理解spring中的BeanFactory和FactoryBean的區別與聯繫

原文地址：http://blog.csdn.net/joenqc/article/details/66479154

首先，這倆都是個接口…

實現 BeanFactory 接口的類代表此類事一個工廠，做用就是配置、新建、管理 各類Bean。

而 實現 FactoryBean 的類代表此類也是一個Bean，類型爲工廠Bean（Spring中共有兩種bean，一種爲普通bean，另外一種則爲工廠bean）。顧名思義，它也是用來管理Bean的，而它自己由spring管理。

一個Bean想要實現 FactoryBean ，必須實現如下三個接口：

1. Object getObject():返回由FactoryBean建立的Bean的實例 2. boolean isSingleton():肯定由FactoryBean建立的Bean的做用域是singleton仍是prototype； 3. getObjectType():返回FactoryBean建立的Bean的類型。

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有一點須要注意，若是將一個實現了FactoryBean的類成功配置到了spring上下文中，那麼經過該類對象的名稱（好比appleFactoryBean）從spring的applicationContext或者beanFactory獲取bean時，獲取到的是appleFactoryBean建立的apple實例，而不是appleFactoryBean本身，若是想經過spring拿到appleFactoryBean，須要在名稱前加 & 符號 ：

out.println(applicationContext.getBean("&appleFactoryBean"))

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這個prefix在BeanFactory接口源碼中有提到：

/** * Used to dereference a {@link FactoryBean} instance and distinguish it from * beans <i>created</i> by the FactoryBean. For example, if the bean named * {@code myJndiObject} is a FactoryBean, getting {@code &myJndiObject} * will return the factory, not the instance returned by the factory. */ String FACTORY_BEAN_PREFIX = "&";

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還有一點須要注意，FactoryBean管理的bean實際上也是由spring進行配置、實例化、管理，所以由FactoryBean管理的bean不能再次配置到spring配置文件中（xml、java類配置、註解均不能夠），不然會報以下異常：

Exception in thread "main" org.springframework.beans.factory.BeanIsNotAFactoryException: Bean named 'appleFactoryBean' is expected to be of type 'org.springframework.beans.factory.FactoryBean' but was actually of type 'java.lang.Object' at org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance(AbstractBeanFactory.java:1612) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:317) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:742) at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866) at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542) at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:84) at com.joen.testspringcontainer.Start.main(Start.java:11) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

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附上一個例子：

spring配置類：

@Configuration @ComponentScan public class Configurations { }

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AppleBean :

//@Component 這裏不能夠加註解 ！！！！！！ public class AppleBean{ }

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AppleFactoryBean :

@Component public class AppleFactoryBean implements FactoryBean{ public Object getObject() throws Exception { return new AppleBean(); } public Class<?> getObjectType() { return AppleBean.class; } public boolean isSingleton() { return false; } }

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啓動類 :

public class Start { public static void main(String[] args){ ApplicationContext applicationContext = new AnnotationConfigApplicationContext(Configurations.class); out.println(applicationContext.getBean("appleFactoryBean"));//獲得的是apple out.println(applicationContext.getBean("&appleFactoryBean"));//獲得的是apple工廠 } }