spring mybatis 动态代理

在spring和mybatis整合使用时候，一般会在配置文件中这样配置：

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<!-- mapper接口包 -->
<property name="basePackage"
value="com.mapper.*" />
<!-- mapper接口需要用到sqlSessionFactory 如果上下文环境只有一个数据源 则不需要配置 -->
<property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" />
</bean>这样配置后当使用mapper包下的dao接口时候，我们只需要自动注入即可，也就是说在spring容器中已经注册了dao的实例，mybatis是如何实现的呢
首先需要说明的是我们也可以对一个dao接口注入一个MapperFactoryBean，那么就会出现这个dao的实例

<bean id="userMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">
<property name="mapperInterface" value="org.mybatis.spring.sample.mapper.UserMapper" />
public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T>
@Override
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);//生成注册代理类
} catch (Throwable t) {
logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", t);
throw new IllegalArgumentException(t);
} finally {
ErrorContext.instance().reset();
}
}
}代理类的生成主要使用了jdk的动态代理，主要的类在org.apache.ibatis.binding包下
那么MapperScannerConfigurer在扫描了所有dao后只需要为每个dao生成对应的MapperFactoryBean，并注册到spring容器中即可生成对应dao的实例

public class MapperScannerConfigurer implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware

父类ClassPathMapperScanner中的doScan方法
@Override
public Set<BeanDefinitionHolder> doScan(String... basePackages) {
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);//设置类类型

definition.getPropertyValues().add("addToConfig", this.addToConfig);

boolean explicitFactoryUsed = false;
if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
explicitFactoryUsed = true;
} else if (this.sqlSessionFactory != null) {
definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
explicitFactoryUsed = true;
}

if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
if (explicitFactoryUsed) {
logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
explicitFactoryUsed = true;
} else if (this.sqlSessionTemplate != null) {
if (explicitFactoryUsed) {
logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
explicitFactoryUsed = true;
}

if (!explicitFactoryUsed) {
if (logger.isDebugEnabled()) {
logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
}
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
}
}

return beanDefinitions;
}