Spring bean加载2--FactoryBean情况处理

Spring bean加载2--FactoryBean情况处理

在Spring bean加载过程中,每次bean实例在返回前都会调用getObjectForBeanInstance来处理FactoryBean的情况.

这边的FactoryBean,Spring设计用于新建复杂bean的,联想下GOF设计模式的创建型,一样的为了解决复杂的bean实例化过程.

其实这边的FactoryBean就是一个factory method[gof定义的意图:定义一个用于创建对象的接口,让子类决定实例化哪个类.Factory Method使一个类的实例化延迟到其子类].

这边的处理主要涉及3个方法,我们来看看3个方法各自的职责:

1. getObjectForBeanInstance: 参数校验之类的准备工作
2. getObjectFromFactoryBean: 单例时,确保实例是全局唯一的
3. doGetObjectFromFactoryBean: 实实在在的实例化

参数校验之类的准备工作##

AbstractBeanFactory的getObjectForBeanInstance处理.

1. 如果name制定要获取FactoryBean本身实例,而beanInstance却又不是FactoryBean,直接抛异常
2. 如果不需要调用getObject实例化,直接返回实例
3. 尝试从缓存中获取实例[FactoryBeanRegistrySupport负责]
4. 准备beanDefinition,委托getObjectFromFactoryBean处理

单例时保障实例全局唯一##

FactoryBeanRegistrySupport的getObjectFromFactoryBean处理

FactoryBeanRegistrySupport负责FactoryBean相关的操作,并缓存FactoryBean的getObject实例化的bean.

1. 判断factory是单例,同时已经new好了(后面需要调用getObject获取目标对象,这边需要factory已经实例化)
2. 单例时,先尝试去缓存找;如果找不到或者不是单例,委托doGetObjectFromFactoryBean实例化一个
3. 由于这样新建就没有机会调用BeanPostProcessor了,所以这边直接调用其postProcessAfterInitialization[职责职责职责,AbstractAutowireCapableBeanFactory干的]
4. 缓冲得到的实例

实例化,干活的##

FactoryBeanRegistrySupport的doGetObjectFromFactoryBean处理

这边做的事很少,就是调用factoryBean的getObject,然后如果实例化得到的对象为空抛异常.

源码摘要##

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 实例的对象,可能是实例本身,也可能是FactoryBean新建的对象
 * @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) {

    // 如果想要获取FactoryBean本身,那么beanInstance必须是FactoryBean的实例
	// 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实例,或者想要获取的就是FactoryBean实例,那么直接返回就好
	// 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) {
        // 获取缓存的实例
		object = getCachedObjectForFactoryBean(beanName);
	}
	if (object == null) {
        // 缓存中没有对象,那么从头准备bean defition实例化一个
		// 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.
 * 获取缓存的,通过FactoryBean暴露出来的对象
 * @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);
    // 类似<重构>中Introduce NUll Object,只是这边的NULL_OBJECT是Object类型的,没有解决Martin说的"不需要询问对象类型,就可以直接调用行为的方法"
	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是单例的同时,还得已经被实例化(感觉这个检验怪怪的,没想明白)
	if (factory.isSingleton() && containsSingleton(beanName)) {
		synchronized (getSingletonMutex()) {
			Object object = this.factoryBeanObjectCache.get(beanName);
			if (object == null) {
                // 缓存中找不到,就自己干吧,实例化
				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,没有作用在实际想要的实例上,这边补一个
                            // 也就是说,BeanPostProcessor的postProcessBeforeInitialization不会作用在FactoryBean上
							object = postProcessObjectFromFactoryBean(object, beanName);
						}
						catch (Throwable ex) {
							throw new BeanCreationException(beanName,
									"Post-processing of FactoryBean's singleton object failed", ex);
						}
					}
                    // 缓存
					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;
}