中postProcessProperties方法给@Value和@Autoword进行注入操作，CommonAnnotationBeanPostProcessor类中的postProcessProperties方法给@Resource的进行注入操作；AutowiredAnnotationBeanPostProcessor类相关依赖注入源码请移步至《Spring之Bean生命周期~依赖注入（2）》，CommonAnnotationBeanPostProcessor类相关依赖注入源码请移步至《Spring之Bean生命周期~依赖注入（3）》
5. 最后，如果当前Bean中的BeanDefinition中设置了PropertyValues，那么最终将是PropertyValues中的值，覆盖@Autowired、@Value、@Resource；