MyBatis是一款优秀的持久层构架，它支持定制化SQL语句、存储过程以及高级映射，可以通过简单的XML或注解配置和映射原生信息，将POJOs映射成数据库中的记录，从而方便进行增删改查的操作(官网)
问题
Spring如何与MyBatis连接起来的？如何去获取Mybatis中的Mapper,以及获取Mapper时干了哪些骚操作？
为了弄清楚上述问题，菜菜首先贴出开发时主要用到的MyBatis配置，随后进行源码解读，本文采用的是注解式配置。
一般MyBatis的配置如下所示：
@Configuration
@EnableTransactionManagement(proxyTargetClass = true)
@PropertySource({"classpath:jdbc.properties"})
@MapperScan(basePackages = "me.ele.eliter.**.mapper")
public class MyBatisConfig {
private static final Log LOG = LogFactory.getLog(MyBatisConfig.class);
@Bean(destroyMethod = "close")
public DataSource dataSource(
@Value("${db.driver}") String driverClass,
@Value("${db.url}") String jdbcUrl,
@Value("${db.username}") String userName,
@Value("${db.password}") String passWord) throws Exception {
ComboPooledDataSource dataSource = new ComboPooledDataSource();
dataSource.setDriverClass(driverClass);
dataSource.setJdbcUrl(jdbcUrl);
dataSource.setUser(userName);
dataSource.setPassword(passWord);
return dataSource;
}
@Bean
public PlatformTransactionManager transactionManager(DataSource dataSource) {
DataSourceTransactionManager dataSourceTransactionManager = new DataSourceTransactionManager(dataSource);
dataSourceTransactionManager.afterPropertiesSet();
return dataSourceTransactionManager;
}
@Bean
public SqlSessionFactoryBean sqlSessionFactoryBean(DataSource dataSource, ApplicationContext applicationContext) throws Exception {
SqlSessionFactoryBean sessionFactory = new SqlSessionFactoryBean();
sessionFactory.setDataSource(dataSource);
sessionFactory.setConfigLocation(applicationContext.getResource("classpath:mybatis-config.xml"));
sessionFactory.afterPropertiesSet();
sessionFactory.setPlugins(new Interceptor[] { new PagerInterceptor()});
return sessionFactory;
}
@Bean
public TransactionTemplate transactionTemplate(PlatformTransactionManager transactionManager) throws Exception {
TransactionTemplate transactionTemplate = new TransactionTemplate(transactionManager);
transactionTemplate.afterPropertiesSet();
return transactionTemplate;
}
}
可以看到MyBatisConfig这个类上有几个重要的注解@Configuration与@MapperScan,
@Configuration是Spring中注解，主要说明该类是配置类，Spring在启动时会执行@Bean的方法，并进行Bean的定义，详细信息请自行搜索。
本文主要讲@MapperScan，首先看MapperScan的定义如下
MapperScan
registerBeanDefinitions调用链
MapperScannerRegistrar源码
接下来看MyBatis扫描干了些什么事？以及如何将这些BeanDefinition注册到IOC容器中。
以下是MapperScannerRegistrar实现registerBeanDefinitions的源码
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
//1.获取了注解MapperScan中的信息
AnnotationAttributes annoAttrs = AnnotationAttributes.fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
/*2.将Spring中的registry放入scanner中，ClassPathMapperScanner继承了Spring的ClassPathBeanDefinitionScanner，
在new 的时候调用了ClassPathBeanDefinitionScanner的构造方法，将register传入了ClassPathBeanDefinitionScanner中
*/
ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
//3.一些属性设置操作
// this check is needed in Spring 3.1
if (resourceLoader != null) {
scanner.setResourceLoader(resourceLoader);
}
Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");
if (!Annotation.class.equals(annotationClass)) {
scanner.setAnnotationClass(annotationClass);
}
Class<?> markerInterface = annoAttrs.getClass("markerInterface");
if (!Class.class.equals(markerInterface)) {
scanner.setMarkerInterface(markerInterface);
}
Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");
if (!BeanNameGenerator.class.equals(generatorClass)) {
scanner.setBeanNameGenerator(BeanUtils.instantiateClass(generatorClass));
}
Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");
if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {
scanner.setMapperFactoryBean(BeanUtils.instantiateClass(mapperFactoryBeanClass));
}
scanner.setSqlSessionTemplateBeanName(annoAttrs.getString("sqlSessionTemplateRef"));
scanner.setSqlSessionFactoryBeanName(annoAttrs.getString("sqlSessionFactoryRef"));
List<String> basePackages = new ArrayList<String>();
for (String pkg : annoAttrs.getStringArray("value")) {
if (StringUtils.hasText(pkg)) {
basePackages.add(pkg);
}
}
for (String pkg : annoAttrs.getStringArray("basePackages")) {
if (StringUtils.hasText(pkg)) {
basePackages.add(pkg);
}
}
for (Class<?> clazz : annoAttrs.getClassArray("basePackageClasses")) {
basePackages.add(ClassUtils.getPackageName(clazz));
}
scanner.registerFilters();
//4.具体执行扫描
scanner.doScan(StringUtils.toStringArray(basePackages));
}
从上面源码中可以看到在Spring调用registerBeanDefinitions时传入了BeanDefinitionRegistry(registry),有了registry就可以将MyBatis中的Mapper的BeanDefinition信息注册到Spring的IOC容器中，下面具体看下registerBeanDefinitions源码，首先是获取了MapperScan注解中的属性信息，接着将Sping中的register传入scanner,这样可将scanner扫描到的BeanDefinition信息注册到IOC容器中，最后是具体的扫描操作，下面看下ClassPathMapperScanner中具体扫描操作的源码
@Override
public Set<BeanDefinitionHolder> doScan(String... basePackages) {
/*首先调用了Spring中的ClassPathBeanDefinitionScanner扫描类进行扫描得到了具体的beanDefinitions信息，
这一步也将扫描到的beanDefinitions信息注册到了Spring的IOC容器中，这里就不进入Spring源码看了*/
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 {
//这里是MyBatis对这些BeanDefinition做了一系列操作为MyBatis的特性服务
processBeanDefinitions(beanDefinitions);
}
return beanDefinitions;
}
private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
GenericBeanDefinition definition;
for (BeanDefinitionHolder holder : beanDefinitions) {
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
/*这里获取到的是Mapper的接口，设置这些Mapper接口的实现类默认为MapperFactoryBean，偷天换日，
（非常重要）下一篇文章中说明为什么Mapper不用写接口实现类就能运行会再次讲到这*/
definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
definition.setBeanClass(this.mapperFactoryBean.getClass());
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() + "'.");
}
//这里设置Bean中的属性自动根据Type注入
//（非常重要，后面的一些属性的注入, 如sqlSessionFactory会自动注入全仗于此）
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
}
}
ClassPathMapperScanner继承了Spring的ClassPathBeanDefinitionScanner，doScan方法
1.调用了ClassPathBeanDefinitionScanner里的doScan方法(扫描方式与Spring的扫描一致)，以上MapperScannerRegistrar源码注释2说明了ClassPathBeanDefinitionScanner拥有了Spring的register,这样在得到MyBatis的Mapper后，就可将其中的BeanDefinition注册到Spring中。
2.通过调用processBeanDefinitions对BeanDefinition做了一些重要处理
2.1 definition.setBeanClass(this.mapperFactoryBean.getClass())设置Mapper接口的class对象或是类的全限定名为MapperFactoryBean(可以理解为实现是MapperFactoryBean),这里是MyBatis只用写接口不用写实现的第一步，具体做是通过JDK动态代理，具体细节见Mybatis源码解析之MapperProxy
2.2 definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE设置了这个bean中的属性是根据Type自动注入，加上2.1后这个也就是说会自动调用MapperFactoryBean中的set方法，sqlSessionFactory就是通过这种方式注入的，进而得到sqlSesion,这方面的详细信息见Mybatis源码解析之SqlSession来自何方
至此，Spring中已经获取到了MyBatis有哪些mapper，并且这些mapper接口的BeanDefinition信息经过了MaBatis的处理，比如设置接口的默认实现类为MapperFactoryBean等等，下一步就是解析MaBatis中配置的xml信息
如有错误，欢迎各位大佬斧正！
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