【spring源码系列】之【FactoryBean类型的接口】

目前我们知道，spring创建bean有多种方式，比如xml方式创建，比如@Component，@Service，@Controler，@Repository注解创建，比如@Autowired依赖注入创建，后续还有通过springboot方式的配置注解@Configuration与@Bean方式结合创建，这里不一一介绍，等分析spring boot源码的时候再做总结。

就spring本身，提供了一种接口方式创建bean，就是本节要讨论的通过FactoryBean接口方式创建。

FactoryBean接口的实现类FactoryBeanDemo:

package com.wzj.FactoryBean;

import org.springframework.beans.factory.FactoryBean;
import org.springframework.stereotype.Service;

@Service
public class FactoryBeanDemo implements FactoryBean {

    @Override
    public Object getObject() throws Exception {
        return new FactoryB();
    }

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

通过FactoryBean实现类，完成自定义类FactoryB的实例化，FactoryB：

package com.wzj.FactoryBean;

import lombok.Data;

@Data
public class FactoryB {
    private String name = "wzj";
}

测试类：

public class TestSpring {

    @Autowired
    private ApplicationContext applicationContext;

    @Test
    public void testFactoryBean() {
        ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring.xml");
        FactoryB factoryB = (FactoryB)applicationContext.getBean("factoryBeanDemo");
        System.out.println(factoryB);

        FactoryBeanDemo factoryBeanDemo = (FactoryBeanDemo)applicationContext.getBean("&factoryBeanDemo");
        System.out.println(factoryBeanDemo);
    }

测试结果：

可以看出，当获取名称为factoryBeanDemo的实例时，得到的是getObject()方法里创建的FactoryB类型的对象，而获取加前缀&的factoryBeanDemo的实例时，得到的是FactoryBeanDemo本身的实例。

step1: FactoryBean 接口的调用入口在实例化和 IOC/DI 做完后，就会调用 FactoryBean 类型的接口如下图所示

				// Create bean instance.
				// 创建bean实例
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, () -> {
						try {
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							// Explicitly remove instance from singleton cache: It might have been put there
							// eagerly by the creation process, to allow for circular reference resolution.
							// Also remove any beans that received a temporary reference to the bean.
							destroySingleton(beanName);
							throw ex;
						}
					});
					// FactoryBean的调用入口
					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}

step2: 如果要获取到 FactoryBean 类本身，就必须加上&符号，比如 beanFactory.getBean("&beanName") ，如下:

	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.
		// 如果为name不为空，且以前缀&打头，直接返回bean本身
		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;
		}

		// 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;
		}

		

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

	String FACTORY_BEAN_PREFIX = "&";

stet3: BeanFactory.getBean("beanName")只能获取到 getObject()方法返回的实例。getObject 方法返回的实例会有单独的缓存存储，跟其他实例不是同一个缓存，对应的缓存是：factoryBeanObjectCache

                // 如果是不是以前缀&打头，并且是FactoryBean类型的
		Object object = null;
		if (mbd != null) {
			mbd.isFactoryBean = true;
		}
		else {
			// 从缓存里拿FactoryBean实例
			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());
			// 缓存没有的话，
			object = getObjectFromFactoryBean(factory, beanName, !synthetic);
		}
		return object;
	}

	protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
		if (factory.isSingleton() && containsSingleton(beanName)) {
			synchronized (getSingletonMutex()) {
				// 先从缓存factoryBeanObjectCache取
				Object object = this.factoryBeanObjectCache.get(beanName);
				// 如果缓存为空，
				if (object == null) {
					// 调用getObject方法
					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)) {
							// 最后放到缓存中
							this.factoryBeanObjectCache.put(beanName, object);
						}
					}
				}
				return object;
			}
		}
                ......

小结：具体代码参考 getSingleton 方法之后 getObjectForBeanInstance

• 如果bean实例不是 FactoryBean 类型的或者 name 以&开始的则直接返回实例。
• 如果bean是 FacotyBean 并且不是以&开头， 会通过方法doGetObjectFromFactoryBean 调用FactoryBean 内部继承实现的 getObject 方法，并且判断一级缓存中如果存在该 bean 实例把实例缓存到factoryBeanObjectCache 对应的 map 中，这个是单独缓存 FactoryBean 类型实例的 map。

灵活创建所需实例对象的时候，通过实现FactoryBean接口的getObject方法定义实例化过程。

比如MyBatis提供mybatis-spring项目中的 org.mybatis.spring.SqlSessionFactoryBean

public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {
	// ...省略其他代码
	
	public SqlSessionFactory getObject() throws Exception {
	if (this.sqlSessionFactory == null) {
	  afterPropertiesSet();
	}

	return this.sqlSessionFactory;
	}
}

sqlSessionFactory是SqlSessionFactoryBean的一个属性，它的赋值是在通过回调afterPropertiesSet()方法进行的。 因为SqlSessionFactoryBean实现了InitializingBean接口，所以在Spring初始化Bean的时候，能回调afterPropertiesSet()方法。

public void afterPropertiesSet() throws Exception {
    // buildSqlSessionFactory()方法会根据mybatis的配置进行初始化。
	this.sqlSessionFactory = buildSqlSessionFactory();
}

在上面的afterPropertiesSet()方法中，buildSqlSessionFactory()方法会根据mybatis的配置，完成客户所需要的的sessionFactory的初始化。