一次性讲清楚spring中bean的生命周期之二：FactoryBean的前世今生

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前言

在《spring中FactoryBean是什么bean》一文中，带着小伙伴学习了spring中的FactoryBean，了解了到了FactoryBean其实是一种生产Bean的bean，也就是FactroyBean的前世是Bean，今生还是Bean，小伙伴要疑惑了都是Bean，但是此Bean非彼Bean。今天带着小伙伴从源码的角度来分析下FactoryBean，重点是getObjectForBeanInstance方法的分析。

前世

在前面说到FactoryBean的前世是一个Bean，是指是一个FactoryBean的实例。先来看下getObjectForBeanInstance方法，

/**
* Get the object for the given bean instance, either the bean
* instance itself or its created object in case of a FactoryBean.
*/
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) { // Don't let calling code try to dereference the factory if the bean isn't a factory.
//1、判断name是否以&开头
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(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.
//beanInstance不是FactoryBean的实例或name以&开头
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
} Object object = null;
//mbd即beanDefinition为空，从缓存中取
if (mbd == null) {
object = getCachedObjectForFactoryBean(beanName);
}
//缓存中没有，则调用FactoryBean的getObject方法，返回其对象
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;
}

看该方法上的注释，

Get the object for the given bean instance, either the bean instance itself or its created object in case of a FactoryBean.

用我蹩脚的英语翻译过来大概是这个意思，

返回给定的bean Instance的一个对象，该对象可能是bean instance或者是由bean instance（是一个FactoryBean）创建的一个对象。

意思很明白了，该方法有可能返回的是一个FactoryBean的实例，也可能是由FactroyBean生产的实例，关键看方法参数中的前两个，

Object beanInstance spring容器中的一个bean

String name 可能含有&前缀的名称

String beanName bean的规范名称

RootBeanDefintion mbd BeanDefinition

看下面的表格更容易理解该方法在各种情况下的返回值，

beanInstance	name	返回值
FactoryBean的实例	带有&	beanInstance
不是FactoryBean的实例	带有&	beanInstance
FactoryBean的实例	不带&	beanInstance生产的对象
不是FactoryBean的实例	不带&	beanInstance

通过上面得表格再结合代码就很容易理解，只要是返回beanInstance对象，那么就是FactroyBean的前世，下面看FactoryBean的今生。

今生

这里有两个方法需要分析，分别是getCachedObjectForFactoryBean和getObjectFromFactoryBean。第一个方法是从缓存中获取，也就是说使用FactoryBean生产的bean会单独放在缓存中，非singletonObjects中，这点务必要注意。

getCachedObjectForFactoryBean

先看下该方法的定义，

@Nullable
protected Object getCachedObjectForFactoryBean(String beanName) {
return this.factoryBeanObjectCache.get(beanName);
}

可以看到很简单就是通过名称从factoryBeanObjectCache中取对象。factoryBeanObjectCache肯定是个map了

/** Cache of singleton objects created by FactoryBeans: FactoryBean name to object. */
private final Map<String, Object> factoryBeanObjectCache = new ConcurrentHashMap<>(16);

getObjectFromFactoryBean

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
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)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
if (isSingletonCurrentlyInCreation(beanName)) {
// Temporarily return non-post-processed object, not storing it yet..
return object;
}
beforeSingletonCreation(beanName);
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
finally {
afterSingletonCreation(beanName);
}
}
//把生产的实例对象放到factoryBeanObjectCache缓存中
if (containsSingleton(beanName)) {
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
else {
//调用getObject方法
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}

该方法就比较复杂了，主要有doGetObjectFromFactoryBean、beforeSingletonCreation、postProcessObjectFromFactoryBean、afterSingletonCreation方法，重要的一个是doGetFromFactroyBean，也就是真正干活生产bean的方法。其定义如下，其余方法可自行查看

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((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//调用FactoryBean中的getObject方法，返回其实例对象
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) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}