如果缓存中没有单例Bean的缓存，则需要从头开始创建单例Bean，这主要是重载getSingleton的重载方法来实现单例Bean的加载。
getSingleton方法
3. 获取单例
/**
* Return the (raw) singleton object registered under the given name,
* creating and registering a new one if none registered yet.
* @param beanName the name of the bean
* @param singletonFactory the ObjectFactory to lazily create the singleton
* with, if necessary
* @return the registered singleton object
*/
public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
Assert.notNull(beanName, "'beanName' 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 != NULL_OBJECT ? singletonObject : null);
}
}
前后处理方法：
3.1
/**
* Callback before singleton creation.
* The default implementation register the singleton as currently in creation.
* @param beanName the name of the singleton about to be created
* @see #isSingletonCurrentlyInCreation
*/
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
3.2
/**
* Callback after singleton creation.
*
The default implementation marks the singleton as not in creation anymore.
* @param beanName the name of the singleton that has been created
* @see #isSingletonCurrentlyInCreation
*/
protected void afterSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
}
}
3.3
/**
* Add the given singleton object to the singleton cache of this factory.
*
To be called for eager registration of singletons.
* @param beanName the name of the bean
* @param singletonObject the singleton object
*/
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
3.4 从3返回
sharedInstances = getSingleton(String beanName, ObjectFactory singletonFactory)
这个方法只是做一些准备以及处理操作，而真正的获取单例bean的方法其实并不是在此方法中实现的，其实现逻辑是在ObjectFactory类型的实例singletonFactory中实现的。
准备及处理操作如下：
首先获取SingletonObjects的对象锁，保证单例的全局唯一性。
检查缓存是否已经加载过。
若没有加载，则记录beanName的正在加载状态
-
加载单例前记录加载状态
beforeSingletonCreation()方法用于记录加载的状态：
调用this.singletonsCurrentlyInCreation.add(beanName)将当前正要创建的bean记录在缓存中，这样便可以对循环依赖进行检测啦 -
通过调用参数传入的ObjectFactory的getObject方法实例化bean
singletonObject = singletonFactory.getObject(); -
加载单例后的处理方法调用
当bean加载结束后需要移除缓存中对该bean的正在加载状态的记录： afterSingletonCreation()方法调用
-
将结果记录至缓存并删除加载bean过程中所有记录的各种辅助状态
调用addSingleton()方法
-
返回处理结果
上述是加载bean的逻辑框架，现在为止还没有对bean加载功能的探索，其实bean的加载逻辑是在传入的ObjectFactory类型的参数singletonFactory中定义的，而ObjectFactory的核心代码只是调用了createBean的方法，接下来就去探究一下这个方法的奥妙。
createBean方法详解
准备创建bean（createBean方法详解）
beans.factory.support.AbstractAutowireCapableBeanFactory
//---------------------------------------------------------------------
// Implementation of relevant AbstractBeanFactory template methods
//---------------------------------------------------------------------
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* @see #doCreateBean
*/
@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.
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);
}
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
createBean方法主要步骤：
根据设置的class属性或者根据className来解析Class。
对override属性进行标记以及验证（是针对Spring配置中lookup-method、replace-method bean的子标签进行处理，因为这两个子标签的加载过程就是将配置统一存放在BeanDefinition中的methodOverride属性里）。
应用实例前的后处理器，给后处理器一个返回代理而不是目标bean实例的机会，如果返回的代理类不为空则直接返回，而不会进行下面的创建bean的过程。
创建bean（doCreateBean方法）。这又是一个比较重要而且复杂的过程，需要仔细分析。
下面重点讲解步骤2、3、4。
步骤2- 处理override属性
用于处理配置中的lookup-method以及replace-method属性。
主要完成的工作：
- 标记
如果一个类中存在若干个重载方法，方法调用以及增强的时候还需要根据参数类型进行匹配，来最终确定当前调用的是哪个方法，Spring在这里将匹配的工作在这里完成匹配的，这样在后续调用的时候变可以直接是我用找到的方法，而不需要进行方法的参数匹配验证了
- 验证
在标记的过程中可以对方法存在性进行验证，一箭双雕。
4.1 处理override属性
beans.factory.support.AbstractBeanFactory(XmlBeanFactory 继承自这个类，拥有这个方法)
/**
* Validate and prepare the method overrides defined for this bean.
* Checks for existence of a method with the specified name.
* @throws BeanDefinitionValidationException in case of validation failure
*/
public void prepareMethodOverrides() throws BeanDefinitionValidationException {
// Check that lookup methods exists.
MethodOverrides methodOverrides = getMethodOverrides();
if (!methodOverrides.isEmpty()) {
Set overrides = methodOverrides.getOverrides();
synchronized (overrides) {
for (MethodOverride mo : overrides) {
prepareMethodOverride(mo);
}
}
}
}
/**
* Validate and prepare the given method override.
* Checks for existence of a method with the specified name,
* marking it as not overloaded if none found.
* @param mo the MethodOverride object to validate
* @throws BeanDefinitionValidationException in case of validation failure
*/
protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
if (count == 0) {
throw new BeanDefinitionValidationException(
"Invalid method override: no method with name '" + mo.getMethodName() +
"' on class [" + getBeanClassName() + "]");
}
else if (count == 1) {
// Mark override as not overloaded, to avoid the overhead of arg type checking.
mo.setOverloaded(false);
}
}
步骤3- 实例化前的的后处理
doCreateBean()方法之前调用了resolveBeforeInstantiation方法对BeanDefinition中的属性进行前置处理。
调用实例化前处理器进行处理。
短路判断：如果前置处理返回的结果不为空，则直接略过后续Bean的创建而直接返回结果，这一特性至关重要，AOP功能就是基于这里的判断的，如果不为空则调用初始化后处理器。
实例化的前置处理
beans.factory.support.AbstractAutowireCapableBeanFactory
/**
* Apply before-instantiation post-processors, resolving whether there is a
* before-instantiation shortcut for the specified bean.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @return the shortcut-determined bean instance, or {@code null} if none
*/
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;
}
4.2.1 初始化前的后处理器
/**
* Apply InstantiationAwareBeanPostProcessors to the specified bean definition
* (by class and name), invoking their {@code postProcessBeforeInstantiation} methods.
* Any returned object will be used as the bean instead of actually instantiating
* the target bean. A {@code null} return value from the post-processor will
* result in the target bean being instantiated.
* @param beanClass the class of the bean to be instantiated
* @param beanName the name of the bean
* @return the bean object to use instead of a default instance of the target bean, or {@code null}
* @see InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation
*/
protected Object applyBeanPostProcessorsBeforeInstantiation(Class beanClass, String beanName) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
if (result != null) {
return result;
}
}
}
return null;
}
4.2.2 实例化后的后处理器
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessAfterInitialization(result, beanName);
if (result == null) {
return result;
}
}
return result;
}
实例化前的后处理器的应用
将AbstractBeanDefinition转换为BeanWrapper前的处理，给子类一个修改BeanDefinition的机会，也就是说当程序经过这个方法之后，bean可能已经不是我们认为的bean了，而是或许成为一个经过处理的代理bean，可能是通过cglib生成的，也可能是通过其他技术生成的，会在AOP中进行讲解，需要清楚的是在bean的实例化前会调用后处理器的方法进行处理。
初始化后的后处理器
Spring保证bean初始化后尽可能将注册的后处理器postProcessAfterInitialization方法应用到该bean中。
步骤4- doCreateBean方法详解
见Spring-IOC-循环依赖检测与Bean的创建