spring SqlSessionFactoryBean创建SqlSessionFactory

原文地址：http://blog.csdn.net/wdyr321/article/details/11763979

Application.xml

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
  <property name="dataSource" ref="dataSource" />
  <property name="configLocation" value="classpath:SQL_MAP_SERVICE_CONFIG.xml" />
bean>

创建SqlSessionFactory

ApplicationContext ctx = new ClassPathXmlApplicationContext("APPLICATION.xml");
SqlSessionFactory sqlSessionFactory=(SqlSessionFactory) ctx.getBean("sqlSessionFactory");

为什么返回的是SqlSessionFactory对象而不是SqlSessionFactoryBean
首先spring在初始化的时候会将所有创建的单例以Map的形式放入singletonObjects，同时调用FactoryBean的getObject()将返回的对象以Map的形式放入factoryBeanObjectCache

如{sqlSessionFactory=org.apache.ibatis.session.defaults.DefaultSqlSessionFactory@d997f9}

然后getBean的时候，spring利用getSingleton从singletonObjects获取单例(这里是SqlSessionFactoryBean对象)，判断是否实现了FactoryBean接口，若实现了就从factoryBeanObjectCache利用beanname(这里是sqlSessionFactory)重新获取，若未实现则返回getSingleton获取的单例

初始化的源码有些复杂，理不清，下面仅仅是getBean的源码：

1、首先调用的是ClassPathXmlApplicationContext父类AbstractRefreshableApplicationContext的父类AbstractApplicationContext的getBean()。

public Object getBean(String name) throws BeansException {
    return getBeanFactory().getBean(name);
}

2、调用AbstractRefreshableApplicationContext的getBeanFactory()返回ConfigurableListableBeanFactory接口实现类DefaultListableBeanFactory

@Override(重写了AbstractApplicationContext的getBeanFactory抽象方法)
    public final ConfigurableListableBeanFactory getBeanFactory() {
        synchronized (this.beanFactoryMonitor) {
            if (this.beanFactory == null) {
                throw new IllegalStateException("BeanFactory not initialized or already closed - " +
                        "call 'refresh' before accessing beans via the ApplicationContext");
            }
            return this.beanFactory;
        }
    }

3、调用DefaultListableBeanFactory的父类AbstractBeanFactory的geBean()

public Object getBean(String name) throws BeansException {
        return doGetBean(name, null, null, false);
    }

4、调用AbstractBeanFactory的doGetBean()

protected  T doGetBean(
            final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {

        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        Object sharedInstance = getSingleton(beanName);//根据bean路径获取单例
        if (sharedInstance != null && args == null) {
            if (logger.isDebugEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

5、调用AbstractBeanFactory的getObjectForBeanInstance()，判断是否实现FactoryBean接口，若未实现则返回上一步创建的单例

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.
        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;
        }//判断是否实现FactoryBean接口

        Object object = null;
        if (mbd == null) {
            object = getCachedObjectForFactoryBean(beanName);//从factoryBeanObjectCache中取出FactoryBean创建的对象
        }
        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;
    }