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下面通过createBean代码了解这个过程
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
//这里判断需要创建的Bean是否可以被实例化,这个类是否可以通过类加载器来载入
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
//如果Bean配置了PostProcessor,那么这里返回的是一个Proxy
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException ex) {
// A previously detected exception with proper bean creation context already...
throw ex;
}
catch (ImplicitlyAppearedSingletonException ex) {
// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
//接着到doCreate中去看看Bean是怎样生成的
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiate the bean.
//用来持有创建出来的Bean对象
BeanWrapper instanceWrapper = null;
//如果是单例,则先把缓存中的同名Bean清除
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
//这里是创建Bean的地方,由createBeanInstance来完成
if (instanceWrapper == null) {
//根据指定bean使用对应的策略创建新的实例,如:工厂方法,构造函数自动注入,简单初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle inters like BeanFactoryAware.
//是否需要提前曝光:单例&允许循环依赖&当前bean正在创建中,检测循环依赖
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");
}
//为避免后期循环依赖,可以在bean初始化完成前将创建实例的ObjectFactory加入工厂
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
//对bean再次依赖引用,主要应用SMartInstantialiationAware BeanPostProcessor,
//其中我们熟知的AOP就是在这里将advice动态织入bean中,若无则直接返回bean,不做任何处理
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
//这里是对Bean的初始化,依赖注入往往在这里发生,这个exposedObject在初始化处理完后悔返回作为依赖注入完成后的Bean
Object exposedObject = bean;
try {
//对bean进行填充,将各个属性值注入,其中可能存在依赖于其他bean的属性,则会递归初始化依赖bean
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
//调用初始化方法,比如init-method
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);
// earlySingletonReference 只有在检测到有循环依赖的情况下才会非空
if (earlySingletonReference != null) {
if (exposedObject == bean) {
//如果exposedObject 没有在初始化方法中被改变,也就是没有被增强
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
//检测依赖
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
//因为bean创建后其所依赖的bean一定是已经创建的,actualDependentBeans非空则表示当前bean创建后其依赖的bean却没有全部创建完,也就是说存在循环依赖
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 {
//根据scope注册bean
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
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依赖注入其实包括两个主要过程
生产Bea所包含的Java对象
Bean对象生成之后,把这些Bean对象的依赖关系设置好
我们从上可以看到与依赖注入关系特别密切的方法有
createBeanInstance
生成Bean包含的Java对象
populateBean.
处理对各种Bean对象的属性进行处理的过程（即依赖关系处理的过程）
