Spring源码 - FactoryBean 应用拓展（附源码解析）

前言

在学习Spring Core中IOC容器时，你确定会接触到BeanFactory这个Spring中最基础的IOC容器。这个应该是你们学习Spring源码时最早接触到的类了。Spring中还存在这一个FactoryBean类，二者拼写上十分类似，而且使用频率都十分得高。在一些Spring面试题，也会问你这二者有什么区别。

这里先说结论：

• BeanFactory：Spring中的IoC容器，全部Spring Bean 的Factory
• FactoryBean：一个Bean，一个不简单的Bean，一个能产生对象或者修饰对象生成的工厂Bean，它的实现与设计模式中的工厂模式和修饰器模式相似

在学习Spring源码和其余开源项目的源码的过程中，发现FactoryBean是一些框架在作集成Spring时常常会使用到的类，本文具体讲述的也是FactoryBean的简单实用和具体应用拓展。

What is FactoryBean

Spring 中有两种类型的Bean，一种是普通Bean，另外一种是工厂Bean 即 FactoryBean。

通常状况下，Spring 经过反射机制利用bean的class属性指定实现类来实例化bean 。在某些状况下，实例化bean 过程比较复杂，若是按照传统的方式，则须要在<bean>中提供大量的配置信息，配置方式的灵活性是受限的， 这时采用编码的方式可能会获得一个简单的方案。Spring 为此提供了一个 org.Springframework.bean.factory.FactoryBean的工厂类接口，用户能够经过实现该接口定制实例化bean的逻辑。（看后面的一些例子会理解更深入）

因此说，当配置一个<bean>的过程很是复杂，建立过程当中涉及到不少其余的bean 和复杂的逻辑，用xml配置比较困难，这时能够考虑用FactoryBean。

接口定义

package org.springframework.beans.factory;

public interface FactoryBean<T> {
    T getObject() throws Exception;
    
    Class<?> getObjectType();
    
    boolean isSingleton();
}

在一些开源框架上的使用

MyBatis-Spring # SqlSessionFactoryBean

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="***" />
        <property name="configLocation" value="***"/>
        <property name="mapperLocations" value="***"/>
    </bean>

public class SqlSessionFactoryBean
    implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {
    ...
}

阿里开源的分布式服务框架 Dubbo # ReferenceBean<T>

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:dubbo="http://dubbo.apache.org/schema/dubbo"
    xsi:schemaLocation="http://www.springframework.org/schema/beans        http://www.springframework.org/schema/beans/spring-beans-4.3.xsd        http://dubbo.apache.org/schema/dubbo        http://dubbo.apache.org/schema/dubbo/dubbo.xsd">
 
    <!-- 消费方应用名，用于计算依赖关系，不是匹配条件，不要与提供方同样 -->
    <dubbo:application name="consumer-of-helloworld-app"  />
 
    <!-- 使用multicast广播注册中心暴露发现服务地址 -->
    <dubbo:registry address="multicast://224.5.6.7:1234" />
 
    <!-- 生成远程服务代理，能够和本地bean同样使用demoService -->
    <dubbo:reference id="demoService" interface="org.apache.dubbo.demo.DemoService" />
</beans>

<dubbo:reference 对应的Bean是com.alibaba.dubbo.config.spring.ReferenceBean 类

public class ReferenceBean<T> extends ReferenceConfig<T> implements FactoryBean, ApplicationContextAware, InitializingBean, DisposableBean {
    ...
}

拓展实践

涉及

• ProduceLocation 生产地区
• Material 材料
• ProductFactoryBean 产品工厂bean
• Product 产品
• Boostrap 启动类
• test-config.xml 测试配置文件

ProduceLocation

@Data
public class ProduceLocation {

    private String locationName;

    private double distanceKm;

    private double pricePerPerKm;
}

Material

@Data
public class Material {

    private String name;

    private double pricePerGram;

    private double weight;
}

Product

@Data
@Builder
public class Product {

    private Material material;

    private ProduceLocation location;

    private double price;
}

ProductFactoryBean

@Setter
@Getter
public class ProductFactoryBean implements FactoryBean<Product> {

    private Material material;

    private ProduceLocation produceLocation;

    @Override
    public Product getObject() throws Exception {
        return Product.builder()
                .location(produceLocation)
                .material(material)
                .price(cal(material, produceLocation))
                .build();
    }

    private double cal(Material material, ProduceLocation produceLocation) {
        return material.getPricePerGram() * material.getWeight()
                + produceLocation.getDistanceKm() * produceLocation.getPricePerPerKm();
    }

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

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

test-config.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd">

    <bean id="produceLocation" class="base.ioc.FactoryBeanDemoSet.ProduceLocation">
        <property name="locationName" value="杭州"/>
        <property name="pricePerPerKm" value="151.01"/>
        <property name="distanceKm" value="3.1"/>
    </bean>
    <bean id="material" class="base.ioc.FactoryBeanDemoSet.Material">
        <property name="name" value="巧克力豆"/>
        <property name="pricePerGram" value="100"/>
        <property name="weight" value="50"/>
    </bean>
    <bean id="product" class="base.ioc.FactoryBeanDemoSet.ProductFactoryBean">
        <property name="material" ref="material"/>
        <property name="produceLocation" ref="produceLocation"/>
    </bean>
</beans>

Boostrap

/**
 * @author Richard_yyf
 * @version 1.0 2019/9/21
 */
public class Bootstrap {

    public static void main(String[] args) throws Exception {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("test-config.xml");
        Product product = (Product) context.getBean("product");
        System.out.println(product.toString());
    }
}

输出

Product(material=Material(name=巧克力豆, pricePerGram=100.0, weight=50.0), location=ProduceLocation(locationName=杭州, distanceKm=3.1, pricePerPerKm=151.01), price=5468.131)

上述的配置固然也能够改用java config 的方式来作。

上述是一个简单业务的示例，你还能够经过FactoryBean来对一些开源工具API使用进行一些封装，好比对httpClient建立过程作了一些封装，例如超时时间、链接池大小、http代理等。

特性

给定一个id=mybean的FactoryBean，getBean("mybean")获得的就是这个FactoryBean建立的对象实例，而getBean("&mybean")获得的确实FactoryBean自身对象。

根据上述的demo，运行以下代码

/**
 * @author Richard_yyf
 * @version 1.0 2019/9/21
 */
public class Bootstrap {

    public static void main(String[] args) throws Exception {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("test-config.xml");
        // Product product = (Product) context.getBean("product");
        // System.out.println(product.toString());
        
        FactoryBean<Product> factoryBean = (ProductFactoryBean) context.getBean("&product");
        System.out.println(factoryBean.getObject().toString());
    }
}

Output

Product(material=Material(name=巧克力豆, pricePerGram=100.0, weight=50.0), location=ProduceLocation(locationName=杭州, distanceKm=3.1, pricePerPerKm=151.01), price=5468.131)

对应源码

先直接锁定对应逻辑源码，

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

        // Don't let calling code try to dereference the factory if the bean isn't a factory.
        // BeanFactoryUtils.isFactoryDereference(name)方法判断name是否以&前缀
        if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
            throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
        }
        
       // 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) || BeanFactoryUtils.isFactoryDereference(name)) {
            return beanInstance;
        }

        Object object = null;
        if (mbd == null) {
            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());
            object = getObjectFromFactoryBean(factory, beanName, !synthetic);
        }
        return object;
    }

1. BeanFactoryUtils.isFactoryDereference(name) 判断是否在获取FactoryBean的引用

   // 不为空且以”&“开头
       public static boolean isFactoryDereference(String name) {
           return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
       }
       String FACTORY_BEAN_PREFIX = "&";

2. 若是进来的beanInstance不是FactoryBean，但调用者是用获取FactoryBean的引用时，抛出BeanIsNotAFactoryException异常
3. 若是调用者是要获取FactoryBean的引用，且beanInstance 是FactoryBean(前面有判断)，则直接返回beanInstance
4. 若是进来的beanInstance是普通bean，直接返回beanInstance
5. 经过进来的FactoryBean 来建立一个对应的bean