本文将从源码的角度来分析SpringBean的创建过程
1. BeanDefinition
大家都知道，在Spring IOC启动时，会扫描类的信息，将相关信息封装为BeanDefinition存入到beanDefinitionMap中，后续用其中的描述信息来生成bean。
2. getBean
首先从AbstractBeanFactory类入手，因为它是IOC容器的顶层实现类，基本上都是用其中的getBean去获取bean的。
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
其中重写了四个getBean方法，仅仅只是传入的参数不同。其中都是调用的doGetBean方法。
在Spring中，方法命名都挺统一，真正干事儿的方法，基本都是以do开头，优秀的代码习惯值得学习！
3. doGetBean
以下是doGetBean方法的代码：
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
String beanName = transformedBeanName(name);
Object beanInstance;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
.tag("beanName", name);
try {
if (requiredType != null) {
beanCreation.tag("beanType", requiredType::toString);
}
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");
}
Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new ScopeNotActiveException(beanName, scopeName, ex);
}
}
}
catch (BeansException ex) {
beanCreation.tag("exception", ex.getClass().toString());
beanCreation.tag("message", String.valueOf(ex.getMessage()));
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
finally {
beanCreation.end();
}
}
return adaptBeanInstance(name, beanInstance, requiredType);
}
我们来一步一步的对代码进行解读：
1、获取去除了&符号的bean名称，再在aliasMap中寻找别名。
2、根据beanName从单例缓存中查找已经存在的对象。
3、如果单例缓存中存在bean，且传入的args为null，则按需打印日志，如果bean是FactoryBean，则返回getObject中的对象，否则返回此bean。
4、但对于单例缓存中不存在bean或者传入的args不为null的处理：
-
如果beanName是正在创建中的原型bean，则抛出错误。
-
由于
AbstractBeanFactory实现了HierarchicalBeanFactory，即父子级联 IOC容器的接口，所以可以对parentBeanFactory进行操作。 -
如果parentBeanFactory不为null，而且当前的beanFactory中不包含此beanName的BeanDefinition，那就去parentBeanFactory中去寻找此bean的定义信息。如果typeCheckOnly为false，则将bean标记为已经创建或者将要创建。
-
获取一个合并了的RootBeanDefinition，如果该bean是一个abstract类型的，就抛出错误。
-
获取当前bean中依赖的（dependsOn）bean，逐个检查，如果A依赖于B，且B又以来于A，那么就抛出BeanCreationException错误。保存Dependent信息，其中
dependenciesForBeanMap保存当前bean依赖于哪些bean，dependentBeanMap保存当前被依赖的bean被哪些bean所依赖。举个例子：A dependsOn B、C， D dependsOn B、E，那么dependenciesForBeanMap就会保存A -> B、C，D -> B、E，而dependentBeanMap中保存的是B -> A、D，C -> A，E -> D。接着调用getBean先获取依赖的bean。 -
对单例bean的处理
- 去单例缓存中找对应的bean。
- 如果没有找到就通过createBean进行创建bean。
- 创建的bean可能是FactoryBean，这时需要调用其getObject获取到bean。
-
对原型bean的处理
- 将当前beanName标记为正在创建中的原型bean，其实就是利用了一个变量名为prototypesCurrentlyInCreation的ThreadLocal。
- 通过createBean进行创建bean。
- 对beanName进行取消创建中的原型bean标记，就是从prototypesCurrentlyInCreation中移除。
- 创建的bean可能是FactoryBean，这时需要调用其getObject获取到bean。
-
对其他scope的处理
- 根据BeanDefinition保存的scope名称从scopes（一个Map）中获取到Scope。
- 调用scope的get方法，不同的Scope有不同的操作方式，但是创建从ObjectFactory中调用getObject来获取bean的途径都是一样的，同Prototype（原型）bean的处理。如SessionScope的处理方式是将beanName和从ObjectFactory取到的bean及范围标识符SCOPE_SESSION存入RequestAttributes中去，每次取时，都要先从RequestAttributes取beanName对应的bean，没有才创建，这就保证了在同一SESSION下bean的唯一性。
5、检查所需类型是否与实际bean实例的类型匹配，必要时进行转换。
为了更加了解每一个细节，我们来对doGetBean中部分方法来做一个详细的说明。
3.1 transformedBeanName
protected String transformedBeanName(String name) {
return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}
public String canonicalName(String name) {
String canonicalName = name;
// Handle aliasing...
String resolvedName;
do {
resolvedName = this.aliasMap.get(canonicalName);
if (resolvedName != null) {
canonicalName = resolvedName;
}
}
while (resolvedName != null);
return canonicalName;
}
// BeanFactoryUtils
public static String transformedBeanName(String name) {
Assert.notNull(name, "'name' must not be null");
if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
return name;
}
return transformedBeanNameCache.computeIfAbsent(name, beanName -> {
do {
beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
}
while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
return beanName;
});
}
这个方法用于获取规范的beanName。
BeanFactoryUtils.transformedBeanName中主要是去除beanName前面的&符号，如入参&&&serviceA&123，经过处理后会返回serviceA&123。并将&&&serviceA&123与serviceA&123的关系保存到transformedBeanNameCache这个Map中。
canonicalName中主要是从别名map中获取最基本的beanName，如果beanName为serviceA的bean具有别名alias1，而别名alias1又保存了一个别名alias2，那么就可以通过alias2得到serviceA。
3.2 getSingleton
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// Quick check for existing instance without full singleton lock
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
synchronized (this.singletonObjects) {
// Consistent creation of early reference within full singleton lock
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
}
}
return singletonObject;
}
这个方法是用于获取单例bean。
先从singletonObjects这个map取，如果还没有生成且这个bean被标记为了正在创建中，则尝试从earlySingletonObjects中取，如果这里面也没有且容器允许早期引用，则先锁住singletonObjects，防止多个线程造成singletonObjects中生成多个对象，从而违反了单例的初衷；再次进行判断（线程同步知识点），如果还是没有，就需要去生成bean了，这里是从singletonFactories中获取到这个bean的ObjectFactory，再调用其getObject方法去创建一个bean。创建完成后，将bean存入到earlySingletonObjects中去，并删除其ObjectFactory对象。
这里讲到的singletonObjects、earlySingletonObjects、singletonObjects就是三个Map，分别称之为第一级、第二级、第三级缓存。为什么要使用如此多的变量呢？这是为了解决Spring中的循环依赖问题，具体将在后续的文章中进行说明。
3.3 getObjectForBeanInstance
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
}
if (mbd != null) {
mbd.isFactoryBean = true;
}
return beanInstance;
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
if (!(beanInstance instanceof FactoryBean)) {
return beanInstance;
}
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
}
else {
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
从BeanInstance中得到Object，因为得到的beanInstance可能是一个FactoryBean，这时需要调用其getObject才能得到真正需要的bean实例。
BeanFactoryUtils.isFactoryDereference就是判断beanName是否以&开头，如果是，那就是要获取这个FactoryBean。如果bean实例不是一个FactoryBean，那么就是普通bean的获取，直接返回。
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
}
else {
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
如果传入的mbd为null，则先尝试从factoryBeanObjectCache缓存中取对象，没有的话再进行创建。如果mbd不为null，则直接进行创建。
创建的流程为：
- 如果mbd为null，且当前上下文中存在bean定义，则合并
BeanDefinition。 - 从
FactoryBean中生成bean，并将其放在factoryBeanObjectCache缓存中。
这里有两个主要的分支方法，继续解读。
3.3.1 getMergedLocalBeanDefinition
protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
// Quick check on the concurrent map first, with minimal locking.
RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
if (mbd != null && !mbd.stale) {
return mbd;
}
return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
}
首先从mergedBeanDefinitions缓存中取，如果能取到合并后的BeanDefinition，且为不可被合并的状态，则直接返回缓存对象。否则进入合并流程。
protected RootBeanDefinition getMergedBeanDefinition(
String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)
throws BeanDefinitionStoreException {
synchronized (this.mergedBeanDefinitions) {
RootBeanDefinition mbd = null;
RootBeanDefinition previous = null;
// Check with full lock now in order to enforce the same merged instance.
if (containingBd == null) {
mbd = this.mergedBeanDefinitions.get(beanName);
}
if (mbd == null || mbd.stale) {
previous = mbd;
if (bd.getParentName() == null) {
// Use copy of given root bean definition.
if (bd instanceof RootBeanDefinition) {
mbd = ((RootBeanDefinition) bd).cloneBeanDefinition();
}
else {
mbd = new RootBeanDefinition(bd);
}
}
else {
// Child bean definition: needs to be merged with parent.
BeanDefinition pbd;
try {
String parentBeanName = transformedBeanName(bd.getParentName());
if (!beanName.equals(parentBeanName)) {
pbd = getMergedBeanDefinition(parentBeanName);
}
else {
BeanFactory parent = getParentBeanFactory();
if (parent instanceof ConfigurableBeanFactory) {
pbd = ((ConfigurableBeanFactory) parent).getMergedBeanDefinition(parentBeanName);
}
else {
throw new NoSuchBeanDefinitionException(parentBeanName,
"Parent name '" + parentBeanName + "' is equal to bean name '" + beanName +
"': cannot be resolved without a ConfigurableBeanFactory parent");
}
}
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanDefinitionStoreException(bd.getResourceDescription(), beanName,
"Could not resolve parent bean definition '" + bd.getParentName() + "'", ex);
}
// Deep copy with overridden values.
mbd = new RootBeanDefinition(pbd);
mbd.overrideFrom(bd);
}
// Set default singleton scope, if not configured before.
if (!StringUtils.hasLength(mbd.getScope())) {
mbd.setScope(SCOPE_SINGLETON);
}
// A bean contained in a non-singleton bean cannot be a singleton itself.
// Let's correct this on the fly here, since this might be the result of
// parent-child merging for the outer bean, in which case the original inner bean
// definition will not have inherited the merged outer bean's singleton status.
if (containingBd != null && !containingBd.isSingleton() && mbd.isSingleton()) {
mbd.setScope(containingBd.getScope());
}
// Cache the merged bean definition for the time being
// (it might still get re-merged later on in order to pick up metadata changes)
if (containingBd == null && isCacheBeanMetadata()) {
this.mergedBeanDefinitions.put(beanName, mbd);
}
}
if (previous != null) {
copyRelevantMergedBeanDefinitionCaches(previous, mbd);
}
return mbd;
}
}
这个方法比较长，让我们来一步一步分析：
- 首先，这个方法有三个参数，
String beanName,BeanDefinition bd,BeanDefinition containingBd，分别为bean名称，beanName对应的BeanDefinition，beanName所包含的BeanDefinition。比如beanName对应的是一个内部bean，那么containingBd则是其外层bean的定义。 - 现在来对mbd进行赋值，如果bd的parentName为null，那就将bd包装成一个RootBeanDefinition，并赋值给mbd。如果parentName不为null，那就需要考虑到父子级联容器的处理了，判断此beanName对应的beanDefinition存在于当前容器中还是父容器中，并从对应的容器中的得到parentName进行merge之后的BeanDefinition，并包装成一个RootBeanDefinition，赋值给mbd。
- 给mbd的scope赋值，默认赋值
SCOPE_SINGLETON，如果外层bean定义不为null，且其scope不是单例，但是这个bean又是单例，则将mbd的scope设置为跟外层bean的scope一致。 - 将合并结果存入mergedBeanDefinitions缓存。
- 复制属性到mbd中。
3.3.2 getObjectFromFactoryBean
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
if (isSingletonCurrentlyInCreation(beanName)) {
// Temporarily return non-post-processed object, not storing it yet..
return object;
}
beforeSingletonCreation(beanName);
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
finally {
afterSingletonCreation(beanName);
}
}
if (containsSingleton(beanName)) {
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}
这里是从FactoryBean中获取到bean，主要有两个分支，一个是如果这个FactoryBean是单例的，且beanName对应的bean是存在于singletonObjects单例缓存中的，那就需要控制从FactoryBean的getObject方法生成的bean也是单例的；如果非单例，那么每次都是使用新生成的bean。
控制单例，最好的方法就是加缓存，这次生成过后，下次再来生成时优先使用缓存中已经存在的对象，没有才去考虑新生成对象。所以这里用了一个factoryBeanObjectCache的Map来作为getObject生成的bean的缓存。
这里生成的过程就是先从FactoryBean的getObject方法的到一个原始的对象，再根据传入的shouldPostProcess去判断是否应该执行BeanPostProcessor的postProcessAfterInitialization方法。这里在对bean执行Bean的后置处理器之前，需要将bean标记为正在创建中（将beanName放入singletonsCurrentlyInCreation的Set集合），执行完后撤销标记（在singletonsCurrentlyInCreation中移除beanName）。
3.4 createBean
这一步是创建bean的具体方法，也是至关重要的一步。
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isTraceEnabled()) {
logger.trace("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.
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.
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.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
这里时创建bean和初始化bean的方法，这里生成bean有两种途径，一种是通过InstantiationAwareBeanPostProcessor后置处理器生成的，另一种是常规通过反射来生成的。
接下来对两个主要的方法来进行解析：
3.4.1 resolveBeforeInstantiation
@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}
如果RootBeanDefinition的beforeInstantiationResolved属性为true，如果此bean定义不是合成的，且存在InstantiationAwareBeanPostProcessor类型的bean后置处理器，那么就调用InstantiationAwareBeanPostProcessor的postProcessBeforeInstantiation方法得到其返回的bean，如果有多个InstantiationAwareBeanPostProcessor，那么取第一个有返回的值。再调用beanPostProcessors这个Map中的BeanPostProcessor的postProcessAfterInitialization方法对bean进行处理。最后返回bean。
3.4.2 doCreateBean
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) {
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 {
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.
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");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
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<String> 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 " +
"'getBeanNamesForType' 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;
}
为了得到instanceWrapper， 首先从factoryBeanInstanceCache中找，看此beanName是否是一个单例的factoryBean，是则移除并取出，如果不是，则进入createBeanInstance去创建一个BeanWrapper。
创建BeanWrapper有多种方式，如工厂方法、构造函数自动装配或简单实例化，createBeanInstance中根据配置去使用不同的策略去创建。
然后是使用MergedBeanDefinitionPostProcessor这种类型的后置处理器去允许修改合并后的BeanDefinition。
接着如果允许bean早期曝光，那就将这个beanName和创建的ObjectFactory对象存入第三级缓存singletonFactories中。
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
这里() -> getEarlyBeanReference(beanName, mbd, bean)实际就是一个ObjectFactory对象。
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) {
exposedObject = bp.getEarlyBeanReference(exposedObject, beanName);
}
}
return exposedObject;
}
其中就是返回已经被new出来的对象，只是这里可以用SmartInstantiationAwareBeanPostProcessor这个后置处理器对new出来的对象进行处理。
接着populateBean，其中是对bean属性的赋值，包括@Autowired等自动注入的属性，有兴趣的可以参考文章《死磕Spring系列：@Autowired工作原理》。
接着initializeBean，对bean进行初始化，大概流程如下：
- 如果bean实现了
BeanNameAware、BeanClassLoaderAware、BeanFactoryAware接口，那么就会分别回调其实现的方法。 - 执行
BeanPostProcessor的postProcessBeforeInitialization方法。 - 如果bean实现了
InitializingBean接口，那么就会回调其afterPropertiesSet方法。如果指定了init方法，则用反射执行。 - 执行
BeanPostProcessor的postProcessAfterInitialization方法。
实例化，初始化完成之后，就需要对之前曝光的操作做处理了。
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<String> 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 " +
"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
此时去调用getSingleton，注意这里getSingleton(beanName, false)，第二个allowEarlyReference参数为false，那就只会从一二级缓存中查找数据，并不会从ObjectFactory中生成对象。
最开始创建A的时候，这里是肯定不能从二级缓存中获得到A bean的，那在什么情况下这里的二级缓存能取到A bean呢？
@Component
public class A {
@Autowired
private B b;
}
@Component
public class B {
@Autowired
private A a;
}
比如在这种循环依赖的场景下，A通过getSingleton去创建A Bean时，会将自身的ObjectFactory放入singletonFactories，到了赋值B的这一步，会去创建B bean，B到了赋值A时，会去调用getSingleton获取A bean，而这时，A会从singletonFactories中获取到早期对象放入earlySingletonObjects中去。A完成了属性b的赋值，就会走到当前这步，而这里的二级缓存earlySingletonObjects中就会有值。
由于现在的exposedObject是经过BeanPostProcessor的处理了的，所以现在的exposedObject和存在二级缓存earlySingletonObjects中的bean可能已经是两个不同的对象了，所以需要进行判断，如果前后两个对象还是还是同一个对象，那就把经过SmartInstantiationAwareBeanPostProcessor处理后的bean给exposedObject用。
如果原始的bean、经过BeanPostProcessor处理的bean、SmartInstantiationAwareBeanPostProcessor处理的bean这三者都不一致，但是这个Bean已经被其他Bean依赖了，但是又不知道到底是使用哪一个bean实例了，所以不得不抛出异常来终止程序运行。
最后注册DisposableBean，返回需要曝光的对象。
经过方法的处理，如果是单例模式，生成的对象已经存在于二级缓存earlySingletonObjects中了，那是什么时候放入到一级缓存中的呢？我们来继续分析。
3.5 getSingleton(beanName, objectFactory)
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
回到这段逻辑，我们进入到这里的getSingleton方法去看看：
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
可以看到，这里逻辑不复杂，就是先从一级缓存singletonObjects中查找，如果有则返回，没有则创建并放入一级缓存。
创建前需要将beanName放入到singletonsCurrentlyInCreation中，来表示这个单例bean正在创建，创建之后从这个Set中移除，表示不处于创建过程中了。
而真正创建的过程，就是从我们上面分析的ObjectFactory中生成的，在生成之后，会将得到的bean进行addSingleton处理。
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
可以得知，得到bean之后，会放入一级缓存singletonObjects，并清除二级缓存earlySingletonObjects，三级缓存singletonFactories中的缓存数据。
4、总结
简而言之，Spring Bean的创建过程就是：
先从缓存中取，如果取不到，就去创建，如果这个bean的定义是在父容器中的，那就去父容器中继续从头开始执行。
将beanName标记为alreadyCreated，将父子容器和外层bean的BeanDefinition进行合并，执行依赖bean的初始化流程，根据singleton、prototype、其他scope进行bean的实例化和初始化。
知识点：
-
可以在未进行bean的实例化之前就通过InstantiationAwareBeanPostProcessor直接返回一个bean。
-
可以在实例化bean之后，初始化之前，通过MergedBeanDefinitionPostProcessor对合并后的BeanDefinition进行处理。
-
在将bean放入二级缓存earlySingletonObjects之前，可以通过SmartInstantiationAwareBeanPostProcessor对生成的bean进行处理。
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