Mybatis源码解析之SpringBoot集成mybatis-spring-boot-starter分析

本文将针对SpringBoot和Mybatis的集成进行解析，若无特殊说明，本文解析的jar包和版本为：org.mybatis.spring.boot:mybatis-spring-boot-starter:1.3.1。

一、jar包介绍

该jar包是mybatis和springboot的集成jar包，因此对mybatis、mybatis-spring和springboot部分的jar包都存在依赖。
jar包介绍

二、配置

在mybatis-spring-boot-starter中，支持的配置内容对应MybatisProperties类。

@ConfigurationProperties(prefix = MybatisProperties.MYBATIS_PREFIX)
public class MybatisProperties {

  public static final String MYBATIS_PREFIX = "mybatis";

  /**
   * Location of MyBatis xml config file.
   */
  private String configLocation;

  /**
   * Locations of MyBatis mapper files.
   */
  private String[] mapperLocations;

  /**
   * Packages to search type aliases. (Package delimiters are ",; \t\n")
   */
  private String typeAliasesPackage;

  /**
   * Packages to search for type handlers. (Package delimiters are ",; \t\n")
   */
  private String typeHandlersPackage;

  /**
   * Indicates whether perform presence check of the MyBatis xml config file.
   */
  private boolean checkConfigLocation = false;

  /**
   * Execution mode for {@link org.mybatis.spring.SqlSessionTemplate}.
   */
  private ExecutorType executorType;

  /**
   * Externalized properties for MyBatis configuration.
   */
  private Properties configurationProperties;

  /**
   * A Configuration object for customize default settings. If {@link #configLocation}
   * is specified, this property is not used.
   */
  @NestedConfigurationProperty
  private Configuration configuration;
}

如下文的例子指定了mapper位置。

mybatis:
  mapper-locations: classpath*:mybatis/mapper/*/*.xml

三、MybatisAutoConfiguration

基于自动配置类MybatisAutoConfiguration对配置内容进行解析。

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

  private static final Logger logger = LoggerFactory.getLogger(MybatisAutoConfiguration.class);

  private final MybatisProperties properties;

  private final Interceptor[] interceptors;

  private final ResourceLoader resourceLoader;

  private final DatabaseIdProvider databaseIdProvider;

  private final List<ConfigurationCustomizer> configurationCustomizers;

  public MybatisAutoConfiguration(MybatisProperties properties,
                                  ObjectProvider<Interceptor[]> interceptorsProvider,
                                  ResourceLoader resourceLoader,
                                  ObjectProvider<DatabaseIdProvider> databaseIdProvider,
                                  ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider) {
    this.properties = properties;
    this.interceptors = interceptorsProvider.getIfAvailable();
    this.resourceLoader = resourceLoader;
    this.databaseIdProvider = databaseIdProvider.getIfAvailable();
    this.configurationCustomizers = configurationCustomizersProvider.getIfAvailable();
  }

  @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)");
    }
  }

  @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();
  }

  @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);
    }
  }

可以看到，在没有手动注入的情况下，根据配置的属性注入了SqlSessionTemplate和SqlSessionFactory，值域SqlSessionFactoryBean依赖的属性DataSource，在springboot的jabc包中的自动配置类DataSourceAutoConfiguration中已经实现了默认的注入。

四、MapperScannerRegistrarNotFoundConfiguration

我们知道，在注入了SqlSessionFactory后，接下来只要注入mapper接口的bean就可以完成mybatis的集成。
在SpringBoot中，同样支持通过MapperScan注解的方式通过MapperFactoryBean注入mapper接口，这一点同spring和mybatis的集成一样。
如果，我们没有注入任何的MapperFactoryBean，那么SpringBoot还支持Mapper注解的单个注入方式。

@org.springframework.context.annotation.Configuration
@Import({ AutoConfiguredMapperScannerRegistrar.class })
@ConditionalOnMissingBean(MapperFactoryBean.class)
public static class MapperScannerRegistrarNotFoundConfiguration {

  @PostConstruct
  public void afterPropertiesSet() {
    logger.debug("No {} found.", MapperFactoryBean.class.getName());
  }
}

在MybatisAutoConfiguration$MapperScannerRegistrarNotFoundConfiguration中可以看到，如果没有MapperFactoryBean类的bean，其可以通过AutoConfiguredMapperScannerRegistrar注入bean。
AutoConfiguredMapperScannerRegistrar实现了ImportBeanDefinitionRegistrar接口，注入bean的方法是registerBeanDefinitions方法。

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

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

  ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);

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

    List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
    if (logger.isDebugEnabled()) {
      for (String pkg : packages) {
        logger.debug("Using auto-configuration base package '{}'", pkg);
      }
    }

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

可以看到，其和MapperScanner注解的注入方式类似，生成了一个扫描类ClassPathMapperScanner，扫描路径下被Mapper注解的mapper接口并注入。ClassPathMapperScanner的源码前面在spring和mybatis的集成成一家进行了简单的分析，不在赘述。