前言
前面一章【Spring源码学习】Spring Bean实例化过程-创建Bean实例，我们着重跟踪了bean实例的创建过程，本章，我们主要跟一下bean实例化后的依赖属性注入的过程。先把代码定位到类AbstractAutowireCapableBeanFactory中的doCreateBean()方法
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//创建实例,,重点看，重要程度：5
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//CommonAnnotationBeanPostProcessor 支持了@PostConstruct，@PreDestroy,@Resource注解
//AutowiredAnnotationBeanPostProcessor 支持 @Autowired,@Value注解
//BeanPostProcessor接口的典型运用，这里要理解这个接口
//对类中注解的装配过程
//重要程度5，必须看
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
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//是否单例bean提前暴露
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
//这里着重理解，对理解循环依赖帮助非常大，重要程度 5 添加三级缓存
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//ioc di，依赖注入的核心方法，该方法必须看，重要程度：5
populateBean(beanName, mbd, instanceWrapper);
//bean 实例化+ioc依赖注入完以后的调用，非常重要，重要程度：5
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);
}
}
......
//省略一些代码
// Register bean as disposable.
try {
//注册bean销毁时的类DisposableBeanAdapter
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}1.addSingletonFactory()
先看看addSingletonFactory()方法。这个方法主要作用就是针对单例的、支持提前暴露引用的且是正在创建的对象的工厂方法放入三级缓存中，这里对于理解循环依赖帮助非常大，尤其是AOP代理。
//DefaultSingletonBeanRegistry
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
//如果一级缓存不存在
if (!this.singletonObjects.containsKey(beanName)) {
//设置三级缓存
this.singletonFactories.put(beanName, singletonFactory);
//删除二级缓存
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}其中，() -> getEarlyBeanReference(beanName, mbd, bean)返回的是一个ObjectFactory匿名对象，ObjectFactory中getObject()的方法是getEarlyBeanReference(beanName, mbd, bean)方法的调用，提前暴露了对象的引用。
2.getEarlyBeanReference()
/**
* Obtain a reference for early access to the specified bean,
* typically for the purpose of resolving a circular reference.
* @param beanName the name of the bean (for error handling purposes)
* @param mbd the merged bean definition for the bean
* @param bean the raw bean instance
* @return the object to expose as bean reference
* 三级缓存对象工厂 ObjectFactory.getObject()调用到这个方法
*/
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
}
}
}
return exposedObject;
}主要看AbstractAutoProxyCreator的getEarlyBeanReference方法
3.getEarlyBeanReference()
public Object getEarlyBeanReference(Object bean, String beanName) {
//缓存的key
Object cacheKey = getCacheKey(bean.getClass(), beanName);
//这个缓存会在后面初始化的后置处理中用到，主要是AOP代理类生成的时候用到
this.earlyProxyReferences.put(cacheKey, bean);
return wrapIfNecessary(bean, beanName, cacheKey);
}
/**
* Wrap the given bean if necessary, i.e. if it is eligible for being proxied.
* @param bean the raw bean instance
* @param beanName the name of the bean
* @param cacheKey the cache key for metadata access
* @return a proxy wrapping the bean, or the raw bean instance as-is
*/
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
return bean;
}
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
//创建当前bean的代理，如果这个bean有advice的话，重点看，重要程度5
// Create proxy if we have advice.
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
//如果有切面，则生成该bean的代理
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
//把被代理对象bean实例封装到SingletonTargetSource对象中
Object proxy = createProxy(
bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}这里不继续展开了，后面将动态代理的时候会详细跟一下这里的逻辑
一、依赖属性注入
定位到AbstractAutowireCapableBeanFactory中的doCreateBean()的populateBean(beanName, mbd, instanceWrapper)方法
try {
//ioc di，依赖注入的核心方法，该方法必须看，重要程度：5
populateBean(beanName, mbd, instanceWrapper);
//bean 实例化+ioc依赖注入完以后的调用，非常重要，重要程度：5
exposedObject = initializeBean(beanName, exposedObject, mbd);
}1.populateBean()
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.
//这里很有意思，写接口可以让所有类都不能依赖注入
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
return;
}
}
}
}
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//......省略一些代码
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
PropertyDescriptor[] filteredPds = null;
//重点看这个if代码块，重要程度 5
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//依赖注入过程，@Autowired的支持
PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
//老版本用这个完成依赖注入过程，@Autowired的支持
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);
}
//这个方法很鸡肋了，建议不看，是老版本用<property name="username" value="Jack"/>
//标签做依赖注入的代码实现，复杂且无用
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}依赖注入核心的地方就在这里了
//依赖注入过程，@Autowired的支持
PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);上一章节中，我们已经说到了@Resource@Autowired 注解的收集，那么这个方法就是
根据收集到的注解进行反射调用，这里以@Autowired 注解的属性为例，注解的方法类似。
1.1.postProcessProperties()
//AutowiredAnnotationBeanPostProcessor
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
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;
}核心方法metadata.inject 循环收集到的metaData 中的list 对象，然后挨个调用里面的InjectedElement 的
inject 方法完成依赖注入。
1.2.inject()
//InjectionMetadata
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection<InjectedElement> checkedElements = this.checkedElements;
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
for (InjectedElement element : elementsToIterate) {
if (logger.isTraceEnabled()) {
logger.trace("Processing injected element of bean '" + beanName + "': " + element);
}
//通过反射来完成依赖注入调用
element.inject(target, beanName, pvs);
}
}
}
protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs) throws Throwable {
//属性注入
if (this.isField) {
Field field = (Field) this.member;
ReflectionUtils.makeAccessible(field);
field.set(target, getResourceToInject(target, requestingBeanName));
}
//方法反射调用
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();
}
}
}通过上述过程对@Autowired 和@Resource 以及@Value 注解修饰的Field 和方法等完成依赖注入。其中value 值的获取，如果依赖的属性是一个引用类型必定会触发该属性的BeanFactory.getBean 操作，从而从spring 容器中获取到对应的实例。方法的依赖注入类似这里就不再赘述，接着上面继续看下getResourceToInject()方法。
1.3.getResourceToInject(), autowireResource()
protected Object getResourceToInject(Object target, @Nullable String requestingBeanName) {
return (this.lazyLookup ? buildLazyResourceProxy(this, requestingBeanName) :
getResource(this, requestingBeanName));
}
protected Object getResource(LookupElement element, @Nullable String requestingBeanName)
throws NoSuchBeanDefinitionException {
if (StringUtils.hasLength(element.mappedName)) {
return this.jndiFactory.getBean(element.mappedName, element.lookupType);
}
if (this.alwaysUseJndiLookup) {
return this.jndiFactory.getBean(element.name, element.lookupType);
}
if (this.resourceFactory == null) {
throw new NoSuchBeanDefinitionException(element.lookupType,
"No resource factory configured - specify the 'resourceFactory' property");
}
//关注这个方法
return autowireResource(this.resourceFactory, element, requestingBeanName);
}
protected Object autowireResource(BeanFactory factory, LookupElement element, @Nullable String requestingBeanName) throws NoSuchBeanDefinitionException {
Object resource;
Set<String> autowiredBeanNames;
String name = element.name;
if (factory instanceof AutowireCapableBeanFactory) {
AutowireCapableBeanFactory beanFactory = (AutowireCapableBeanFactory) factory;
DependencyDescriptor descriptor = element.getDependencyDescriptor();
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");
}
}
else {
resource = beanFactory.resolveBeanByName(name, descriptor);
autowiredBeanNames = Collections.singleton(name);
}
}
else {
//重点在这里，通过getBean()方式获取依赖所需要的bean
resource = factory.getBean(name, element.lookupType);
autowiredBeanNames = Collections.singleton(name);
}
if (factory instanceof ConfigurableBeanFactory) {
ConfigurableBeanFactory beanFactory = (ConfigurableBeanFactory) factory;
for (String autowiredBeanName : autowiredBeanNames) {
if (requestingBeanName != null && beanFactory.containsBean(autowiredBeanName)) {
beanFactory.registerDependentBean(autowiredBeanName, requestingBeanName);
}
}
}
return resource;
}到此，populateBean()关于bean的依赖属性注入流程跟踪完毕，下一章，我们去跟一下bean的初始化流程，敬请期待哦！^_^
