在前面的文章Spring源码之IoC容器的Bean创建和依赖注入，DefaultListableBeanFactory容器为例中提到获取Bean的过程。
通过getBean方法调用doGetBean方法，如果从缓存中获得Bean，通过getObjectForBeanInstance方法完成FactoryBean的相关处理，取得FactoryBean的处理结果，也就是最终的Bean。
那么getObjectForBeanInstance方法做了哪些操作，一起看看
1、如果不是FactoryBean的调用（Bean的name如果以“&”开头，说明是想获得FactoryBean本身，或者缓存中获取到的Bean实例不是FactoryBean的子类），则结束处理返回beanInstance
2、以上说明getBean不是为了获取FactoryBean本身，并且缓存中获取到的Bean实例是FactoryBean的子类，这时候通过getObjectFromFactoryBean从FactoryBean中得到Bean
3、在doGetObjectFromFactoryBean方法中，调用factory的getObject方法来从FactoryBean中得到Bean
4、这里返回的是作为工厂的FactoryBean的生产的产品，而不是FactoryBean本身。
beanInstance如果实现了FactoryBean接口，那么根据该Bean的名称获取到的实际上是getObject()返回的对象，而不是这个Bean自身实例，如果要获取这个Bean自身实例，那么需要在名称前面加上’&'符号。
getObjectForBeanInstance方法
protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
} else if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
} else {
if (mbd != null) {
mbd.isFactoryBean = true;
}
return beanInstance;
}
} else if (!(beanInstance instanceof FactoryBean)) {
return beanInstance;
} else {
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
} else {
object = this.getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
FactoryBean<?> factory = (FactoryBean)beanInstance;
if (mbd == null && this.containsBeanDefinition(beanName)) {
mbd = this.getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = mbd != null && mbd.isSynthetic();
object = this.getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
}
getObjectFromFactoryBean方法
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && this.containsSingleton(beanName)) {
synchronized(this.getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
object = this.doGetObjectFromFactoryBean(factory, beanName);
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
} else {
if (shouldPostProcess) {
if (this.isSingletonCurrentlyInCreation(beanName)) {
return object;
}
this.beforeSingletonCreation(beanName);
try {
object = this.postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable var14) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's singleton object failed", var14);
} finally {
this.afterSingletonCreation(beanName);
}
}
if (this.containsSingleton(beanName)) {
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
} else {
Object object = this.doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = this.postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable var17) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", var17);
}
}
return object;
}
}
doGetObjectFromFactoryBean方法
private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, String beanName) throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = this.getAccessControlContext();
try {
object = AccessController.doPrivileged(factory::getObject, acc);
} catch (PrivilegedActionException var6) {
throw var6.getException();
}
} else {
object = factory.getObject();
}
} catch (FactoryBeanNotInitializedException var7) {
throw new BeanCurrentlyInCreationException(beanName, var7.toString());
} catch (Throwable var8) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", var8);
}
if (object == null) {
if (this.isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}