Spring-IOC-FactoryBean检测与获取Bean

主要讲解getObjectForInstance方法，对FactoryBean验证，并获取需要的Bean实例。

• 无论是从缓存中获得bean还是根据不同的scope策略加载bean，都需要在得到bean实例后调用getObjectForInstance方法来检测一下正确性；即检测当前bean是否是FactoryBean类型的bean，如果是，那么需要调用该bean对应的FactoryBean实例中的getObject()方法作为返回值。

• 无论是从缓存中获取的bean还是通过不同scope策略加载的bean都只是原始的bean状态，并不一定是我们最终想要的bean，举个例子，假如我们需要对工厂bean进行处理，那么这里得到的其实是工厂bean的初始状态，但是我们真正需要的是工厂bean中定义的factory-method方法中返回的bean，而getObjectForInstance方法就是完成这个工作的。

主要步骤：

1. 对FactoryBean正确性的验证

beanName含有 &前缀，但是又不是FactoryBean，则验证不通过并抛出异常。

2. 对非FactoryBean或者beanName含有&前缀的FactoryBean则不做任何处理，直接返回。
3. 如果是beanName不含有&前缀的FactoryBean，则先尝试从factoryBeanObjectCache中获取由Factory创建的bean。

factoryBeanObjectCache用于存放被FactoryBean创建的单例bean对象，保存的映射为 FactoryBean -> object。

4. factoryBeanObjectCache中不存在，则将FactoryBean中获取bean的工作委托给getObjectFormFactoryBean方法（主要是调用FactoryBean.getObject()方法）。

FactoryBean检测与获取Bean
beans.factory.support.AbstractBeanFactory(XmlBeanFactory 继承自这个类，拥有这个方法)
    /**
     * Get the object for the given bean instance, either the bean
     * instance itself or its created object in case of a FactoryBean.
     * @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) {

        //1. 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());
        }

        //2. 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;
        }

        // 3.
        Object object = null;
        if (mbd == null) {
            object = getCachedObjectForFactoryBean(beanName);
        }
        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());
            // 4. 
            object = getObjectFromFactoryBean(factory, beanName, !synthetic);
        }
        return object;
    }

接下来主要看一下第四步是如何操作的？

beans.factory.support.FactoryBeanRegistrySupport(XmlBeanFactory 继承自这个类，拥有这个方法)
    /**
     * 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) {
        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 (object != null && shouldPostProcess) {
                            try {
                                object = postProcessObjectFromFactoryBean(object, beanName);
                            }
                            catch (Throwable ex) {
                                throw new BeanCreationException(beanName,
                                        "Post-processing of FactoryBean's singleton object failed", ex);
                            }
                        }
                        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