当前位置 博文首页 > 北漂程序员：一次性讲清楚spring中bean的生命周期之二：FactoryB
在《spring中FactoryBean是什么bean》一文中，带着小伙伴学习了spring中的FactoryBean，了解了到了FactoryBean其实是一种生产Bean的bean，也就是FactroyBean的前世是Bean，今生还是Bean，小伙伴要疑惑了都是Bean，但是此Bean非彼Bean。今天带着小伙伴从源码的角度来分析下FactoryBean，重点是getObjectForBeanInstance方法的分析。
在前面说到FactoryBean的前世是一个Bean，是指是一个FactoryBean的实例。先来看下getObjectForBeanInstance方法，
/** * Get the object for the given bean instance, either the bean * instance itself or its created object in case of a FactoryBean. */ 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.
//1、判断name是否以&开头 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.
//beanInstance不是FactoryBean的实例或name以&开头 if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) { return beanInstance; } Object object = null;
//mbd即beanDefinition为空，从缓存中取 if (mbd == null) { object = getCachedObjectForFactoryBean(beanName); }
//缓存中没有，则调用FactoryBean的getObject方法，返回其对象 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()); object = getObjectFromFactoryBean(factory, beanName, !synthetic); } return object; }
看该方法上的注释，
Get the object for the given bean instance, either the bean instance itself or its created object in case of a FactoryBean.
用我蹩脚的英语翻译过来大概是这个意思，
返回给定的bean Instance的一个对象，该对象可能是bean instance或者是由bean instance（是一个FactoryBean）创建的一个对象。
意思很明白了，该方法有可能返回的是一个FactoryBean的实例，也可能是由FactroyBean生产的实例，关键看方法参数中的前两个，
Object beanInstance spring容器中的一个bean
String name 可能含有&前缀的名称
String beanName bean的规范名称
RootBeanDefintion mbd BeanDefinition
看下面的表格更容易理解该方法在各种情况下的返回值，
| beanInstance | name | 返回值 |
| FactoryBean的实例 | 带有& | beanInstance |
| 不是FactoryBean的实例 | 带有& | beanInstance |
| FactoryBean的实例 | 不带& | beanInstance生产的对象 |
| 不是FactoryBean的实例 | 不带& | beanInstance |
通过上面得表格再结合代码就很容易理解，只要是返回beanInstance对象，那么就是FactroyBean的前世，下面看FactoryBean的今生。
这里有两个方法需要分析，分别是getCachedObjectForFactoryBean和getObjectFromFactoryBean。第一个方法是从缓存中获取，也就是说使用FactoryBean生产的bean会单独放在缓存中，非singletonObjects中，这点务必要注意。
先看下该方法的定义，
@Nullable protected Object getCachedObjectForFactoryBean(String beanName) { return this.factoryBeanObjectCache.get(beanName); }
可以看到很简单就是通过名称从factoryBeanObjectCache中取对象。factoryBeanObjectCache肯定是个map了
/** Cache of singleton objects created by FactoryBeans: FactoryBean name to object. */ private final Map<String, Object> factoryBeanObjectCache = new ConcurrentHashMap<>(16);
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 (shouldPostProcess) { if (isSingletonCurrentlyInCreation(beanName)) { // Temporarily return non-post-processed object, not storing it yet.. return object; } beforeSingletonCreation(beanName); try { object = postProcessObjectFromFactoryBean(object, beanName); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Post-processing of FactoryBean's singleton object failed", ex); } finally { afterSingletonCreation(beanName); } }
//把生产的实例对象放到factoryBeanObjectCache缓存中 if (containsSingleton(beanName)) { this.factoryBeanObjectCache.put(beanName, object); } } } return object; } } else {
//调用getObject方法 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; } }
该方法就比较复杂了，主要有doGetObjectFromFactoryBean、beforeSingletonCreation、postProcessObjectFromFactoryBean、afterSingletonCreation方法，重要的一个是doGetFromFactroyBean，也就是真正干活生产bean的方法。其定义如下，其余方法可自行查看
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 {
//调用FactoryBean中的getObject方法，返回其实例对象 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; }
在上文中的注释部分已经看到最终调用了getObject方法，也就是返回的是FactoryBean中getObject方法的返回值。
主要分析了FactoryBean的底层源码，判断是返回FactoryBean的实例还是返回其生产的实例，主要看bean的类型是否为FactoryBean和名称中是否带&。
推荐：《spring中FactoryBean是什么bean》
《一次性讲清楚spring中bean的生命周期之一：getSingleton方法 》
