springboot集成mybatis源码

1：springboot集成mybatis，在pom.xml中引入下面依赖性即可：

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

2：在mybatis-spring-boot-starter依赖项里，除了引入 spring-boot-starter，spring-boot-starter-jdbc、mybatis-spring之外，还会引入一个自动配置转载项：

 <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-jdbc</artifactId>
    </dependency>
    <dependency>
      <groupId>org.mybatis.spring.boot</groupId>
      <artifactId>mybatis-spring-boot-autoconfigure</artifactId>
    </dependency>

3：mybatis-spring-boot-autoconfigure 引入自动加载项：MybatisAutoConfiguration。代码如下：

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

4：在MybatisAutoConfiguration里，会创建 SqlSessionFactory 和 SqlSessionTemplate 两个对象。其前提是需要有 DataSource 实例。

  @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 (this.properties.getTypeAliasesSuperType() != null) {
      factory.setTypeAliasesSuperType(this.properties.getTypeAliasesSuperType());
    }
    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);
    }
  }

5：mybatis提成到spring有2个重要的注解，@MapperScan 和 @Mapper。@MapperScan注解一般用在启动类，@Mapper 一般用在Mapper接口类上。
这两个注解的功能类似，二者只用其一即可。如果两者同时使用，则@Mapper 注解不生效。
@Mapper注解通过 AutoConfiguredMapperScannerRegistrar 处理，如下：

/**
   * {@link org.mybatis.spring.annotation.MapperScan} ultimately ends up
   * creating instances of {@link MapperFactoryBean}. If
   * {@link org.mybatis.spring.annotation.MapperScan} is used then this
   * auto-configuration is not needed. If it is _not_ used, however, then this
   * will bring in a bean registrar and automatically register components based
   * on the same component-scanning path as Spring Boot itself.
   */
  @org.springframework.context.annotation.Configuration
  @Import({ AutoConfiguredMapperScannerRegistrar.class })
  @ConditionalOnMissingBean(MapperFactoryBean.class)
  public static class MapperScannerRegistrarNotFoundConfiguration implements InitializingBean {

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

@MapperScan 注解通过 MapperScannerRegistrar 处理，代码如下：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
public @interface MapperScan {

由于@MapperScan 注解处理后会生成 MapperFactoryBean 实例，而@Mapper是在 MapperFactoryBean 不存在的情况下生效，所以2个是互斥的。

6：@MapperScan 和 @Mapper 注解都是通过 ClassPathMapperScanner 这个Scanner 来扫描代码，如下：

      ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
      if (this.resourceLoader != null) {
        scanner.setResourceLoader(this.resourceLoader);
      }
      scanner.setAnnotationClass(Mapper.class);
      scanner.registerFilters();
      scanner.doScan(StringUtils.toStringArray(packages));

而一般情况下，Mapper 是个接口。在Spring的默认情况下，接口interface是不会生成 BeanDefinition 对象。而这里为了生成 BeanDefinition 对象，则重写了 isCandidateComponent 方法，如下：

  @Override
  protected boolean isCandidateComponent(AnnotatedBeanDefinition beanDefinition) {
    return beanDefinition.getMetadata().isInterface() && beanDefinition.getMetadata().isIndependent();
  }

7：由于自定义的Mapper一般是接口，但是接口是不能实例化的，所以在mybatis里面，BeanDefinition 把定义的实例化类型变为 MapperFactoryBean。所以实际上实例化的是 MapperFactoryBean 对象(自定义的Mapper实例作为构造函数的入参传进去)。获取自定义的Mapper实例的时候，通过 MapperFactoryBean 的 getObject() 方法获取。代码如下：

    for (BeanDefinitionHolder holder : beanDefinitions) {
      definition = (GenericBeanDefinition) holder.getBeanDefinition();
      definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
      definition.setBeanClass(this.mapperFactoryBean.getClass());

      definition.getPropertyValues().add("addToConfig", this.addToConfig);