9、Spring之Bean生命周期~依赖注入（总）
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
xml
<!-- 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：
java
// 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：
java
/**
* 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@Autowired
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}通过上述代码我们可以看到：
- 首先，第一个判断是检验Bean对象不是null；
- 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；
- 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）》；
- 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）》
- 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；