转：Spring FactoryBean源码浅析

http://blog.csdn.net/java2000_wl/article/details/7410714

在Spring BeanFactory容器中管理两种bean  

1.标准Java Bean  

2,另一种是工厂Bean,   即实现了FactoryBean接口的bean  它不是一个简单的Bean 而是一个生产或修饰对象生成的工厂Bean

在向Spring容器获得bean时  对于标准的java Bean  返回的是类自身的实例 

而FactoryBean 其返回的对象不一定是自身类的一个实例，返回的是该工厂Bean的getObject方法所返回的对象

一个简单的例子

public class SayHelloFactoryBeanImpl implements FactoryBean {

    /**
     * 返回该工厂生成的bean
     */
    public Object getObject() throws Exception {
        return new ChinaSayHelloServiceImpl();
    }

    /**
     * getObject返回对象对应的Class
     */
    public Class getObjectType() {
        return ChinaSayHelloServiceImpl.class;
    }

    /**
     * getObject返回的对象 是否是一个单例
     */
    public boolean isSingleton() {
        return false;
    }
}

配置文件
<bean id="sayHelloFactoryBean" class="com.xx.service.impl.SayHelloFactoryBeanImpl" />

ApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"classpath:applicationContext-server.xml"}, true);
//bean的 getObject方法 返回的对象
Object object = context.getBean("sayHelloFactoryBean");
System.out.println(object);

控制台输出

com.xx.service.impl.ChinaSayHelloServiceImpl@1f66cff

容器返回的是 bean getObject方法返回对象  而不是SayHelloFactoryBeanImpl自身的实例 当然可以用“&”符号转义 获得FactoryBean的自身实例

ApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"classpath:applicationContext-server.xml"}, true);
        //可以用转义符"&"来获得FactoryBean本身实例
        System.out.println(context.getBean("&sayHelloFactoryBean"));

控制台输出

com.xx.service.impl.SayHelloFactoryBeanImpl@75e4fc

下面看看FactoryBean是怎么实现的

Spring  FactoryBean接口定义

public interface FactoryBean {

    Object getObject() throws Exception;

    Class getObjectType();

    boolean isSingleton();
}

bean的实例化 是在AbstractBeanFactory getBean方法发生的

public Object getBean(String name, Class requiredType, Object[] args) throws BeansException {
        return doGetBean(name, requiredType, args, false);
    }

protected Object doGetBean(final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException {
        // bean name处理  如果是以"&"开头  截取掉
        final String beanName = transformedBeanName(name);
        Object bean = null;
        //单例的bean 只实例化一次  第一次实例化后会放到一个Map中 即singletonObjects map集合中  下次使用的时候直接拿
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
            // FactoryBean 相关处理 在此方法发生
            //name 调用getBean时传入的参数
            //beanName 截取"&"后的name
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            ...略
        }
        return bean;
｝

protected Object getObjectForBeanInstance(
        Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
    // 如果不是FactoryBean的相关调用 结束处理
    //isFactoryDereference 方法判断name 是不是以"&"开始  如果以"&"开始 返回true
    if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
        throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
    }
    if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
        return beanInstance;
    }
    Object object = null;
    //bean的定义为null
    if (mbd == null) {
        //缓存集合factoryBeanObjectCache中包含  当前bean getObject方法返回的实例  不需要在调用 直接返回
        object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
        FactoryBean factory = (FactoryBean) beanInstance;
        //containsBeanDefinition方法->  bean的定义map beanDefinitionMap集合中 是否包含该bean的定义
        if (mbd == null && containsBeanDefinition(beanName)) {
            mbd = getMergedLocalBeanDefinition(beanName);
        }
        boolean synthetic = (mbd != null && mbd.isSynthetic());
        // FactoryBean getObject触发   并缓存到factoryBeanObjectCache集合中
        object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
}

protected Object getObjectFromFactoryBean(FactoryBean factory, String beanName, boolean shouldPostProcess) {
    //当前的factoryBean是否单例  并且  缓存singletonObjects‘Map中包含FactoryBean的自身实例
    if (factory.isSingleton() && containsSingleton(beanName)) {
        synchronized (getSingletonMutex()) {
            // factoryBeanObjectCache 缓存的是 getObject返回的对象
            Object object = this.factoryBeanObjectCache.get(beanName);
            if (object == null) {
                //getObject方法调用
                object = doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
                //缓存 getObject方法返回的实例对象
                this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
            }
            return (object != NULL_OBJECT ? object : null);
        }
    }
    else {
        //getObject方法调用
        return doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
    }
}

private Object doGetObjectFromFactoryBean(
        final FactoryBean factory, final String beanName, final boolean shouldPostProcess)
        throws BeanCreationException {
    AccessControlContext acc = AccessController.getContext();
    return AccessController.doPrivileged(new PrivilegedAction() {
        public Object run() {
            Object object;

            try {
                // getObject方法调用
                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);
            }
            if (object == null && isSingletonCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(
                        beanName, "FactoryBean which is currently in creation returned null from getObject");
            }

            if (object != null && shouldPostProcess) {
                try {
                    object = postProcessObjectFromFactoryBean(object, beanName);
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(beanName, "Post-processing of the FactoryBean's object failed", ex);
                }
            }

            return object;
        }
    }, acc);
}

当一个受Spring容器管理的bean  如果实现了FactoryBean接口  在bean实例化(getBean)阶段  Spring会调用该bean的getObejct方法 返回的不一定是自身的实例

Spring 框架中有很多FactoryBean   例如RmiProxyFactoryBean, SqlMapClientFactoryBean. LocalSessionFactoryBean等都是通过FactoryBean getObject方法驱动起来的.对bean的生产 修饰做了很好的封装。