说道FactoryBean，不少人会拿它跟BeanFactory作比较，但是实际上他们没有多大关系；我们简单介绍一下两者
一、BeanFactory和FactoryBean区别
BeanFactory
BeanFactory：这就是一个Factory，是一个IOC容器或者叫对象工厂，它里面存着很多的bean。例如默认的实现方式DefaultListableBeanFactory；我们把IOC容器可以比作一个水桶，IOC容器里面的所有bean就是装的水；
FactoryBean
FactoryBean：是一个Java Bean，但是它是一个能生产对象的工厂Bean，把IOC容器比作水桶，那么Java Bean就是水桶里面的水，但是这个FactoryBean一种比较特殊的水，可以把它看成是一个水球，这个水球里面也包含了水，我们可以通过IOC取这个水球，但是也可以直接取水球里面的水;(用法就是 用&表示取水球，取水球里的水用正常的获取方式就行了；如果我们用&获取到了水球之后，可以通过这个水球的 getObject方法获取水球里面的水;)
在之前的文章中我们已经分析过BeanFactory的源码了，今天我们单独分析FactoryBean
代码入口
public static void main(String[] args){
ClassPathResource resource = new ClassPathResource("SpringContextConfig.xml");
DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
reader.loadBeanDefinitions(resource);
MyFactoryBean beanA = (MyFactoryBean)factory.getBean("&fb");
TestFb beanB = (TestFb)factory.getBean("fb");
TestFb beanC =beanA.getObject();
System.out.print(beanB==beanC);
System.out.print(beanA);
}AbstractBeanFactory
AbstractBeanFactory extends FactoryBeanRegistrySupport
断点进去最后访问到AbstractBeanFactory的getObjectForBeanInstance方法；方法中的几个参数分别表示
beanInstance:
通过Object sharedInstance = getSingleton(beanName);获取到的，这个就是实例化的对象，就算用我们getBean的时候beanName传的是fb，不是&fb，这里返回的就是FactoryBean的实例对象;就是上面比喻的水球;
name:就是传进来的name ；没有过滤&字符的；
beanName:过滤了&字符的
/**专门缓存从FactoryBeans中生成的对象的；除了第一次要getObject()，后面直接从这个mao中取就行了 Cache of singleton objects created by FactoryBeans: FactoryBean name --> object */
private final Map<String, Object> factoryBeanObjectCache = new ConcurrentHashMap<String, Object>(16);
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// 如果name 是&开头的，但是beanInstance又不是FactoryBean类型的就抛出异常
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
//如果不是FactoryBean 例如name=fb；直接返回
//如果name是&开头并且FactoryBean类型的例如name=&fb,fb也是FactoryBean类型； 直接返回
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
// beanInstance是FactoryBean类型的并且name不是是以&开头的;例如name=fb 就要走下面的流程
//beanInstance不是FactoryBean类型并且name
Object object = null;
if (mbd == null) {
//检查之前是否已经生成放入缓存了，如果有 直接返回
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// 获取合并之后的BeanDefinition
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
//TODO....暂时不知道这里是怎么设置的，回头分析
boolean synthetic = (mbd != null && mbd.isSynthetic());
/** *1.返回FactoryBean.getObject()对象 *2.执行后置处理器接口的方法BeanPostProcessor中的 *postProcessAfterInitialization(result, beanName);方法； */
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
FactoryBeanRegistrySupport
public abstract class FactoryBeanRegistrySupport extends DefaultSingletonBeanRegistry{
/** * 从FactoryBean中获取Bean * @param factory the FactoryBean instance * @param beanName the name of the bean * @param shouldPostProcess whether the bean is subject to post-processing * @return the object obtained from the FactoryBean * @throws BeanCreationException if FactoryBean object creation failed * @see org.springframework.beans.factory.FactoryBean#getObject() */
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
//判断factory是单例的并且beanName已经被创建
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
//如果已经创建过直接返回
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)
//TODO....这里回头分析...
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (object != null && shouldPostProcess) {
try {
//执行所有后置处理器接口的方法BeanPostProcessor中的postProcessAfterInitialization(result, beanName);方法；
//这里单独写一篇文章将后置处理器BeanPostProcessor
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
}
//将通过factoryBean.getObject方法得到的对象存到 factoryBeanObjectCache中 this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
}
}
return (object != NULL_OBJECT ? object : null);
}
}
else {//如果不是单例对象
//调用FactoryBean的getObject方法返回实例，也就是从水球里面取出水；
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (object != null && shouldPostProcess) {
try {
//执行后置处理器
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}
//调用FactoryBean的getObject放法
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
@Override
public Object run() throws Exception {
return factory.getObject();
}
}, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//可以看到最终调用了getObject方法；这是需要实现类自己实现
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null && isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
return object;
}
二、总结
总结一下整个FactoryBean源码的流程
1.判断是否需要调用FactoryBean的getObject方法来获取实例
判断依据是:①.如果name有前缀&，直接返回
②.如果没有前缀&，调用getObject方法获取实例
2.如果是通过调用GetObject方法获取实例，则还要执行一下后置处理器
执行后置处理器接口的方法BeanPostProcessor中的
postProcessAfterInitialization(result, beanName);方法；
三、分析过程有一些需要单独分析，占个坑
1. mbd.isSynthetic()中的值是什么时候设置的？有什么用？
TODO…
2.BeanPostProcessor后置处理器讲解
3.上面Object alreadyThere = this.factoryBeanObjectCache.get(beanName);这段代码的意思？什么情况下要先查这个？
TODO…..
