Spring中FactoryBean的作用和实现原理

BeanFactory与FactoryBean，相信很多刚翻看Spring源码的同学跟我一样很好奇这俩货怎么长得这么像，分别都是干啥用的。

1. BeanFactory是Spring中Bean工厂的顶层接口，也是我们常说的SpringIOC容器，它定下了IOC容器的一些规范和常用方法并管理着Spring中所有的Bean；
2. FactoryBean首先它是一个Bean，但又不仅仅是一个Bean。它是一个能生产或修饰对象生成的工厂Bean，类似于设计模式中的工厂模式和装饰器模式。它能在需要的时候生产一个对象，且不仅仅限于它自身，它能返回任何Bean的实例。

我们都知道在Spring中我们的Bean都会被Spring的IOC容器所管理，在AbstractApplicationContext中有一个很重要的方法：refresh()，项目启动或重启的时候refresh()会调用getBean()初始化所有的Bean，这个getBean()最终会指向AbstractBeanFactory中的getBean()方法。
在AbstractBeanFactory中，不管是根据名称还是根据类型，getBean()最终都会调用doGetBean()，在doGetBean()方法中一开始就获取了名称beanName和实例sharedInstance，这个方法太长，这里就贴前面两行。

        String beanName = this.transformedBeanName(name);Object sharedInstance = this.getSingleton(beanName);

transformedBeanName(name)是为了获取Bean真正的名称，它会去掉name前面的’&'，而getSingleton(beanName)是从父类容器singletonObjects中取的这个Bean的实例。在Spring中还有很多这样的容器，比如DefaultListableBeanFactory中的beanDefinitionMap，它就是的IOC容器真正保存Bean的地方，它是一个HashMap。类似的还有FactoryBeanRegistrySupport中的factoryBeanObjectCache等。
回到doGetBean()方法中，拿到sharedInstance后，后面的一大堆操作做了单例、多例等判断，最终会走到this.getObjectForBeanInstance()，关键部分就在这个方法中，进入方法代码。

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.if (BeanFactoryUtils.isFactoryDereference(name)) {if (beanInstance instanceof NullBean) {return beanInstance;}if (!(beanInstance instanceof FactoryBean)) {throw new BeanIsNotAFactoryException(beanName, 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.if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {return beanInstance;}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());object = getObjectFromFactoryBean(factory, beanName, !synthetic);}return object;}

在上面的代码中有两个判断分别是beanInstance instanceof FactoryBean和BeanFactoryUtils.isFactoryDereference(name)，前面判断的是beanInstance是否属于FactoryBean或其子类的实例，后面这个方法判断name是否不为空且以&开头。

public static boolean isFactoryDereference(@Nullable String name) {return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));}

如果beanInstance不属于FactoryBean或其子类的实例，或者name是以&开头就直接返回实例对象beanInstance，
否则继续往下走。各种if … ==null判断是为了提高性能，咱们只挑关键部分看object = getObjectFromFactoryBean(factory, beanName, !synthetic);

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);}}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;}}

看源码的时候如果我们一直追究所有的细节那会让我们会越陷越深，掉入细节的无底洞，稍不留神脑回路跟不上就会蒙圈。我们要学会找源码中的关键部分看，弄懂主要流程和本次看源码的目的的那部分就行。等我们对Spring整体有了一个很好的理解之后，再回头看之前不懂的代码就会豁然开朗。

在上面这个方法中不管是走上面的if分支还是到下边的else中，关键部分就是object = this.doGetObjectFromFactoryBean(factory, beanName)这段代码调用，继续点进去

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) {if (isSingletonCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName, "FactoryBean which is currently in creation returned null from getObject");}object = new NullBean();}return object;}

在这个方法中有一行关键代码：object = factory.getObject(); 这个factory就是我们传入的beanInstance实例。绕了这么一大圈，getBean方法返回的居然是我们实现FactoryBean接口定义的getObject方法。那么跟一开始对FactoryBean的描述印证了，FactoryBean是一个能生产或修饰对象生成的工厂Bean。一个Bean如果实现了FactoryBean接口，那么根据该Bean的名称获取到的实际上是getObject()返回的对象，而不是这个Bean自身实例，如果要获取这个Bean自身实例，那么需要在名称前面加上’&'符号。

一般情况下，Spring通过反射机制利用的class属性指定实现类实例化Bean，在某些情况下，实例化Bean过程比较复杂，如果按照传统的方式，则需要在中提供大量的配置信息。配置方式的灵活性是受限的，这时采用编码的方式可能会得到一个简单的方案。Spring为此提供了一个org.springframework.bean.factory.FactoryBean的工厂类接口，用户可以通过实现该接口定制实例化Bean的逻辑。FactoryBean接口对于Spring框架来说占用重要的地位，Spring自身就提供了70多个FactoryBean的实现。它们隐藏了实例化一些复杂Bean的细节，给上层应用带来了便利。从Spring3.0开始，FactoryBean开始支持泛型，即接口声明改为FactoryBean的形式

package com.lvyuanj.core.service;//接口
public interface FactoryBeanService {/*** 测试FactoryBean*/void testFactoryBean();
}

package com.lvyuanj.core.service.Impl;import com.lvyuanj.core.service.FactoryBeanService;//实现类
public class FactoryBeanServiceImpl implements FactoryBeanService {@Overridepublic void testFactoryBean() {System.out.println("我是FactoryBean的一个测试类。。。。");}
}

package com.lvyuanj.core.service.Impl;import com.lvyuanj.core.service.FactoryBeanService;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.stereotype.Component;@Component
public class MyFactoryBean implements FactoryBean<FactoryBeanService> {@Overridepublic FactoryBeanService getObject() throws Exception {//这个Bean是我们自己new的，这里我们就可以控制Bean的创建过程了return new FactoryBeanServiceImpl();}@Overridepublic Class<?> getObjectType() {return FactoryBeanService.class;}@Overridepublic boolean isSingleton() {return true;}
}

@Testpublic void test2() throws Exception {ApplicationContext applicationContext = new AnnotationConfigApplicationContext(MyConfig.class);String[] beanDefinitionNames = applicationContext.getBeanDefinitionNames();for (String beanName : beanDefinitionNames) {System.out.println("beanName: " + beanName);}FactoryBeanService factoryBeanService = (FactoryBeanService) applicationContext.getBean("MyFactoryBean");System.out.println(factoryBeanService);MyFactoryBean factoryBeanService3 = (MyFactoryBean) applicationContext.getBean("&myFactoryBean");System.out.println(factoryBeanService3);}