在Spring bean加載過程中,每次bean實例在返回前都會調用getObjectForBeanInstance來處理FactoryBean的情況.
這邊的FactoryBean,Spring設計用於新建復雜bean的,聯想下GOF設計模式的創建型,一樣的為了解決復雜的bean實例化過程.
其實這邊的FactoryBean就是一個factory method[gof定義的意圖:定義一個用於創建對象的接口,讓子類決定實例化哪個類.Factory Method使一個類的實例化延遲到其子類].
這邊的處理主要涉及3個方法,我們來看看3個方法各自的職責:
AbstractBeanFactory的getObjectForBeanInstance處理.
FactoryBeanRegistrySupport的getObjectFromFactoryBean處理
FactoryBeanRegistrySupport負責FactoryBean相關的操作,並緩存FactoryBean的getObject實例化的bean.
FactoryBeanRegistrySupport的doGetObjectFromFactoryBean處理
這邊做的事很少,就是調用factoryBean的getObject,然後如果實例化得到的對象為空拋異常.
AbstractBeanFactory
/** * Get the object for the given bean instance, either the bean * instance itself or its created object in case of a FactoryBean. * 返回bean 實例的對象,可能是實例本身,也可能是FactoryBean新建的對象 * @param beanInstance the shared bean instance * @param name name that may include factory dereference prefix * @param beanName the canonical bean name * @param mbd the merged bean definition * @return the object to expose for the bean */
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// 如果想要獲取FactoryBean本身,那麼beanInstance必須是FactoryBean的實例
// 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());
}
// 如果instance不是FactoryBean實例,或者想要獲取的就是FactoryBean實例,那麼直接返回就好
// 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) {
// 獲取緩存的實例
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// 緩存中沒有對象,那麼從頭准備bean defition實例化一個
// 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;
}FactoryBeanRegistrySupport
/** * Obtain an object to expose from the given FactoryBean, if available * in cached form. Quick check for minimal synchronization. * 獲取緩存的,通過FactoryBean暴露出來的對象 * @param beanName the name of the bean * @return the object obtained from the FactoryBean, * or {@code null} if not available */
protected Object getCachedObjectForFactoryBean(String beanName) {
Object object = this.factoryBeanObjectCache.get(beanName);
// 類似<重構>中Introduce NUll Object,只是這邊的NULL_OBJECT是Object類型的,沒有解決Martin說的"不需要詢問對象類型,就可以直接調用行為的方法"
return (object != NULL_OBJECT ? object : null);
}
/** * Obtain an object to expose from the given FactoryBean. * @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) {
// factory是單例的同時,還得已經被實例化(感覺這個檢驗怪怪的,沒想明白)
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
// 緩存中找不到,就自己干吧,實例化
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 (object != null && shouldPostProcess) {
try {
// 外面BeanPostProcessor作用在factory上,沒有作用在實際想要的實例上,這邊補一個
// 也就是說,BeanPostProcessor的postProcessBeforeInitialization不會作用在FactoryBean上
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
}
// 緩存
this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
}
}
return (object != NULL_OBJECT ? object : null);
}
}
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (object != null && shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}
/** * Obtain an object to expose from the given FactoryBean. * @param factory the FactoryBean instance * @param beanName the name of the bean * @return the object obtained from the FactoryBean * @throws BeanCreationException if FactoryBean object creation failed * @see org.springframework.beans.factory.FactoryBean#getObject() */
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 {
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;
}
/** * Post-process the given object that has been obtained from the FactoryBean. * The resulting object will get exposed for bean references. * <p>The default implementation simply returns the given object as-is. * Subclasses may override this, for example, to apply post-processors. * @param object the object obtained from the FactoryBean. * @param beanName the name of the bean * @return the object to expose * @throws org.springframework.beans.BeansException if any post-processing failed */
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) throws BeansException {
return object;
}AbstractAutowireCapableBeanFactory
/** * Applies the {@code postProcessAfterInitialization} callback of all * registered BeanPostProcessors, giving them a chance to post-process the * object obtained from FactoryBeans (for example, to auto-proxy them). * @see #applyBeanPostProcessorsAfterInitialization */
@Override
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
return applyBeanPostProcessorsAfterInitialization(object, beanName);
}
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessAfterInitialization(result, beanName);
if (result == null) {
return result;
}
}
return result;
}
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