理解spring中的BeanFactory和FactoryBean的区别与联系

首先，这俩都是个接口…

实现 BeanFactory 接口的类表明此类事一个工厂，作用就是配置、新建、管理 各种Bean。

而 实现 FactoryBean 的类表明此类也是一个Bean，类型为工厂Bean（Spring中共有两种bean，一种为普通bean，另一种则为工厂bean）。顾名思义，它也是用来管理Bean的，而它本身由spring管理。

一个Bean想要实现 FactoryBean ，必须实现以下三个接口：

1. Object getObject():返回由FactoryBean创建的Bean的实例

2. boolean isSingleton():确定由FactoryBean创建的Bean的作用域是singleton还是prototype；

3. getObjectType():返回FactoryBean创建的Bean的类型。

有一点需要注意，如果将一个实现了FactoryBean的类成功配置到了spring上下文中，那么通过该类对象的名称（比如appleFactoryBean）从spring的applicationContext或者beanFactory获取bean时，获取到的是appleFactoryBean创建的apple实例，而不是appleFactoryBean自己，如果想通过spring拿到appleFactoryBean，需要在名称前加 & 符号 ：

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

这个prefix在BeanFactory接口源码中有提到：

/**
* 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 = "&";

还有一点需要注意，FactoryBean管理的bean实际上也是由spring进行配置、实例化、管理，因此由FactoryBean管理的bean不能再次配置到spring配置文件中（xml、java类配置、注解均不可以），否则会报如下异常：

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)

附上一个例子：

spring配置类：

@Configuration
@ComponentScan
public class Configurations {
}

AppleBean :

//@Component  这里不可以加注解 ！！！！！！
public class AppleBean{

}

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;
    }
}

启动类 :

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