骚话@Autowired的底层工作原理,spring的自动装配

前情回顾

Spring拓展接口之BeanPostProcessor,我们来看看它的底层实现中讲到了spring对BeanPostProcessor的底层支持,并且知道了BeanPostProcessor的两个方法:postProcessBeforeInitialization、postProcessAfterInitialization的执行时机,没看的小伙伴可以回过头去看看。本来spring的自动装配是打算放到上一篇博文中详细讲解的,可后来觉得篇幅可能太大了,既然你们都感觉出来了,那我也就明人不说暗话了,之所以没放到上篇讲解,确实是因为篇幅太大了。好了,我们言归正传,之所以没放到上篇来讲,篇幅只是原因之

一,最主要的原因是发现我犯错了! 犯什么错了呢?

我想当然了! 理所当然的认为自动装配是在AutowiredAnnotationBeanPostProcessor的postProcessBeforeInitialization或postProcessAfterInitialization中实现的,我们来看下AutowiredAnnotationBeanPostProcessor类继承图

骚话@Autowired的底层工作原理,spring的自动装配

它间接实现了BeanPostProcessor,我们再去看下那两个方法(在父类InstantiationAwareBeanPostProcessorAdapter中)

@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
 return bean;
}
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
 return bean;
}
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竟然啥也没干,只是简单的return bean; 当自己深以为然的认知被推翻时,那感觉真是毙了狗了

所以自动装配不能和BeanPostProcessor放一块讲,不得不开两篇来分开讲,我们都知道:强扭的瓜不甜!

自动装配简单示例

我们先来看一个简单的自动装配的示例,完整实例代码:spring-boot-BeanPostProcessor

AnimalServiceImpl

@Service
public class AnimalServiceImpl implements IAnimalService {
 @Autowired
 private Dog dog;
 @Resource
 private Cat cat;
 @Inject
 private Pig pig;
 @Override
 public void printName() {
 System.out.println(dog.getName());
 System.out.println(cat.getName());
 System.out.println(pig.getName());
 }
}
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AnimalTest

@RunWith(SpringRunner.class)
@SpringBootTest(classes={Application.class})
public class AnimalTest {
 @Autowired
 private IAnimalService animalService;
 @Test
 public void test() {
 animalService.printName();
 }
}
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运行结果

骚话@Autowired的底层工作原理,spring的自动装配

我们在AnimalConfig中只是将Dog、Cat、Pig的实例注册到了spring容器,那为什么AnimalServiceImpl实例能够直接应用这些实例了,我们并没有手动的将这些实例赋值到AnimalServiceImpl实例呀? 这其实就是spring提供的自动装配功能,虽然我们没有手动的将这些实例赋值到AnimalServiceImpl实例,但是我们发现AnimalServiceImpl的属性实例上多了一些注解:@Autowired、@Resource、@Inject,spring通过这些注解自动完成了属性实例的注入,而不需要我们手动的去赋值了;那么spring是如何实现自动装配的呢? 我们慢慢往下看(注意:后文主要以@Autowired为例来讲解)

自动装配源码解析

AutowiredAnnotationBeanPostProcessor的实例化与注册

不管怎么说,AutowiredAnnotationBeanPostProcessor终归还是一个BeanPostProcessor,那么它的实例化与注册(注册到spring的beanFactory)过程与BeanPostProcessor的实例化与注册一样,在spring的启动过程中,刷新上下文(refresh)的时候,会调用registerBeanPostProcessors(beanFactory)方法完成BeanPostProcessor的实例化与注册,后续再调用finishBeanFactoryInitialization(beanFactory)实例化非延迟加载的单例bean时,会用到上述注册的BeanPostProcessor

AutowiredAnnotationBeanPostProcessor的构造方法值得我们看看

public AutowiredAnnotationBeanPostProcessor() {
 this.autowiredAnnotationTypes.add(Autowired.class);
 this.autowiredAnnotationTypes.add(Value.class);
 try {
 this.autowiredAnnotationTypes.add((Class)
 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.
 }
}
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默认情况下,AutowiredAnnotationBeanPostProcessor支持@Autowired和@Value,如果类路径下有java.inject.Inject(也就是引入了javax.inject.jar),那么也支持@Inject注解,是不是与我们最初的认知有些不一样?

将支持的注解放到了autowiredAnnotationTypes属性中,后续会用到该属性

bean的实例化与依赖注入

默认情况下,spring会把spring容器中的bean当成non-lazy-init singleton来处理(有些特殊的bean除外),也就是说会在spring的启动过程中就会逐个实例化这些bean,并对这些bean进行依赖注入;当我们真正用到这些bean的时候,直接用就行,不用再去实例化,也不用再去注入bean的相关依赖,spring是不是很厉害?

具体是不是说的这样,大家准备好花生、瓜子和啤酒,好戏即将开始

我们先找到正确的入口,然后用下图省略掉无聊的前戏,直接进入高潮:doCreateBean

骚话@Autowired的底层工作原理,spring的自动装配

doCreateBean内容如下

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实例
 instanceWrapper = createBeanInstance(beanName, mbd, args);
 }
 final Object bean = instanceWrapper.getWrappedInstance();
 Class beanType = instanceWrapper.getWrappedClass();
 if (beanType != NullBean.class) {
 mbd.resolvedTargetType = beanType;
 }
 // Allow post-processors to modify the merged bean definition.
 // 允许后置处理器来修改bean定义
 synchronized (mbd.postProcessingLock) {
 if (!mbd.postProcessed) {
 try {
 // 调用MergedBeanDefinitionPostProcessor的postProcessMergedBeanDefinition方法
 // AutowiredAnnotationBeanPostProcessor实现了MergedBeanDefinitionPostProcessor,即MergedBeanDefinitionPostProcessor的MergedBeanDefinitionPostProcessor会被调用
 applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
 }
 catch (Throwable ex) {
 throw new BeanCreationException(mbd.getResourceDescription(), beanName,
 "Post-processing of merged bean definition failed", ex);
 }
 mbd.postProcessed = true;
 }
 }
 // Eagerly cache singletons to be able to resolve circular references 立即缓存单例以便能够解析循环引用
 // even when triggered by lifecycle interfaces like BeanFactoryAware.
 boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
 isSingletonCurrentlyInCreation(beanName));
 if (earlySingletonExposure) {
 if (logger.isDebugEnabled()) {
 logger.debug("Eagerly caching bean '" + beanName +
 "' to allow for resolving potential circular references");
 }
 addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
 }
 // Initialize the bean instance.
 Object exposedObject = bean;
 try {
 // 填充bean,包含依赖注入
 populateBean(beanName, mbd, instanceWrapper);
 // 初始化bean,BeanPostProcessor的两个方法在此中被调用
 exposedObject = initializeBean(beanName, exposedObject, mbd);
 }
 catch (Throwable ex) {
 if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
 throw (BeanCreationException) ex;
 }
 else {
 throw new BeanCreationException(
 mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
 }
 }
 if (earlySingletonExposure) {
 Object earlySingletonReference = getSingleton(beanName, false);
 if (earlySingletonReference != null) {
 if (exposedObject == bean) {
 exposedObject = earlySingletonReference;
 }
 else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
 String[] dependentBeans = getDependentBeans(beanName);
 Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
 for (String dependentBean : dependentBeans) {
 if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
 actualDependentBeans.add(dependentBean);
 }
 }
 if (!actualDependentBeans.isEmpty()) {
 throw new BeanCurrentlyInCreationException(beanName,
 "Bean with name '" + beanName + "' has been injected into other beans [" +
 StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
 "] in its raw version as part of a circular reference, but has eventually been " +
 "wrapped. This means that said other beans do not use the final version of the " +
 "bean. This is often the result of over-eager type matching - consider using " +
 "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
 }
 }
 }
 }
 // Register bean as disposable.
 try {
 registerDisposableBeanIfNecessary(beanName, bean, mbd);
 }
 catch (BeanDefinitionValidationException ex) {
 throw new BeanCreationException(
 mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
 }
 return exposedObject;
}
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我们重点看下posProcessMergedBeanDefinition方法和populateBean方法

posProcessMergedBeanDefinition

骚话@Autowired的底层工作原理,spring的自动装配

可以看到会读取bean的field和method上的注解,并判断该注解是否在autowiredAnnotationTypes中,如果在则将field封装成AutowiredFiledElement对象、将method封装成AutoWiredMethodElement对象,并存放到InjectionMetadata对象的Set checkedElements属性中,最后将该InjectionMetadata对象缓存到了AutowiredAnnotationBeanPostProcessor的Map injectionMetadataCache属性中;说白了就是将bean中被@Autowried(当然还包括@Value、@Inject)修饰的field、method找出来,封装成InjectionMetadata对象并缓存起来,就这么简单。不仅仅是上图中的animalServiceImpl这一个bean,spring中所有的非延迟加载的bean都会走这个创建流程。是不是很简单,是不是干劲十足了

populateBean

骚话@Autowired的底层工作原理,spring的自动装配

调用AutowiredAnnotationBeanPostProcessor的postProcessPropertyValues方法,从injectionMetadataCache中获取当前bean的依赖信息,比如animalServiceImpl依赖的dog、pig(有人可能会有这样的疑问:cat呢? cat是被@Resource修饰的,而@Resource不是由AutowiredAnnotationBeanPostProcessor支持,后续会讲由谁支持),然后逐个将依赖bean注入到目标bean(将dog、pig实例注入到animalServiceImpl实例中);依赖bean从哪来呢?还是从beanFactory中获取,如果不存在,则又回到bean的创建过程把依赖bean(dog、pig)创建出来,流程与创建animalServiceImpl实例一模一样,也就说在animalServiceImpl实例的依赖注入过程中会把dog、pig对象也创建出来,而不是等到spring逐个实例化bean的过程中轮到dog、pig才实例化dog、pig,那后续轮到dog、pig时怎么办了,spring会把创建的bean缓存起来,下次就直接从缓存中取了。上图只演示Field的,Method也差不太多,就不演示了,都是通过反射实现的 。

总结

1、bean的创建与初始化

(1)instanceWrapper = createBeanInstance(beanName, mbd, args) 创建目标bean实例;

(2)applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName) 寻找目标bean的依赖;

(3)populateBean(beanName, mbd, instanceWrapper) 填充目标bean,完成依赖注入; (这里的循环依赖,有兴趣的可以自行去琢磨下)

(4)initializeBean(beanName, exposedObject, mbd) 初始化目标bean

2、自动装配与自动配置

自动配置一般而言说的是spring的@Autowired,是spring的特性之一,而自动配置是springboot的@Configuration,是springboot的特性之一

3、Spring支持几下几种自动装配的注解

@Autowired、@Inject、@Resource以及@Value,用的最多的应该是@Autowired(至少我是这样的),@Inject和@Value也是由AutowiredAnnotationBeanPostProcessor支持,而@Resource是由CommonAnnotationBeanPostProcessor支持(还支持@PostConstruct、@PreDestroy等注解)

关于@Value与@Autowired,不知道大家是否清楚他们之间的区别,不清楚的可以看看:Spring: @Value vs. @Autowired或者spring的官方文档,总结下:@Value >= @Autowired,只是平时应用中,@Value更多的是用来注入配置值(如:@Value("${db.url}")),而@Autowired则是bean对象的注入


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转载自juejin.im/post/5d822b02f265da03da24cc5a