Spring源码分析（十四）从bean的实例中获取对象

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。