001|搭上SpringBoot自动注入源码分析专车

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本系列为SpringBoot深度源码专车系列，第一篇发车！

该趟专车是开往Spring Boot自动注入原理源码分析的专车

• Spring Boot何时注入@Autowired标注的属性？



• 如果注入类型的Bean存在多个Spring Boot是如何处理的？

  
  专车示例
  



• 定义接口

  public interface PersonService {
String hello(String name);
}
  




• 定义接口的一个实现

  @Service(value = "studentService")
public class StudentServiceImpl implements PersonService {
@Override
public String hello(String name) {
return "[student service] hello " + name;
}
}
  




• 定义接口的另一个实现

  @Service(value = "teacherService")
public class TeacherServiceImpl implements PersonService {
@Override
public String hello(String name) {
return "[teacher service] hello " + name;
}
}
  




• 定义控制器

  @RestController
public class TestController {
@Autowired
private PersonService studentService;
@Autowired
private PersonService teacherService;
@GetMapping("/hello")
public String hello(@RequestParam(name = "name") String name) {
return studentService.hello(name) + "=======>" + teacherService.hello(name);
}
}

  以上示例代码很简单，创建了一个接口，接口有两个实现类，然后在控制器中注入实现类，从而完成业务方法的调用。接下来我们就开始对源码进行分析

  
  

在分析代码之前我们先回忆一下操作对象的步骤：

• 首先我们会实例化一个对象


• 然后调用对象的set方法来设置对象的属性


• 有了上面的基础知识，接下来就开始揭秘旅程

在分析源码的时候最关键的一步就是寻找程序的入口，有了入口我们就成功了一半，那么如何寻找程序的入口？针对此处的源码分析，我们可以在TestController类上打一个断点，然后查看调用链

基于调用链路，我们看到有一个doCreateBean方法，该方法就是用来创建bean的，也就是我们上面提到的实例化对象部分

AbstractAutowireCapableBeanFactory#doCreateBean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 创建bean
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class beanType = instanceWrapper.getWrappedClass();
// ...省略部分代码
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 填充bean，也就是我们上面提到的调用对象的set方法设置对象属性
populateBean(beanName, mbd, instanceWrapper);
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
// ...省略部分代码
return exposedObject;
}
填充bean

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
// ...省略代码
PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
// 遍历所有的后置处理器
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
// 通过断点分析我们可以得知此处调用的是AutowiredAnnotationBeanPostProcessor#postProcessProperties
PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
pvsToUse = ibp.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);
}
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
处理属性

AutowiredAnnotationBeanPostProcessor#postProcessProperties

public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
// 查找当前bean需要注入的元数据信息，以TestController为例，那么需要注入的就是studentService和teacherService两个属性
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
// 注入属性
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;
}

注入属性 AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject

protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
// 获取属性，此处的属性就是studentService
Field field = (Field) this.member;
// 属性对应的value
Object value;
if (this.cached) {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
else {
DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
desc.setContainingClass(bean.getClass());
Set autowiredBeanNames = new LinkedHashSet<>(1);
Assert.state(beanFactory != null, "No BeanFactory available");
TypeConverter typeCOnverter= beanFactory.getTypeConverter();
try {
// 解析属性依赖
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
synchronized (this) {
if (!this.cached) {
if (value != null || this.required) {
this.cachedFieldValue = desc;
registerDependentBeans(beanName, autowiredBeanNames);
if (autowiredBeanNames.size() == 1) {
String autowiredBeanName = autowiredBeanNames.iterator().next();
if (beanFactory.containsBean(autowiredBeanName) &&
beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
this.cachedFieldValue = new ShortcutDependencyDescriptor(
desc, autowiredBeanName, field.getType());
}
}
}
else {
this.cachedFieldValue = null;
}
this.cached = true;
}
}
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
// 给属性设置值，完成注入功能
field.set(bean, value);
}
}

解析属性依赖 DefaultListableBeanFactory#resolveDependency

public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
if (Optional.class == descriptor.getDependencyType()) {
return createOptionalDependency(descriptor, requestingBeanName);
}
else if (ObjectFactory.class == descriptor.getDependencyType() ||
ObjectProvider.class == descriptor.getDependencyType()) {
return new DependencyObjectProvider(descriptor, requestingBeanName);
}
else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
}
else {
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
// 解析依赖
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}

解析属性依赖 DefaultListableBeanFactory#doResolveDependency

public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectiOnPoint= ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
// ...省略代码
// 解析多个Bean，比如Array、List、Map类型，有兴趣可以自己查看分析
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
// 根据类型获取候选对象，针对studentService而言，该属性的类型为PersonService
// PersonService有2个实现类，StudentServiceImpl和TeacherServiceImpl
// 所以此处获取结果为StudentServiceImpl对象和TeacherServiceImpl对象
Map matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
// 重点处理，如果存在多个匹配的bean
if (matchingBeans.size() > 1) {
// 从已经匹配的bean中选择一个符合的bean
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {
// 如果bean必须注入或者存在多个匹配的bean，则抛出异常
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
}
else {
// In case of an optional Collection/Map, silently ignore a non-unique case:
// possibly it was meant to be an empty collection of multiple regular beans
// (before 4.3 in particular when we didn't even look for collection beans).
return null;
}
}
// 根据bean名称获取对应的示例
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();
}
if (autowiredBeanNames != null) {
autowiredBeanNames.add(autowiredBeanName);
}
if (instanceCandidate instanceof Class) {
instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;
if (result instanceof NullBean) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
result = null;
}
if (!ClassUtils.isAssignableValue(type, result)) {
throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
}
// 返回对应的示例对象
return result;
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}

此处主要根据类型获取所有匹配的bean，如果匹配的bean有多个，那么最后会选择一个符合条件的bean名称，然后将对应的bena实例返回，调用set方法进行进行注入，到此注入的原理本该结束了。但是还是要分析一下Spring Boot是如何选择出符合条件的bean？

选择符合条件的bean DefaultListableBeanFactory#determineAutowireCandidate

protected String determineAutowireCandidate(Map candidates, DependencyDescriptor descriptor) {
Class requiredType = descriptor.getDependencyType();
// 如果bean对应的primary属性为true，则返回bean对应的名称
String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);
if (primaryCandidate != null) {
return primaryCandidate;
}
// 如果候选bean使用javax.annotation.Priority标注，返回高优先级bean对应的名称
String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);
if (priorityCandidate != null) {
return priorityCandidate;
}
// Fallback
// 如果匹配bean的名称和需要注入的属性名称一致，则返回匹配bean的名称
for (Map.Entry entry : candidates.entrySet()) {
String candidateName = entry.getKey();
Object beanInstance = entry.getValue();
if ((beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) ||
matchesBeanName(candidateName, descriptor.getDependencyName())) {
return candidateName;
}
}
return null;
}

获取符合条件bean名称总结：

• 依据Bean的primary属性


• 依据javax.annotation.Priority



• 依据注入属性的名称

  
  专车总结
  


• Bean实例化完成后，填充Bean


• 调用AutowiredAnnotationBeanPostProcessor#postProcessProperties处理属性


• 获取所有需要注入的属性


• 根据注入属性的类型从IOC容器中查找匹配实例


• 如果匹配实例存在多个，根据primary属性--->javax.annotation.Priority注解--->注入属性名称依次过滤，返回符合条件的Bean名称


• 过滤之后，存在一个符合条件的Bean名称，则返回对应的实例，否则抛出异常
  专车回顾

回顾一下开头的2个问题：

• Spring Boot何时注入@Autowired标注的属性？


• 如果注入类型的Bean存在多个Spring Boot是如何处理的？
  第一个问题：是在Bean实例化后，填充Bean的时候注入@Autowired标注的属性

第二个问题：如果存在多个类型的Bean，会根据primary--->javax.annotation.Priority--->名称依次过滤，得到最终匹配的bean名称

———— e n d ————
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