摘要：本文结合《Spring源码深度解析》来分析Spring 5.0.6版本的源代码。若有描述错误之处，欢迎指正。
在getBean方法中，getObjectForBeanlnstance是个髙频率使用的方法，无论是从缓存中获得bean还是根据不同的scope策略加载bean。总之，我们得到bean的实例后要做的第一步就是调用这个方法来检测一下正确性，其实就是用于检测当前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()作为返回值。
无论是从缓存中获取到的bean还是通过不同的scope策略加载的bean都只是最原始的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getObjectForBeanlnstance方法就是完成这个工作的。
/** * 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, @Nullable RootBeanDefinition mbd) { // Don't let calling code try to dereference the factory if the bean isn't a factory. // 如果指定的name是工厂相关（以&为前缀）且 beanInstance又不是FactoryBean类型则验证不通过 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. // 现在我们有了个bean的实例，这个实例可能会是正常的bean或者是FactoryBean // 如果是FactoryBean我们使用它创建实例，似是如果用户想要直接获取工厂实例而不是工厂的getObject方法对应的实例，那么传人的name应该加入前缀& if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) { return beanInstance; } // 加栽 FactoryBean Object object = null; if (mbd == null) { // 尝试从缓存中加栽bean object = getCachedObjectForFactoryBean(beanName); } if (object == null) { // Return bean instance from factory. //到这里已经明确知道beanInstance—定是FactoryBean类型 FactoryBean<?> factory = (FactoryBean<?>) beanInstance; // Caches object obtained from FactoryBean if it is a singleton. // containsBeanDefinition检测beanDefinitionMap中也就是在所有已经加载的类中检测是否定义beanName if (mbd == null && containsBeanDefinition(beanName)) { // 将存储 XML 配置文件的 GenericBeanDefinition 转换为 RootBeanDefinition, 如果指定beanName是子Bean的话同时会合并父类的相关属性 mbd = getMergedLocalBeanDefinition(beanName); } // 是否是用户定义的而不是应用程序本身定义的 boolean synthetic = (mbd != null && mbd.isSynthetic()); object = getObjectFromFactoryBean(factory, beanName, !synthetic); } return object; }
从上面的代码来看，其实这个方法并没有什么重要的信息，大多是些辅助代码以及一些功能性的判断，而真正的核心代码却委托给了getObjectFromFactoryBean，我们来看看 getObjectForBeanlnstance 所做的工作。
（1）对FactoryBean正确性的验证。
（2）对非FactoryBean不做任何处理。
（3）对bean进行转换。
（4）将从 Factory 中解析 bean 的工作委托给 getObjectFromFactoryBean。
/** * 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) { // 如果是单例模式 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 { // 调用ObjectFactory的后处理器 object = postProcessObjectFromFactoryBean(object, beanName); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Post-processing of FactoryBean's singleton object failed", ex); } finally { afterSingletonCreation(beanName); } } if (containsSingleton(beanName)) { this.factoryBeanObjectCache.put(beanName, object); } } } return object; } } else { 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; } }
很遗憾，在这个代码中我们还是没有看到想要看到的代码，在这个方法里只做了一件事情， 就是返回的bean如果是单例的，那就必须要保证全局唯一，同时，也因为是单例的，所以不必重复创建，可以使用缓存来提高性能，也就是说已经加载过就要记录下来以便于下次复用， 否则的话就直接获取了。
在doGetObjectFromFactoryBean方法中我们终于看到了我们想要看到的方法，也就足object = factory.getObjcct()，是的，就是这句代码，我们的历程犹如剥洋葱一样，一层一层的直到最内部的代码实现，虽然很简单。
/** * 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 { // 需要权限验证 if (System.getSecurityManager() != null) { AccessControlContext acc = getAccessControlContext(); try { object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc); } catch (PrivilegedActionException pae) { throw pae.getException(); } } else { // 直接调用getObject方法 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; }
上面我们已经讲述了FactoryBean的调用方法，如果bean声明为FactoryBean类型，则当提取bean时提取的并不是FactoryBean，而是FactoryBean中对应的getObject方法返冋的bean，而doGetObjectFromFactoryBean正是实现这个功能的。但是，我们看到在上面的方法中除了调用object = factory.getObject()得到我们想要的结果后并没有直接返回，而是接下来又做了些后处理的操作，这个又是做什么用的呢？于是我们跟踪进入AbstractAutowireCapableBeanFactory 类的 postProcessObjectFromFactoryBean 方法：
@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()) { Object current = beanProcessor.postProcessAfterInitialization(result, beanName); if (current == null) { return result; } result = current; } return result; }
对于后处理器的使用我们还未过多接触，后续章节会使用大量篇幅介绍，这里，我们只需了解在Spring获取bean的规则中有这样一条：尽可能保证所有bean初始化后都会调用注册的BeanPostProcessor的postProcessAfterlnitialization方法进行处理，在实际开发过程中大可以针对此特性设计自己的业务逻辑。