Mybatis实现【7】 --基于接口编程的原理

MyBatis可以仅通过申明接口并在annotation上注明sql，即可省略配置文件的编写。

这里Mapper是不需要实现类，我们来探究下MyBatis是如何做这层代理的。

bean的注入

1、声明需要注入的包

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">       <!--basePackage指定要扫描的包，可指定多个包，包与包之间用逗号或分号分隔-->       <property name="basePackage" value="com.cyou.fz.*.*.dao,com.cyou.fz.*.*.*.dao,com.cyou.fz.*.*.*.*.dao"/>   </bean>

MapperScannerConfig类描述是：

//BeanDefinitionRegistryPostProcessor that searches recursively starting from a base package for * interfaces and registers them as MapperFactoryBean. Note that only interfaces with at * least one method will be registered; concrete classes will be ignored

MapperScannerConfigurer的Scanner类负责搜索basePackage类下所有的MapperClass并将其注册至spring的beanfinitionHolder中，其注册的classBean为MapperFactoryBean。

1.1、与Spring框架整合注入Bean

MapperScannerConfigurer通过BeanDefinitionRegistryPostProcessor并实现postProcessBeanDefinitionRegistry(...)方法，在Spring初始化时候将Bean及该Bean的一些属性信息（scope、className、beanName等）保存至BeanDefinitionHolder中。

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {   if (this.processPropertyPlaceHolders) {     processPropertyPlaceHolders();   }   ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);   //...   scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS)); }

实际上调用ClassPathMapperScanner的doscan（）方法扫描包并注册

public Set<BeanDefinitionHolder> doScan(String... basePackages) {    //在super.doScan()中将所有Mapper接口的class注册至BeanDefinitionHolder并放回一个set对象    Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);    if (beanDefinitions.isEmpty()) {      logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");    } else {      for (BeanDefinitionHolder holder : beanDefinitions) {        GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition();        if (logger.isDebugEnabled()) {          logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName()               + "' and '" + definition.getBeanClassName() + "' mapperInterface");        }        // the mapper interface is the original class of the bean        // but, the actual class of the bean is MapperFactoryBean        definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());        definition.setBeanClass(MapperFactoryBean.class);        ///...    return beanDefinitions;  }

MapperFactoryBean直接实现了Spring的FactoryBean及InitializingBean接口，作为专门生产Mapper实例的工厂

1.2、通过MapperFactoryBean获取Mapper实例

1.2.1、在创建Mapper实例时，首先在MapperFactoryBean中执行afterPropertiesSet():

public final void afterPropertiesSet() throws IllegalArgumentException, BeanInitializationException {        // Let abstract subclasses check their configuration.        checkDaoConfig();        //...    }

checkDaoConfig()如下：

//检测当前需要创建的mapperInterface在Configuration中是否存在，不存在则添加protected void checkDaoConfig() {    super.checkDaoConfig();    notNull(this.mapperInterface, "Property 'mapperInterface' is required");    Configuration configuration = getSqlSession().getConfiguration();    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {      try {        configuration.addMapper(this.mapperInterface);      } catch (Throwable t) {        logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", t);        throw new IllegalArgumentException(t);      } finally {        ErrorContext.instance().reset();      }    }  }

1.2.2、获取Mapper实例

在执行完afterPropertiesSet（）方法后，执行getObject（）方法来获得Mapper实例：

public T getObject() throws Exception {  return getSqlSession().getMapper(this.mapperInterface);}

通过调用链，最终调用MapperRegistry的getMapper（）方法：

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {  //这里的knowMappers保存之前afterPropertiesSet中保存进来的mapperInterface  final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);  if (mapperProxyFactory == null)    throw new BindingException("Type " + type + " is not known to the MapperRegistry.");  try {    return mapperProxyFactory.newInstance(sqlSession);  } catch (Exception e) {    throw new BindingException("Error getting mapper instance. Cause: " + e, e);  }}

在mapperProxyFactory的newInstance中：

public T newInstance(SqlSession sqlSession) {   final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);   return newInstance(mapperProxy); }protected T newInstance(MapperProxy<T> mapperProxy) {   return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); }

这里创建了MapperProxy对象。并与“Mybatis实现【4】”中查询的执行过程对接起来。

该系列文章参考如下书籍及文章：

《Java Persistence with MyBatis 》

《http://www.cnblogs.com/hzhuxin/p/3349836.html》

《http://www.iteye.com/topic/1112327》

《http://www.iteye.com/blogs/subjects/mybatis_internals》

《http://denger.me/2011/05/mybatis-and-spring-interface-integrated/》