spring中有两种类型的Bean：一种是普通的JavaBean；另一种就是工厂Bean（FactoryBean），这两种Bean都受Spring的IoC容器管理，但它们之间却有一些区别。
普通的JavaBean不再多说，我们将其定义好，然后在配置文件中定义并配置其依赖关系，就可以通过IoC容器的getBean获取到。
那么FactoryBean呢？
FactoryBean跟普通Bean不同，它是实现了FactoryBean<T>接口的Bean，
1.通过BeanFactory类的getBean方法直接获取到的并不是该FactoryBean的实例，而是该FactoryBean中方法getObject返回的对象。
2.但我们可以通过其它途径获取到该FactoryBean的实例，方法就是在通过getBean方法获取实例时在参数name前面加上“&”符号即可。
FactoryBean接口提供的方法如下：
public interface FactoryBean<T> { //获取FactoryBean初始化的Bean实例 T getObject() throws Exception; //获取Bean实例的类型 Class<?> getObjectType(); //判断是否是单例模式 boolean isSingleton(); }
通常会通过这种方式获取到容器中的bean：
ApplicationContext appCtx = new ClassPathXmlApplicationContext("applicationContext.xml"); appCtx.getBean("myFactoryBean"); appCtx.getBean("&myFactoryBean");通过跟代码，会定位到org.springframework.beans.factory.support.AbstractBeanFactory#getBean(java.lang.String)方法：
@Override public Object getBean(String name) throws BeansException { return doGetBean(name, null, null, false); }doGetBean方法中，会调用getObjectForBeanInstance来获取给定的bean实例的对象，
protected <T> T doGetBean( final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException { .........//省略部分代码 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); .........//省略部分代码 return (T) bean; }getObjectForBeanInstance方法在AbstractBeanFactory的getObjectForBeanInstance方法
获取给定bean实例的对象，对于FactoryBean，可以是bean实例本身，也可以是它创建的对象。
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bean name是以&开始的，会直接返回beanInstance if (BeanFactoryUtils.isFactoryDereference(name)) { if (beanInstance instanceof NullBean) { return beanInstance; } if (!(beanInstance instanceof FactoryBean)) { //beanInstance不是FactoryBean，抛异常 throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass()); } if (mbd != null) { mbd.isFactoryBean = true; } return beanInstance; } // 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)) { return beanInstance; } Object object = null; if (mbd != null) { mbd.isFactoryBean = true; } else { 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()); //从给定的FactoryBean获取要公开的对象 object = getObjectFromFactoryBean(factory, beanName, !synthetic); } return object; }其中BeanFactoryUtils.isFactoryDereference(name)源码如下，只是判断了开始是否是&开头
String FACTORY_BEAN_PREFIX = "&"; public static boolean isFactoryDereference(@Nullable String name) { return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)); }getObjectFromFactoryBean方法如下，用来从FactoryBean中获取bean
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; } }
