前言
前几篇文章已经说明Spring对BeanDefinition的解析与维护，在beans->bean标签定义下的bean 默认会通过 org.springframework.beans.factory.support.DefaultListableBeanFactory.registerBeanDefinition(String beanName, BeanDefinition beanDefinition)方法注册到BeanFactory中，我们到目前为止，在BeanDefinition解析的链条中并没有发现有实例化Bean的行为，毕竟我们使用Bean时可以简单到使用getBean方法就能拿到一个实例化对象，那么Bean是何时被初始化的？带着这个疑问继续分析Spring源码，找到Bean初始化的真相。
源码
按照前几文的逻辑链，beanDefinition是在刷新ApplicationContext时注册进来的，那么回到org.springframework.context.support.AbstractApplicationContext.refresh() 方法
*下边有两个接口，如果不清楚作用和意义的话，可能影响阅读，可以看道某友的博文《Spring的BeanFactoryPostProcessor和BeanPostProcessor》 写的不错
org.springframework.beans.factory.config.BeanFactoryPostProcessor
org.springframework.beans.factory.config.BeanPostProcessor
借用道友的图，和部分文字（话说我之前怎么想不到这么说明问题呢……） 
“而BeanPostProcessor的执行，取决于配置文件中bean的定义，如果定义的bean是singleton并且不是抽象类，也不延迟初始化，则BeanPostProcessor是在第11步中执行；而对于prototype的bean，BeanPostProcessor是在程序getBean的时候执行的。”
singleton 非延迟加载（non-lazy-init）
按文字说明来到第11步finishBeanFactoryInitialization(beanFactory)
其源码如下，其他都不重要，主要看beanFactory.preInstantiateSingletons(); 这一行
/**
* Finish the initialization of this context's bean factory,
* initializing all remaining singleton beans.
*/
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(new StringValueResolver() {
@Override
public String resolveStringValue(String strVal) {
return getEnvironment().resolvePlaceholders(strVal);
}
});
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
// *重要 看这里
beanFactory.preInstantiateSingletons();
}
最后一行在做了各种准备之后，调用beanFactory.preInstantiateSingletons();方法“Instantiate all remaining (non-lazy-init) singletons.”我们谷歌一下，其人话为“实例化所有剩下的（非懒惰init）单例。”这就很好理解了，所有非延迟加载的单例bean都在这个方法中实例化，那么是如何实例化的呢?ctrl+alt+b 试一下。正好直接看到这个方法实现只有一个，就是那位老朋友org.springframework.beans.factory.support.DefaultListableBeanFactory这个类，下面分析一下这个类
@Override
public void preInstantiateSingletons() throws BeansException {
if (this.logger.isDebugEnabled()) { //日志（这是一种好的日志处理方式，当没有开启debug级别时，就会省下一次字符串拼接），spring源码里到处都是这样的处理，可以学习。
this.logger.debug("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
//拷贝一个新的list ，this.beanDefinitionNames 干嘛的，上文大概已有说明，可以结合《Spring 源码阅读 BeanFactory(二) 之registerBeanDefinition方法》一块看
List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) { //遍历beanName
//这里先解释一下getMergedLocalBeanDefinition方法的含义，因为这个方法会常常看到。Bean定义公共的抽象类是AbstractBeanDefinition，普通的Bean在Spring加载Bean定义的时候，实例化出来的是GenericBeanDefinition，而Spring上下文包括实例化所有Bean用的AbstractBeanDefinition是RootBeanDefinition，这时候就使用getMergedLocalBeanDefinition方法做了一次转化，将非RootBeanDefinition转换为RootBeanDefinition以供后续操作。
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {//非抽象的，单例的，非懒加载的
if (isFactoryBean(beanName)) { //Spring中有两种类型的Bean，一种是普通Bean，另一种是工厂Bean，即FactoryBean, 工厂Bean 再起炉灶
final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
@Override
public Boolean run() {
return ((SmartFactoryBean<?>) factory).isEagerInit();
}
}, getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
else { //非工厂Bean 调用该方法
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
// 暂时不重要，MARK
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
smartSingleton.afterSingletonsInstantiated();
return null;
}
}, getAccessControlContext());
}
else {
//MARK
smartSingleton.afterSingletonsInstantiated();
}
}
}
}这里主要的就是getBean（beanName）了。
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}getBean方法内部调用了一个doGetBean方法，其源码如下，简单机翻一下其文档注释Return an instance, which may be shared or independent, of the specified bean. “返回指定bean的一个可能是共享的或独立的实例。”
“`
/**
* Return an instance, which may be shared or independent, of the specified bean.
* @param name the name of the bean to retrieve
* @param requiredType the required type of the bean to retrieve
* @param args arguments to use when creating a bean instance using explicit arguments
* (only applied when creating a new instance as opposed to retrieving an existing one)
* @param typeCheckOnly whether the instance is obtained for a type check,
* not for actual use
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
@SuppressWarnings(“unchecked”)
protected T doGetBean(
final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);//是对你传入的bean的name的转换，代码内部用到了BeanFactoryUtils.transformedBeanName(name)方法，该方法主要是去掉以String FACTORY_BEAN_PREFIX = "&";开头的bean的name，截取&后面的部分，最后会查看private final Map<String, String> aliasMap = new ConcurrentHashMap<String, String>(16);别名map中是否已经存在这个bean name 存在则沿用以前的bean name
Object bean;
// Eagerly check singleton cache for manually registered singletons.
/*
尝试检查缓存中是否存在实例,方法内调用 org.springframework.beans.factory.support.DefaultSingletonBeanRegistry
protected Object getSingleton(String beanName,boolean allowEarlyReference)
allowEarlyReference 传值为true
*/
//
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) { //日志处理 continue
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = 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 (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final 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);
getBean(dep);
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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;
}
}
});
bean = 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);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final 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, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}