(一)Spring IoC源码-2.bean的加载-03从FactoryBean实例中获取目标实例

本文已收录于【Spring源码札记专栏】。

关联文章： (一)Spring IoC源码-2.bean的加载-01整体概览 (一)Spring IoC源码-2.bean的加载-02从缓存中获取单例bean (一)Spring IoC源码-2.bean的加载-03从bean实例中获取对象 (一)Spring IoC源码-2.bean的加载-04创建bean

无论是已经加载到了单例bean还是创建bean后，都需要通过bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);从bean实例中获取目标对象。

无论是从缓存中获取到的bean还是通过不同的scope策略创建的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，即FactoryBean实例进行处理，那么这里得到的其实是工厂bean的初始状态，但我们真正想要的是工厂bean中定义的factory-method方法中返回的bean。getObjectForBeanInstance方法就是完成这个工作的。

@Nullable protected Object getObjectForBeanInstance( @Nullable Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) { if (beanInstance == null) { return null; } // 验证参数是否合法 if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) { throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass()); } //如果不是bean实例不是FactoryBean，直接返回 if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) { return beanInstance; } Object object = null; //尝试从缓存中获取对象 if (mbd == null) { object = getCachedObjectForFactoryBean(beanName); } //如果没能从缓存中获取到对象 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()); //将获取对象的工作交给getObjectFromFactoryBean方法 object = getObjectFromFactoryBean(factory, beanName, !synthetic); } //返回获取的对象 return object; }

从上面的代码来看，这个方法将核心的功能委托给了getObjectFromFactoryBean方法实现。

@Nullable protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) { //如果是单例模式 if (factory.isSingleton() && containsSingleton(beanName)) { synchronized (getSingletonMutex()) { //从缓存中获取对象 Object object = this.factoryBeanObjectCache.get(beanName); //如果从缓存中没有获取到 if (object == null) { //委托给doGetObjectFromFactoryBean方法 object = doGetObjectFromFactoryBean(factory, beanName); // 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; } }

再看doGetObjectFromFactoryBean方法，看到以do开头的方法，我们就知道这就是我们要找的方法。

@Nullable 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((PrivilegedExceptionAction<Object>) () -> 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; }

如果bean声明为FactoryBean类型，则当提取bean时候提取的不是FactoryBean，而是FactoryBean中的getObject方法返回的bean。

从上面的源码中将FactoryBean的实现原理总结如下。 通过上述的分析，我们知道最后返回的是通过FactoryBean中对应的getObject方法返回的bean。这是不是和工厂方法模式有些相似呢。下图是工厂方法模式的UML图。 对比一下，FactoryBean可以看做是IFactory；getObject方法可以看做getProduct方法；而FactoryBean的具体实现，比如TransactionProxyFactoryBean，可以看做具体的工厂实现FactoryA；通过getObject方法生成的TransactionProxy可以看做是具体的产品实现PorductA。

最后总结下FactoryBean与BeanFactory的区别。

BeanFactory是用于管理bean的一个工厂。

FactoryBean是一种特殊的bean。在BeanFactory中管理两种bean，一种是标准的Java bean，另一种是工厂bean, 即实现了FactoryBean接口的bean。

通过beanFactory.getBean(beanName)从BeanFactory获取bean实例时，对于标准的Java bean，返回的是类自身的实例。而对于工厂bean，返回的不是自身的实例，而是该工厂bean的getObject方法所返回的实例。如果想要获取工厂bean的实例，可以通过getBean(&+beanName)这种方法来获取。