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;
@Before
public void setUp() throws Exception{
ClassPathResource resource = new ClassPathResource("mybatis-config.xml");
InputStream inputStream = resource.getInputStream();
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
}
@Test
public 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
@ConditionalOnMissingBean
public 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
@ConditionalOnMissingBean
public 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 {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//这步相当于调用sqlSessionFactory的openSesion方法获得SqlSession
SqlSession 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 #22
closeSqlSession(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SqlSession
closeSqlSession(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MapperFactoryBean
Class<? 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 3.1
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();
//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~