MyBatis整合Spring原理分析

假如不结合Spring框架，我们使用MyBatis时的一个典型使用方式如下：

public class UserDaoTest {private SqlSessionFactory sqlSessionFactory;@Beforepublic void setUp() throws Exception{ClassPathResource resource = new ClassPathResource("mybatis-config.xml");InputStream inputStream = resource.getInputStream();sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);}@Testpublic void selectUserTest(){String id = "{0003CCCA-AEA9-4A1E-A3CC-06D884BA3906}";SqlSession sqlSession = sqlSessionFactory.openSession();CbondissuerMapper cbondissuerMapper = sqlSession.getMapper(CbondissuerMapper.class);Cbondissuer cbondissuer = cbondissuerMapper.selectByPrimaryKey(id);System.out.println(cbondissuer);sqlSession.close();}}

我们首先需要SqlSessionFactory，然后通过SqlSessionFactory的openSession方法获得SqlSession。通过SqlSession获得我们定义的接口的动态代理类（MapperProxy）。当我们整合Spring框架时，我们使用MyBatis的方式简单的“难以想象”：

@Autowore
private CbondissuerMapper cbondissuerMapper;

非常Spring形式的使用方式，直接注入就可以了。那这个是怎么实现的呢？Spring是怎么把上面略显复杂的模板代码省略的呢？我的第一直觉是Spring在启动的时候做了Mybatis的初始化工作，然后一次性获取了所有Mapper接口的动态代理实现类并将其放入Spring容器中进行管理。

下面来验证下自己的猜想对不对。

MyBatis整合Spring原理分析

下面以Spring Boot中的MyBatisAutoConfigration为列子做下简单分析：

  //SqlSessionFactoryBean的最终作用就是解析MyBati配置文件，并最终生成Configration对象，然后生成DefaultSqlSessionFactory,并加入Spring的容器管理。可以看出SqlSessionFactoryBean的作用和SqlSessionFactoryBeanBuilder的作用很像。@Bean@ConditionalOnMissingBeanpublic SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {SqlSessionFactoryBean factory = new SqlSessionFactoryBean();factory.setDataSource(dataSource);factory.setVfs(SpringBootVFS.class);if (StringUtils.hasText(this.properties.getConfigLocation())) {factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));}Configuration configuration = this.properties.getConfiguration();if (configuration == null && !StringUtils.hasText(this.properties.getConfigLocation())) {configuration = new Configuration();}if (configuration != null && !CollectionUtils.isEmpty(this.configurationCustomizers)) {for (ConfigurationCustomizer customizer : this.configurationCustomizers) {customizer.customize(configuration);}}factory.setConfiguration(configuration);if (this.properties.getConfigurationProperties() != null) {factory.setConfigurationProperties(this.properties.getConfigurationProperties());}if (!ObjectUtils.isEmpty(this.interceptors)) {factory.setPlugins(this.interceptors);}if (this.databaseIdProvider != null) {factory.setDatabaseIdProvider(this.databaseIdProvider);}if (StringUtils.hasLength(this.properties.getTypeAliasesPackage())) {factory.setTypeAliasesPackage(this.properties.getTypeAliasesPackage());}if (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());}if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {factory.setMapperLocations(this.properties.resolveMapperLocations());}return factory.getObject();}//创建SqlSessionTemplate这个Bean，这个类动态代理了DefaultSqlSession，所以最后还是调用了DefaultSqlSession，下面会重点分析下SqlSessionTemplate@Bean@ConditionalOnMissingBeanpublic SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {ExecutorType executorType = this.properties.getExecutorType();if (executorType != null) {return new SqlSessionTemplate(sqlSessionFactory, executorType);} else {return new SqlSessionTemplate(sqlSessionFactory);}}

以下是SqlSessionTemplate的源代码，由于源码较多，只贴出重点部分：

public class SqlSessionTemplate implements SqlSession, DisposableBean {private final SqlSessionFactory sqlSessionFactory;private final ExecutorType executorType;//SqlSessionTemplate动态代理了DefaultSqlSession，所用对sqlSessionProxy的调用都会经过代理对象private final SqlSession sqlSessionProxy;private final PersistenceExceptionTranslator exceptionTranslator;....public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,PersistenceExceptionTranslator exceptionTranslator) {notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");notNull(executorType, "Property 'executorType' is required");this.sqlSessionFactory = sqlSessionFactory;this.executorType = executorType;this.exceptionTranslator = exceptionTranslator;this.sqlSessionProxy = (SqlSession) newProxyInstance(SqlSessionFactory.class.getClassLoader(),new Class[] { SqlSession.class },new SqlSessionInterceptor());}....//对sqlSessionProxy的CRUD调用都会先调用到这里private class SqlSessionInterceptor implements InvocationHandler {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {//这步相当于调用sqlSessionFactory的openSesion方法获得SqlSessionSqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,SqlSessionTemplate.this.executorType,SqlSessionTemplate.this.exceptionTranslator);try {//调用DefaultSqlSession的CRUD方法Object result = method.invoke(sqlSession, args);if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {sqlSession.commit(true);}return result;} catch (Throwable t) {Throwable unwrapped = unwrapThrowable(t);if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {// release the connection to avoid a deadlock if the translator is no loaded. See issue #22closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);sqlSession = null;Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);if (translated != null) {unwrapped = translated;}}throw unwrapped;} finally {if (sqlSession != null) {//最后强制关闭SqlSessioncloseSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);}}}}
}

到此为止，整个Spring中获取SqlSessionFactory、获得SqlSession、执行CRUD、以及关闭SqlSession的流程都进行分析了。还有一个有疑问的地方就是Mapper接口的实现类是在什么时候动态生成的。

MapperScan的秘密

我们知道MapperScan是用来扫描Mapper接口的，所以很自然的想到去MapperScan这个类里面一探究竟。

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
//秘密在MapperScannerRegistrar里面
@Import(MapperScannerRegistrar.class)
public @interface MapperScan {String[] value() default {};//设置扫描的包String[] basePackages() default {};Class<?>[] basePackageClasses() default {};Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;Class<? extends Annotation> annotationClass() default Annotation.class;Class<?> markerInterface() default Class.class;String sqlSessionTemplateRef() default "";String sqlSessionFactoryRef() default "";//指定自定义的MapperFactoryBeanClass<? extends MapperFactoryBean> factoryBean() default MapperFactoryBean.class;}

下面就看下MapperScannerRegistrar里面做了什么：

public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {AnnotationAttributes annoAttrs = AnnotationAttributes.fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));//创建了一个ClassPathMapperScanner，并根据@MapperScan里面的配置设置属性ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);// this check is needed in Spring 3.1if (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();//开始扫描scanner.doScan(StringUtils.toStringArray(basePackages));}

以上创建了一个ClassPathMapperScanner，并根据@MapperScan里面的配置设置属性，开始扫描。下面再看ClassPathMapperScanner。

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {GenericBeanDefinition definition;//对扫描到的BeanDefinition做进一步处理for (BeanDefinitionHolder holder : beanDefinitions) {definition = (GenericBeanDefinition) holder.getBeanDefinition();if (logger.isDebugEnabled()) {logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + definition.getBeanClassName() + "' mapperInterface");}definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59//这边是重点，将扫描到的Mapper接口的BeanClass设置为MapperFactoryBrean//MapperFactoryBrean是工厂Bean，用于生成MapperProxy；//Spring在自动注入Mapper的时候会自动调用这个工厂Bean的getObject方法，生成MapperProxy并放入Spring容器。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() + "'.");}definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);}}}

到此Spring自动生成Mapper接口实现类的过程也分析完了。

简单总结

通过以上分析，印证了一开始的猜想：Spring在启动的时候做了Mybatis的初始化工作，然后一次性获取了所有Mapper接口的动态代理实现类并将其放入Spring容器中进行管理。大致流程如下：

• 配置SqlSessionFactoryBean，这个Bean的最终作用是创建DefaultSqlSessionFactory，并将DefaultSqlSessionFactory加入Spring容器管理；
• 创建SqlSessionTemplate，这个对象代理了MyBatis中的DefaultSqlSession，通过SqlSessionTemplate调用任何CRUD方法都会经历openSession、调用DefaultSqlSession的CRUD方法和关闭Session的过程；

Mapper接口生成的流程大致如下：

• @MapperScan注解扫描到所有的Mapper接口生成相应的BeanDefinition；
• ClassPathMapperScanner的处理方法对这些BeanDefinition做进一步配置，其中最终要的一步就是将这些BeanDefinition的Beanclass属性设置为MapperFactoryBean（这是一个工厂Bean，专门用于生成Mapper的动态代理实现MapperProxy，一个Dao接口会对应一个MapperFactoryBean）；
• Spring在检测到需要自动注入Mapper时通过层层调用，最终会调用到MapperFactoryBean这个工厂Bean的getObject方法生成对应的MapperProxy，并将这个对象纳入Spring管理。

以上大致就是MyBatis整合进Spring的原理。我们发现其实质和传统的Mybatis使用是一样的，只不过是通过Spring做了一些自定配置等。

分析的有点乱，先这样吧~