SpringBoot整合MyBatis原理

张申傲

发布于 2020-09-03 15:59:00

7310

发布于 2020-09-03 15:59:00

文章被收录于专栏：漫漫架构路漫漫架构路

SpringBoot整合MyBatis原理

一. 自动配置

SpringBoot提供了MyBatis的自动配置类MybatisAutoConfiguration，可以自动注册SqlSessionFactory、SqlSessionTemplate等组件，开发人员只需在配置文件中指定相关属性即可。

@Configuration
@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })
@ConditionalOnBean(DataSource.class)
@EnableConfigurationProperties(MybatisProperties.class)
@AutoConfigureAfter(DataSourceAutoConfiguration.class)
public class MybatisAutoConfiguration {

  private static Log log = LogFactory.getLog(MybatisAutoConfiguration.class);

  @Autowired
  private MybatisProperties properties;

  @Autowired(required = false)
  private Interceptor[] interceptors;

  @Autowired
  private ResourceLoader resourceLoader = new DefaultResourceLoader();

  @Autowired(required = false)
  private DatabaseIdProvider databaseIdProvider;

  //检查配置文件路径
  @PostConstruct
  public void checkConfigFileExists() {
    if (this.properties.isCheckConfigLocation() && StringUtils.hasText(this.properties.getConfigLocation())) {
      Resource resource = this.resourceLoader.getResource(this.properties.getConfigLocation());
      Assert.state(resource.exists(), "Cannot find config location: " + resource
                   + " (please add config file or check your Mybatis " + "configuration)");
    }
  }

  //注册SqlSessionFactory
  @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()));
    }
    factory.setConfiguration(properties.getConfiguration());
    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();
  }

  //注册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);
    }
  }

  //注册ClassPathMapperScanner，进行Mapper的自动扫描，也可以使用@MapperScan代替
  public static class AutoConfiguredMapperScannerRegistrar
    implements BeanFactoryAware, ImportBeanDefinitionRegistrar, ResourceLoaderAware {

    private BeanFactory beanFactory;

    private ResourceLoader resourceLoader;

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

      log.debug("Searching for mappers annotated with @Mapper'");

      ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);

      try {
        if (this.resourceLoader != null) {
          scanner.setResourceLoader(this.resourceLoader);
        }

        List<String> pkgs = AutoConfigurationPackages.get(this.beanFactory);
        for (String pkg : pkgs) {
          log.debug("Using auto-configuration base package '" + pkg + "'");
        }

        scanner.setAnnotationClass(Mapper.class);
        scanner.registerFilters();
        scanner.doScan(StringUtils.toStringArray(pkgs));
      } catch (IllegalStateException ex) {
        log.debug("Could not determine auto-configuration " + "package, automatic mapper scanning disabled.");
      }
    }

    @Override
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
      this.beanFactory = beanFactory;
    }

    @Override
    public void setResourceLoader(ResourceLoader resourceLoader) {
      this.resourceLoader = resourceLoader;
    }
  }

  //如果程序中没有显式注入MapperFactoryBean，就打印日志
  @Configuration
  @Import({ AutoConfiguredMapperScannerRegistrar.class })
  @ConditionalOnMissingBean(MapperFactoryBean.class)
  public static class MapperScannerRegistrarNotFoundConfiguration {

    @PostConstruct
    public void afterPropertiesSet() {
      log.debug(String.format("No %s found.", MapperFactoryBean.class.getName()));
    }
  }

}

SpringBoot为我们自动注册了相应的组件：

SqlSessionFactoryBean：用于构建MyBatis的SqlSessionFactory
SqlSessionTemplate：MyBatis的代理类，将SqlSession与Spring的事务进行了整合
ClassPathMapperScanner：提供MyBatis的Mapper的自动扫描

二. SqlSessionFactoryBean

SqlSessionFactoryBean是一个工厂Bean，其作用就是加载用户自定义的配置，然后使用MyBatis的API创建一个SqlSessionFactory，具体逻辑如下：

protected SqlSessionFactory buildSqlSessionFactory() throws IOException {

  Configuration configuration;

  XMLConfigBuilder xmlConfigBuilder = null;
  if (this.configuration != null) {
    configuration = this.configuration;
    if (configuration.getVariables() == null) {
      configuration.setVariables(this.configurationProperties);
    } else if (this.configurationProperties != null) {
      configuration.getVariables().putAll(this.configurationProperties);
    }
  } else if (this.configLocation != null) {
    xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
    configuration = xmlConfigBuilder.getConfiguration();
  } else {
    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug("Property `configuration` or 'configLocation' not specified, using default MyBatis Configuration");
    }
    configuration = new Configuration();
    configuration.setVariables(this.configurationProperties);
  }

  if (this.objectFactory != null) {
    configuration.setObjectFactory(this.objectFactory);
  }

  if (this.objectWrapperFactory != null) {
    configuration.setObjectWrapperFactory(this.objectWrapperFactory);
  }

  if (this.vfs != null) {
    configuration.setVfsImpl(this.vfs);
  }

  if (hasLength(this.typeAliasesPackage)) {
    String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
                                                           ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
    for (String packageToScan : typeAliasPackageArray) {
      configuration.getTypeAliasRegistry().registerAliases(packageToScan,
                                                           typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Scanned package: '" + packageToScan + "' for aliases");
      }
    }
  }

  if (!isEmpty(this.typeAliases)) {
    for (Class<?> typeAlias : this.typeAliases) {
      configuration.getTypeAliasRegistry().registerAlias(typeAlias);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Registered type alias: '" + typeAlias + "'");
      }
    }
  }

  if (!isEmpty(this.plugins)) {
    for (Interceptor plugin : this.plugins) {
      configuration.addInterceptor(plugin);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Registered plugin: '" + plugin + "'");
      }
    }
  }

  if (hasLength(this.typeHandlersPackage)) {
    String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
                                                              ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
    for (String packageToScan : typeHandlersPackageArray) {
      configuration.getTypeHandlerRegistry().register(packageToScan);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Scanned package: '" + packageToScan + "' for type handlers");
      }
    }
  }

  if (!isEmpty(this.typeHandlers)) {
    for (TypeHandler<?> typeHandler : this.typeHandlers) {
      configuration.getTypeHandlerRegistry().register(typeHandler);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Registered type handler: '" + typeHandler + "'");
      }
    }
  }

  if (this.databaseIdProvider != null) {//fix #64 set databaseId before parse mapper xmls
    try {
      configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
    } catch (SQLException e) {
      throw new NestedIOException("Failed getting a databaseId", e);
    }
  }

  if (this.cache != null) {
    configuration.addCache(this.cache);
  }

  if (xmlConfigBuilder != null) {
    try {
      xmlConfigBuilder.parse();

      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Parsed configuration file: '" + this.configLocation + "'");
      }
    } catch (Exception ex) {
      throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  if (this.transactionFactory == null) {
    this.transactionFactory = new SpringManagedTransactionFactory();
  }

  configuration.setEnvironment(new Environment(this.environment, this.transactionFactory, this.dataSource));

  if (!isEmpty(this.mapperLocations)) {
    for (Resource mapperLocation : this.mapperLocations) {
      if (mapperLocation == null) {
        continue;
      }

      try {
        XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
                                                                 configuration, mapperLocation.toString(), configuration.getSqlFragments());
        xmlMapperBuilder.parse();
      } catch (Exception e) {
        throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
      } finally {
        ErrorContext.instance().reset();
      }

      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'");
      }
    }
  } else {
    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found");
    }
  }

  return this.sqlSessionFactoryBuilder.build(configuration);
}

三. SqlSessionTemplate

SqlSessionTemplate是Spring提供的一个对MyBatis的SqlSession的一个增强类，它的作用就是将SqlSession与当前的事务所绑定，而且是线程安全的，一个SqlSessionTemplate可以被多个dao所共享。

SqlSessionTemplate基于动态代理模式，内部委托了一个SqlSession对象，并且在其基础上进行了增强。代码如下：

//传入的SqlSessionFactory
private final SqlSessionFactory sqlSessionFactory;

//Executor类型
private final ExecutorType executorType;

//SqlSession的动态代理
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;
  
  //创建SqlSession的动态代理
  this.sqlSessionProxy = (SqlSession) newProxyInstance(
    SqlSessionFactory.class.getClassLoader(),
    new Class[] { SqlSession.class },
    new SqlSessionInterceptor());
}

在创建SqlSessionTemplate传入SqlSessionFactory，而通过SqlSessionFactory创建的SqlSession其实是一个动态代理类，其增强的逻辑在SqlSessionInterceptor中定义，如下：

private class SqlSessionInterceptor implements InvocationHandler {
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    //获取当前事务下绑定的SqlSession，由于Spring通过ThreadLocal将线程与事务绑定，所以也可以认为获取的是当前线程绑定的SqlSession
    SqlSession sqlSession = getSqlSession(
      SqlSessionTemplate.this.sqlSessionFactory,
      SqlSessionTemplate.this.executorType,
      SqlSessionTemplate.this.exceptionTranslator);
    try {
      //执行SqlSession的方法
      Object result = method.invoke(sqlSession, args);
      
      //如果当前在非事务环境下允许，则强制commit一下，因为有些数据库要求在close()方法前要先调用commit()或rollback()
      if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
        sqlSession.commit(true);
      }
      return result;
    } catch (Throwable t) {
      Throwable unwrapped = unwrapThrowable(t);
      throw unwrapped;
    } finally {
      //关闭sqlSession
      if (sqlSession != null) {
        closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
      }
    }
  }
}

这样一来，Spring就将事务与SqlSession整合到了一起。

四. Mapper自动扫描

通过@MapperScan注解，可以指定Mapper自动扫描的包路径。自动扫描的处理是通过ClassPathMapperScanner实现的，其doScan()方法如下：

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;
}

ClassPathMapperScanner会将所有的Mapper扫描进来，并且将每个Mapper包装成一个类型为MapperFactoryBean的BeanDefinition，注册到IoC容器中。 MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor，是一个BeanFactoryPostProcessor，它的功能就是在容器启动阶段动态向容器中注册BeanDefinition。经过MapperScannerConfigurer处理后，所有Mapper接口的BeanDefinition就以MapperFactoryBean的形式注册到Spring IoC容器中了。代码可见ClassPathMapperScanner#processBeanDefinitions()：

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
      //这里实际注册的BeanDefinition是Mapper接口所对应的MapperFactoryBean
      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() + "'.");
        }
        definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
      }
    }
  }

五. MapperFactoryBean

MapperFactoryBean也是一个工厂Bean，可以通过它创建Mapper实例：

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

这样一来，所有程序中定义的Mapper就都以Bean的形式加载到Spring的IoC容器中了。

六. Spring对MyBatis生命周期的修改

SqlSession 在MyBatis原生API中，SqlSession的生命周期是方法级别的，在每个方法中创建一个新的SqlSession实例，用完了之后就销毁。而Spring对MyBatis原生的SqlSession进行了事务的增强，通过一个单例的SqlSessionTemplate，保证每次获取的SqlSession其实都是与当前线程绑定的。
Mapper MyBatis原生API中，通过SqlSession#getMapper()获取到的Mapper，是一个MapperProxy动态代理类，它的生命周期与SqlSession一致，都是方法级别的。但是Spring通过MapperFactoryBean创建的Mapper是单例的，也就是将其声明周期提升到了全局级别。

七. MyBatis一级缓存失效问题

我们知道MyBatis有两级缓存，其中一级缓存是默认开启的。但是在Spring整合了MyBatis后，却经常出现一级缓存失效的问题，其原因是在SqlSessionTemplate对SqlSession进行了代理后，在非事务环境下，每次执行完SqlSession的操作后都会进行一次commit()，且最后后关闭SqlSession，因此会清空一级缓存(强调下仅在非事务环境下，在事务中调用不会出现这个问题)，源码如下：

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)) {
          //在非事务环境下执行，会强制commit()
          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 {
      	//关闭SqlSession
      	//在非事务环境下，就是直接调用SqlSession.close()方法
      	//在事务环境下，并不会关闭SqlSession,而仅是对当前事务的引用计数-1
        if (sqlSession != null) {
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }

八. SpringBoot整合MyBatis简单演示

引入MyBatis的Starter依赖

<dependency>
  <groupId>org.mybatis.spring.boot</groupId>
  <artifactId>mybatis-spring-boot-starter</artifactId>
  <version>1.1.1</version>
</dependency>

在application.properties中增加MyBatis相关配置

#数据源配置
spring.datasource.type=com.alibaba.druid.pool.DruidDataSource
spring.datasource.url=jdbc:mysql://localhost:3306/test
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.datasource.username=xxx
spring.datasource.password=xxx

#MyBatis配置
mybatis.type-aliases-package=xxx
mybatis.mapper-locations=classpath:mappers/*.xml