Spring bean 之 FactoryBean

carl_zhao

原文地址:http://www.carlzone.cn/spring...

Spring中有两种类型的Bean,一种是普通Bean,另一种是工厂Bean,即FactoryBean。Spring FactoryBean是创建复杂的bean,一般的bean直接用xml配置即可,如果一个bean的创建过程中涉及到很多其他的bean和复杂的逻辑,用xml配置比较困难,这时可以考虑用FactoryBean.
这两种Bean都被容器管理,但工厂Bean跟普通Bean不同,其返回的对象不是指定类的一个实例,其返回的是该FactoryBean的getObject方法所返回的对象。在Spring框架内部,有很多地方有FactoryBean的实现类,它们在很多应用如(Spring的AOP、ORM、事务管理)及与其它第三框架(ehCache)集成时都有体现,下面简单分析FactoryBean的用法。

1、FactoryBean用法

1)实现FactoryBean接口

/**
 * Created by Carl on 2016/8/12.
 */
public class FactoryBeanTest implements FactoryBean<Object> {

    private boolean flag;

    public void setFlag(boolean flag){
        this.flag = flag;
    }

    // 返回这个Bean的实例
    @Override
    public Object getObject() throws Exception {
        return flag ? "carl" : new Date();
    }

    // 返回这个类类型
    @Override
    public Class<?> getObjectType() {
        return flag ? String.class : Date.class;
    }

    // 是否为单例
    @Override
    public boolean isSingleton() {
        return true;
    }
}

2)配置XML将Bean纳入Spring管理

<?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.xsd">

    <bean id="factoryBeanTest1" class="com.weimob.carl.user.dto.FactoryBeanTest">
        <property name="flag" value="true" />
    </bean>

    <bean id="factoryBeanTest2" class="com.weimob.carl.user.dto.FactoryBeanTest">
        <property name="flag" value="false" />
    </bean>

</beans>

3)Test

public class Main {

    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("application-test.xml");
        String string = context.getBean("factoryBeanTest1", String.class);
        Date date = context.getBean("factoryBeanTest2", Date.class);
        System.out.println(string);
        System.out.println(date);
    }

}

通过简单的测试可知,该类输出如下:
这里写图片描述

2、实现原理

这里写图片描述
大家都知道应该知道BeanFactory在Spring IOC中的作用.它定义了Spring容器的基本方法。其中就包含getBean.由上面的方法调用图我们就可以看到BeanFactory与FactoryBean的关系。下面我们具体看一看代码实现:
1)org.springframework.beans.factory.support.AbstractBeanFactory#getObjectForBeanInstance

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

    // 如果这里不是对FactoryBean的调用,那么结束处理
    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());
        // 这里从FactoryBean中得到Bean
        object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
}

2)org.springframework.beans.factory.support.FactoryBeanRegistrySupport#getObjectFromFactoryBean

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
    if (factory.isSingleton() && containsSingleton(beanName)) {
        synchronized (getSingletonMutex()) {
            // 从cache中获取这个对象
            Object object = this.factoryBeanObjectCache.get(beanName);
            if (object == null) {
                // 从FactoryBean获取这个对象
                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 (object != null && shouldPostProcess) {
                        try {
                            object = postProcessObjectFromFactoryBean(object, beanName);
                        }
                        catch (Throwable ex) {
                            throw new BeanCreationException(beanName,
                                    "Post-processing of FactoryBean's singleton object failed", ex);
                        }
                    }
                    this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
                }
            }
            return (object != NULL_OBJECT ? object : null);
        }
    }
    else {
        // 从FactoryBean获取这个对象
        Object object = doGetObjectFromFactoryBean(factory, beanName);
        if (object != null && shouldPostProcess) {
            try {
                object = postProcessObjectFromFactoryBean(object, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
            }
        }
        return object;
    }
}

3)org.springframework.beans.factory.support.FactoryBeanRegistrySupport#doGetObjectFromFactoryBean

private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
        throws BeanCreationException {
    Object object;
    try {
        if (System.getSecurityManager() != null) {
            AccessControlContext acc = getAccessControlContext();
            try {
                object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                    @Override
                    public Object run() throws Exception {
                            // 最终调用FactoryBean.getObject()方法
                            return factory.getObject();
                        }
                    }, acc);
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            // 最终调用FactoryBean.getObject()方法
            object = factory.getObject();
        }
    }
    catch (FactoryBeanNotInitializedException ex) {
        throw new BeanCurrentlyInCreationException(beanName, ex.toString());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
    }

    // Do not accept a null value for a FactoryBean that's not fully
    // initialized yet: Many FactoryBeans just return null then.
    if (object == null && isSingletonCurrentlyInCreation(beanName)) {
        throw new BeanCurrentlyInCreationException(
                beanName, "FactoryBean which is currently in creation returned null from getObject");
    }
    return object;
}

现在大家是不是对FactoryBean与BeanFactory这2个在Spring中非常重要的2个对象理解的很清楚了。

3、FactoryBean的存在价值

上面返回的已经是作为工厂的FactoryBean生产的产品,而不是FactoryBean本身。这种FactoryBean的机制可以为我们提供一个很好的封装机制,比如封装Proxy、RMI/JNDI等。通过对FactoryBean实现过程的原理进行分析,相信大家会对getObject有很深刻的印象。这个方法就是主要的FactoryBean的接口,需要实现特定的工厂的生产过程,至于这个生产过程是怎么和IoC容器整合的,就是在上面的分析的内容。

4、FactoryBean与设计模式

下图是一个典型的工厂模式的UML图。在这里我们可以看看设计模式中的工厂模式,做一个对比,以加深对这些代码的理解。
这里写图片描述
对比两者的实现,可以看到FactoryBean类似于AbstractFactory抽象工厂,getObjectForBeanInstance()方法类似于createProductA()这样的生产接口,而具体的FactoryBean实现,如TransactionProxyFactoryBean,就是具体的工厂实现,其生成出的TransactionProxy就是"抽象工厂"模式对应的ConcreteProduct.有了抽象工厂设计模式的参考与对比。对FactoryBean的设计和实现就更容易理解一些了。

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