Spring Framework 源码阅读（三）：认识FactoryBean

​​FactoryBean​​​和​​BeanFactory​​​虽然从命名上看起来相似，但作用却大不相同，​​BeanFactory​​​负责创建和管理​​Spring​​​容器中的​​bean​​​，为​​bean​​​从初始化到销毁建立了一整套统一的流程，让使用者可以不需要关注复杂的流程，只需关注​​bean​​​本身的逻辑。而​​FactoryBean​​​可以提供自定义​​bean​​​初始化流程的功能，因此​​FactoryBean​​​可以用于创建一类​​bean​​。

FactoryBean

public interface FactoryBean<T> {

  String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";

  @Nullable
  T getObject() throws Exception;

  @Nullable
  Class<?> getObjectType();

  default boolean isSingleton() {
    return true;
  }
}

• 如果一个​​bean​​​实现了​​FactoryBean​​​接口，那么该​​bean​​​将作为某一类​​bean​​​的工厂，而不是直接作为一个​​bean​​。
• 实现​​FactoryBean​​​接口的​​bean​​​不能用作普通​​bean​​​。​​FactoryBean​​​是以​​bean​​​定义的，但是​​bean​​​引用的对象始终是​​getObject​​方法创建的对象。
• ​​FactoryBean​​​（不是单指​​FactoryBean​​​接口）可以支持单例和原型，并且可以根据需要以懒加载的形式创建对象，也可以在启动时立即创建对象。​​SmartFactoryBean​​接口允许更细粒度的行为元数据。
• ​​FactoryBean​​​接口在​​Spring​​​框架中大量使用，例如​​ProxyFactoryBean（AOP）​​​或​​JndiObjectFactoryBean​​​。它也可用于定制​​component​​。

​​SmartFactoryBean​​接口源码：

public interface SmartFactoryBean<T> extends FactoryBean<T> {

  default boolean isPrototype() {
    return false;
  }

  default boolean isEagerInit() {
    return false;
  }
}

创建module

先在​​Spring Framework​​​源码中增加一个​​application module​​，这在之前的博文中已经介绍过了，这里就不再赘述：

• ​​Spring Framework 源码阅读（一）：Aware接口及其实现类的作用​​

​​IMessageService​​接口：

package com.kaven.service;

/**
 * @Author: ITKaven
 * @Date: 2021/09/25 14:04
 * @Leetcode: https://leetcode-cn.com/u/kavenit
 * @Notes:
 */

public interface IMessageService {
  default String getMessage() {
    return "default message";
  }
}

​​MessageServiceImpl​​实现类：

package com.kaven.service.impl;

import com.kaven.service.IMessageService;

/**
 * @Author: ITKaven
 * @Date: 2021/09/25 14:05
 * @Leetcode: https://leetcode-cn.com/u/kavenit
 * @Notes:
 */

public class MessageServiceImpl implements IMessageService {

  public String message;

  public MessageServiceImpl(String message) {
    this.message = message;
  }

  @Override
  public String getMessage() {
    return message;
  }
}

​​MessageServiceFactoryBean​​​类（实现​​FactoryBean​​接口）：

package com.kaven.factory;

import com.kaven.service.IMessageService;
import com.kaven.service.impl.MessageServiceImpl;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.stereotype.Component;

/**
 * @Author: ITKaven
 * @Date: 2021/10/28 20:01
 * @Leetcode: https://leetcode-cn.com/u/kavenit
 * @Notes:
 */

@Component("myFactoryBean")
public class MessageServiceFactoryBean implements FactoryBean<IMessageService> {
  @Override
  public IMessageService getObject() throws Exception {
    return new MessageServiceImpl("Hello Spring");
  }

  @Override
  public Class<?> getObjectType() {
    return IMessageService.class;
  }

  @Override
  public boolean isSingleton() {
    return false;
  }
}

​​Application​​启动类：

package com.kaven;

import com.kaven.factory.MessageServiceFactoryBean;
import com.kaven.service.IMessageService;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;

import java.util.Arrays;

/**
 * @Author: ITKaven
 * @Date: 2021/09/25 13:54
 * @Leetcode: https://leetcode-cn.com/u/kavenit
 * @Notes:
 */

@ComponentScan({"com.kaven"})
public class Application {

  public static void main(String[] args) {
    ApplicationContext applicationContext = new AnnotationConfigApplicationContext(Application.class);

    IMessageService messageServiceBean = (IMessageService) applicationContext.getBean("myFactoryBean");
    System.out.println(messageServiceBean.getMessage());

    IMessageService messageServiceBean2 = (IMessageService) applicationContext.getBean("myFactoryBean");
    System.out.println("messageServiceBean == messageServiceBean2: " + (messageServiceBean == messageServiceBean2));

    MessageServiceFactoryBean messageServiceFactoryBean = (MessageServiceFactoryBean) applicationContext.getBean("&myFactoryBean");
    System.out.println("isSingleton: " + messageServiceFactoryBean.isSingleton());

    System.out.println(applicationContext.getBeanDefinitionCount());
    Arrays.stream(applicationContext.getBeanDefinitionNames()).forEach(System.out::println);
  }
}

运行输出：

> Task :application:Application.main()
Hello Spring
messageServiceBean == messageServiceBean2: false
isSingleton: false
6
org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
application
myFactoryBean

很显然输出结果符合预期，通过​​FactoryBean​​​的​​bean​​​名称获取​​bean​​​，只会获取该​​FactoryBean​​​中​​getObject()​​​方法返回的实例，也就是这里的​​MessageServiceImpl​​​实例，如果想要获取​​FactoryBean​​​本身的​​bean​​​，需要在​​FactoryBean​​​的​​bean​​​名称前拼接一个​​&​​字符，在代码中已经有演示了。

想要了解为什么会创建下面这​​4​​​个​​bean​​，可以阅读这篇博客：

• ​​Spring Framework 源码阅读（二）：BeanDefinition的作用​​

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory

源码分析

现在开始分析为什么实现了​​FactoryBean​​​接口，就可以自定义创建某一类​​bean​​​的流程了，以及获取​​FactoryBean​​​本身的​​bean​​​，为什么要在​​FactoryBean​​​的​​bean​​​名称前拼接一个​​&​​字符。

自定义的​​bean​​​，大部分会在​​DefaultListableBeanFactory​​​类的​​preInstantiateSingletons​​方法中被开始创建（删除了部分代码）：

@Override
  public void preInstantiateSingletons() throws BeansException {
    List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

    for (String beanName : beanNames) {
      RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
      if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
        if (isFactoryBean(beanName)) {
          Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
          if (bean instanceof FactoryBean) {
            FactoryBean<?> factory = (FactoryBean<?>) bean;
            boolean isEagerInit;
            if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
              isEagerInit = AccessController.doPrivileged(
                  (PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
                  getAccessControlContext());
            }
            else {
              isEagerInit = (factory instanceof SmartFactoryBean &&
                  ((SmartFactoryBean<?>) factory).isEagerInit());
            }
            if (isEagerInit) {
              getBean(beanName);
            }
          }
        }
        else {
          getBean(beanName);
        }
      }
    }
  }

​​Debug​​如下图所示：

关注这一行（​​!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()​​​结果是​​true​​​，这些在​​AbstractBeanDefinition​​​抽象类中设置了默认值，博主在上一篇博客中进行了介绍，并且​​isFactoryBean​​​方法也会返回​​true​​​，因为它就是​​FactoryBean​​）：

Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);

String FACTORY_BEAN_PREFIX = "&";

很显然​​beanName​​​就是​​FactoryBean​​​的​​bean​​​名称，而​​FACTORY_BEAN_PREFIX + beanName​​​就是​​&myFactoryBean​​​，所以这里是获取​​FactoryBean​​​本身的​​bean​​。

后面还会调用​​AbstractBeanFactory​​​抽象类的​​getObjectForBeanInstance​​方法（删除了部分代码）：

protected Object getObjectForBeanInstance(
      Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

    if (BeanFactoryUtils.isFactoryDereference(name)) {
      if (beanInstance instanceof NullBean) {
        return beanInstance;
      }
      if (!(beanInstance instanceof FactoryBean)) {
        throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
      }
      if (mbd != null) {
        mbd.isFactoryBean = true;
      }
      return beanInstance;
    }

    if (!(beanInstance instanceof FactoryBean)) {
      return beanInstance;
    }

    Object object = null;
    if (mbd != null) {
      mbd.isFactoryBean = true;
    }
    else {
      object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
      FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
      if (mbd == null && containsBeanDefinition(beanName)) {
        mbd = getMergedLocalBeanDefinition(beanName);
      }
      boolean synthetic = (mbd != null && mbd.isSynthetic());
      object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
  }

​​getObjectForBeanInstance​​​方法会进行判断选择分支，会有三个分支，如果是获取​​FactoryBean​​​本身的​​bean​​​（​​getObjectForBeanInstance​​​方法中的​​beanInstance​​​参数其实在前面的调用链中已经被创建，如上图所示的​​sharedInstance​​​变量，这里就不详细介绍，不然显得太杂乱，博主会在以后的博客中详细介绍​​bean​​的创建流程和生命周期），就会进入第一个分支：

if (BeanFactoryUtils.isFactoryDereference(name)) {
      if (beanInstance instanceof NullBean) {
        return beanInstance;
      }
      if (!(beanInstance instanceof FactoryBean)) {
        throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
      }
      if (mbd != null) {
        mbd.isFactoryBean = true;
      }
      return beanInstance;
    }

​​BeanFactoryUtils​​​抽象类的​​isFactoryDereference​​​方法（就是判断​​name​​​参数在不为​​null​​​的情况下有没有​​&​​前缀）：

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

因此第一个分支就是返回​​FactoryBean​​​本身的​​bean​​。

第二个分支（​​beanInstance​​​参数不是​​FactoryBean​​​实例，就是返回跟​​factory​​​不相关的​​bean​​）：

if (!(beanInstance instanceof FactoryBean)) {
      return beanInstance;
    }

第三个分支（很显然是返回​​FactoryBean​​​对应的那一类​​bean​​，下面会详细介绍）：

Object object = null;
    if (mbd != null) {
      mbd.isFactoryBean = true;
    }
    else {
      object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
      FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
      if (mbd == null && containsBeanDefinition(beanName)) {
        mbd = getMergedLocalBeanDefinition(beanName);
      }
      boolean synthetic = (mbd != null && mbd.isSynthetic());
      object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }

回到获取​​FactoryBean​​​本身​​bean​​​的​​preInstantiateSingletons​​方法中的这个部分：

if (bean instanceof FactoryBean) {
            FactoryBean<?> factory = (FactoryBean<?>) bean;
            boolean isEagerInit;
            if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
              isEagerInit = AccessController.doPrivileged(
                  (PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
                  getAccessControlContext());
            }
            else {
              isEagerInit = (factory instanceof SmartFactoryBean &&
                  ((SmartFactoryBean<?>) factory).isEagerInit());
            }
            if (isEagerInit) {
              getBean(beanName);
            }
          }

​​SmartFactoryBean​​​接口的源码在上面已经展示过了，非常简单，就是继承了​​FactoryBean​​​接口，并且增加了两个方法，比如上面出现的​​isEagerInit()​​​方法，这些源码都是相互联系的，大家还是要亲自​​Debug​​​体验一下。因此这里的​​isEagerInit​​​变量是​​false​​​，因为​​MessageServiceFactoryBean​​​类实现的是​​FactoryBean​​​接口，所以实现​​FactoryBean​​​接口的​​MessageServiceFactoryBean​​​类不会立即（懒加载）创建它对应的那一类​​bean​​​（​​MessageServiceImpl​​​实例）。如果实现​​SmartFactoryBean​​​接口，并且让​​isEagerInit​​​方法返回​​true​​​，在创建​​FactoryBean​​​本身的​​bean​​​后，也会立即创建​​FactoryBean​​​对应的那一类​​bean​​，大家可以动手试一试。

因此，在​​ApplicationContext applicationContext = new AnnotationConfigApplicationContext(Application.class);​​​这行代码结束后，​​FactoryBean​​​本身的​​bean​​​就已经在​​Spring​​​容器中了，以便后续可以创建该​​FactoryBean​​​对应的那一类​​bean​​。

之后通过​​FactoryBean​​​的​​bean​​​名称来获取​​bean​​​，就是该​​FactoryBean​​​对应的那一类​​bean​​​（通过​​getObject​​方法返回的实例）。

IMessageService messageServiceBean = (IMessageService) applicationContext.getBean("myFactoryBean");

​​Bebug​​如下图所示：

会调用​​AbstractBeanFactory​​​抽象类的​​getObjectForBeanInstance​​​方法（删除了部分代码），并且​​factory​​​参数就是之前创建好的​​FactoryBean​​​本身的​​bean​​​（上面说过这里的​​beanInstance​​​参数就是​​FactoryBean​​​本身的​​bean​​​，并且已经在之前的调用链中被创建好了，这里只是强转为​​FactoryBean<?>​​​类型，方便后续调用​​getObject​​方法）。

protected Object getObjectForBeanInstance(
      Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
      
    Object object = null;
    if (mbd != null) {
      mbd.isFactoryBean = true;
    }
    else {
      object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
      FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
      if (mbd == null && containsBeanDefinition(beanName)) {
        mbd = getMergedLocalBeanDefinition(beanName);
      }
      boolean synthetic = (mbd != null && mbd.isSynthetic());
      object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
  }

重点在这一行：

object = getObjectFromFactoryBean(factory, beanName, !synthetic);

调用了​​FactoryBeanRegistrySupport​​​抽象类的​​getObjectFromFactoryBean​​方法（删除了部分代码）：

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);
          Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
          if (alreadyThere != null) {
            object = alreadyThere;
          }
          else {
            if (shouldPostProcess) {
              if (isSingletonCurrentlyInCreation(beanName)) {
                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;
    }
  }

这里的​​factory.isSingleton()​​​会返回​​false​​，因此会执行下面这一行：

Object object = doGetObjectFromFactoryBean(factory, beanName);

调用了​​FactoryBeanRegistrySupport​​​抽象类的​​doGetObjectFromFactoryBean​​方法：

private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, 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();
      }
    }
    return object;
  }

最终会执行​​object = factory.getObject();​​​，而​​factory​​​参数就是之前创建好的​​FactoryBean​​​本身的​​bean​​​，因此会调用​​FactoryBean​​​中的​​getObject​​​方法，创建了与该​​FactoryBean​​​对应的那一类​​bean​​。

​​Debug​​如下图所示：

阅读源码需要耐心，一步步进行​​Debug​​，每个人的理解不同，如果博主有说错的地方或者大家有不同的见解，欢迎大家评论补充。