从BeanDefinition到Bean
当 BeanDefinition 注册完毕以后， Spring Bean 工厂就可以随时根据需要进行实例化了。对于 XmlBeanFactory 来说，实例化默认是延迟进行的，也就是说在 getBean 的时候才会；而对于 ApplicationContext 来说，实例化会在容器启动后通过 AbstractApplicationContext 中 reflash 方法自动进行，主要经过方法链： reflesh() à finishBeanFactoryInitialization (factory) à DefaultListableBeanFactory.preInstantiateSingletons (), 在这里会根据注册的 BeanDefinition 信息依此调用 getBean(beanName) 。而真正实例化的逻辑和 BeanFactory 是“殊途同归”的，所有有关 Bean 实例化都可以从 getBean(beanName) 入手。
AbstractBeanFactory 有四个 getBean 的重载方法，不管调用哪个方法最终都是会调用另一个 doGetBean 方法：
public Object getBean(String name) throws BeansException {
return getBean(name, null, null);
}
public Object getBean(String name, Class requiredType) throws BeansException {
return getBean(name, requiredType, null);
}
public Object getBean(String name, Object[] args) throws BeansException {
return getBean(name, null, args);
}
/**
* 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 if creating a prototype using explicit arguments to a
* static factory method. It is invalid to use a non-null args value in any other case.
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
public Object getBean(String name, Class requiredType, Object[] args) throws BeansException {
return doGetBean(name, requiredType, args, false);
}
doGetBean方法比较长一点，见下面注释：
protected Object doGetBean(
final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
//bean name处理，去除FactoryBean前缀等
final String beanName = transformedBeanName(name);
Object bean = null;
//先从singleton缓存中查看是否已经实例化过该Bean，根据是否有缓存分为两个分支分别处理
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// 分支一，若缓存中获取到了并且该BeanDefinition信息表明该bean是singleton的，直接将获取到的缓存Bean
//(有可能是半成品)交给getObjectForBeanInstance处理
/*.........省略logger部分代码............*/
//调用getObjectForBeanInstance处理
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}else {
// 分之二：没有缓存，则需要从头实例化该bean
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);}
// 检查BeanDefinition是否在当前工厂或父工厂
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// 父工厂getBean
return parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
//将bean加入“正在创建”的集合，完成后会remove,对应afterSingletonCreation/afterPrototypeCreation方法
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 解决依赖关系，将依赖的bean提前实例化
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (int i = 0; i < dependsOn.length; i++) {
String dependsOnBean = dependsOn[i];
getBean(dependsOnBean);
registerDependentBean(dependsOnBean, beanName);
}
}
// 这里又需要根据bean的类型分为三种情况：singleton、prototype、request/session
if (mbd.isSingleton()) {
//通过自定义ObjectFactory实例化Bean，此结果可能是半成品(是FactoryBean等)
sharedInstance = getSingleton(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
try {
//真正实例化装配的逻辑在createBean方法中
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;
}
}
});
//上一步半成品的Bean交给getObjectForBeanInstance方法处理
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
//真正实例化装配的逻辑在createBean方法中
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
//上一步半成品的Bean交给getObjectForBeanInstance方法处理
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
//request、session 的bean
String scopeName = mbd.getScope();
final Scope scope = (Scope) this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
//真正实例化装配的逻辑在createBean方法中
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
//上一步半成品的Bean交给getObjectForBeanInstance方法处理
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);
}
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null &&
!requiredType.isAssignableFrom(bean.getClass())) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return bean;
}
通过注释，可以整理出较清晰的逻辑： 检查Bean缓存，已经有缓存的Bean对象(有可能是半成品)则交给getObjectForBeanInstance方法处理；否则先根据Bean的生命周期类型分别实例化，每种情况大致都分两步,第一步都交给createBean方法生产一个半成品的bean对象，然后同样是将半成品的bean交给getObjectForBeanInstance方法处理。所以关键的逻辑就在这两个方法了，下面以singleton为例看看这两个方法，具体代码：
sharedInstance = getSingleton(beanName, new ObjectFactory() {
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);
getSingleton方法做的工作主要是实例化bean之前和之后回调beforeSingletonCreation/afterSingletonCreation、实例化bean、以及将bean对象缓存起来，具体实例化bean是通过回调匿名对象ObjectFactory的getObject方法实现的，从代码中明显看到主要是createBean方法。这里要特别注意下，缓存的bean对象是createBean生产的，这个方法生产的bean只是“半成品”，有可能是个factoryBean，真正返回给客户端使用的bean还必须进行下一步getObjectBeanInstance处理。所以缓存的bean对象可以认为是“半成品”，这就和前文代码中的注释相呼应(缓存中若取到了bean，必须进行getObjectBeanInstance处理)。
对createBean方法实际上在“Spring IOC之BeanFactory”中已经有过分析，所以流程就不再简单重复，直接按步骤：
1. resolveBeanClass(mbd, beanName);
2. mbd.prepareMethodOverrides();//Spring IOC之BeanFactory已有解释，不重复
3. Object bean = resolveBeforeInstantiation(beanName, mbd);
解释下，这里主要对于一些代理的bean做处理，我们平常经常会配置一些AOP模块，对于需要aop增强的bean实际上都会经过spring代理织入，而这些bean的代理逻辑应该就在这里处理了，具体看看这部分的代码：
/**
* 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</code> 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.hasBeanClass() && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
bean = applyBeanPostProcessorsBeforeInstantiation(mbd.getBeanClass(), beanName);
if (bean != null) {
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
mbd.beforeInstantiationResolved = Boolean.valueOf(bean != null);
}
return bean;
}
/**
* Apply InstantiationAwareBeanPostProcessors to the specified bean definition
* (by class and name), invoking their <code>postProcessBeforeInstantiation</code> methods.
* <p>Any returned object will be used as the bean instead of actually instantiating
* the target bean. A <code>null</code> 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</code>
* @throws BeansException if any post-processing failed
* @see InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation
*/
protected Object applyBeanPostProcessorsBeforeInstantiation(Class beanClass, String beanName)
throws BeansException {
for (Iterator it = getBeanPostProcessors().iterator(); it.hasNext();) {
BeanPostProcessor beanProcessor = (BeanPostProcessor) it.next();
if (beanProcessor instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) beanProcessor;
Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
if (result != null) {
return result;
}
}
}
return null;
}
//AbstractAutoProxyCreator
public Object postProcessBeforeInstantiation(Class beanClass, String beanName) throws BeansException {
Object cacheKey = getCacheKey(beanClass, beanName);
if (!this.targetSourcedBeans.contains(cacheKey)) {
if (this.advisedBeans.contains(cacheKey) || this.nonAdvisedBeans.contains(cacheKey)) {
return null;
}
if (isInfrastructureClass(beanClass, beanName) || shouldSkip(beanClass, beanName)) {
this.nonAdvisedBeans.add(cacheKey);
return null;
}
}
// Create proxy here if we have a custom TargetSource.
// Suppresses unnecessary default instantiation of the target bean:
// The TargetSource will handle target instances in a custom fashion.
TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
if (targetSource != null) {
this.targetSourcedBeans.add(beanName);
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
return null;
}
对于AutoProxy部分代码要放到spring aop部分中去写了，这里只要明白如果是代理bean在postProcessBeforeInstantiation阶段会返回一个proxy，这时候bean已经算是实例化好了，再调用applyBeanPostProcessorsAfterInitialization即BeanPostProcessor.postProcessAfterInitialization回调进行属性的设置。最后的结果就可以返回bean对象了，只是一个Proxy Bean Object.
4. Object beanInstance = doCreateBean(beanName, mbd, args);//如果是代理bean就不会走到这一步
解释下：对于非代理的bean，基本上实例化的逻辑就在doCreateBean这个方法了，这个方法在“Spring IOC之BeanFactory”中已经有过分析，所以不重复解释了，贴一个bean生命周期的流程图做为createBean方法的结尾：
上图不是我所画，来源于spring IOC容器介绍。
半成品出来之后还需要调用getObjectForBeanInstance进一步处理，这个方法：
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// 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) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
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;
}
开始部分是逻辑检查，是否是FactoryBean,后面是从factoryBean中获取实际的Bean。获取bean也是先检查缓存，没有缓存再getObjectFromFactoryBean获取，详细再doGetObjectFromFactoryBean方法中，最后将获取的bean缓存起来。代码就不贴了。