MapperScan的工作，Spring-Mybatis怎么自动getMapper

@MapperScan

@Import(MapperScannerRegistrar.class)
@Repeatable(MapperScans.class)
public @interface MapperScan {
    ...
}

导入了MapperScannerRegistrar，这里面registerBeanDefinitions方法中new了一个ClassPathMapperScanner，调用doScan方法进行spring包扫描，会返回对应的BeanDefinition。

//ClassPathMapperScanner
@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 {
      processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
  }

在ClassPathMapperScanner中doScan会调用父类的doScan方法，这里面会通过BeanDefinition注册Bean

registerBeanDefinition(definitionHolder, this.registry);

在ClassPathMapperScanner的processBeanDefinitions中会设置BeanDefinition的构造参数，BeanClass等属性。

到现在为止,Spring的BeanFactory中已经有了很多MapperFactoryBean（每一个都是不同的Mapper接口类型），然后MapperFactoryBean继承了DaoSupport,在afterPropertiesSet方法中会执行checkDaoConfig();

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

		// Let concrete implementations initialize themselves.
		try {
			initDao();
		}
		catch (Exception ex) {
			throw new BeanInitializationException("Initialization of DAO failed", ex);
		}
	}

在MapperFactoryBean的checkDaoConfig方法中，会注册Mapper到Configuration的mapperRegistry中。

//MapperFactoryBean
@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);
      } catch (Exception e) {
        logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", e);
        throw new IllegalArgumentException(e);
      } finally {
        ErrorContext.instance().reset();
      }
    }
  }

然后从容器中拿Mapper的Bean的时候，实际调用MapperFactoryBean的getObject方法，会拿到对应得Mapper。

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

后面就是Mybatis中的内容，会通过MapperProxyFactory new 一个MapperProxy（InnovationHandler），然后里面有MapperMethod的逻辑。

这里走Mybatis的逻辑。

//DefaultSqlSession
@Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

//Configuration
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

//MapperRegistry
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    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);
    }
  }

MapperFactoryBean这中间还有封装，MapperFactoryBean继承了SqlSessionDaoSupport，SqlSessionDaoSupport中装饰了SqlSessionTemplate.（SqlSessionTemplate代理了SqlSession）

SqlSessionTemplate装饰了SqlSessionFactory，通过SqlSessionFactory，Proxy newInstance了一个SqlSession实例（sqlSessionProxy），使用了SqlSessionInterceptor（InnovationHandler）这个代理。在这里面进行SQLSession的管理，然后在给SQLSession本身执行。

SqlSessionTemplate

MybatisAutoConfiguration 注入了SqlSessionTemplate Bean在IOC容器中。

SqlSessionTemplate 实际上是Spring对于SqlSession的管理，自动创建，自动关闭。调用SqlSessionTemplate 的方法实际上是调用sqlSessionProxy[SqlSessionInterceptor为InnovationHandler]的方法。

//SqlSessionTemplate
@Override
  public int delete(String statement) {
    return this.sqlSessionProxy.delete(statement);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public int delete(String statement, Object parameter) {
    return this.sqlSessionProxy.delete(statement, parameter);
  }

//MybatisAutoConfiguration
@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);
    }
  }

在SqlSessionInterceptor中会自动创建sqlSession，自动关闭sqlSession。

SqlSessionInterceptor会尝试获取当前线程的sqlsession,如果没有就使用sqlsessionfactory创建。

//SqlSessionTemplate
private class SqlSessionInterceptor implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          // force commit even on non-dirty sessions because some databases require
          // a commit/rollback before calling close()
          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) {
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }