Mybatis源码解读-SpringBoot中配置加载和Mapper的生成

本文 mybatis-spring-boot探讨在springboot工程中mybatis相关对象的注册与加载。

建议先了解mybatis在spring中的使用和springboot自动装载机制，再看此文章。

@MapperScan和 @Mapper能一起用吗？

在讨论自动装配方式之前，先看看mybatis最简洁的demo

public static void main(String[] args) throws Exception {
    // 配置文件路径
    String resource = "mybatis-config.xml";
    InputStream inputStream = Resources.getResourceAsStream(resource);
    // 1.读取配置，创建SqlSessionFactory
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    // 2.通过工厂获取SqlSession
    SqlSession session = sqlSessionFactory.openSession();
    try {
        // 3.获取mapper代理对象
        StudentMapper mapper = session.getMapper(StudentMapper.class);
        // 4.执行查询，此处才真正连接数据库
        System.out.println(mapper.selectByName("张三"));
    } finally {
        // 5.关闭连接
        session.close();
    }
}

可以看到，首先需要创建SqlSessionFactory和SqlSession，在springboot中，这两者通过自动装配完成。

在mybatis-spring-boot-autoconfigure-x.x.x.jar的spring.factories中，可以看到自动装配注入了 MybatisAutoConfiguration类

public class MybatisAutoConfiguration implements InitializingBean {
  ......

  @Bean
  @ConditionalOnMissingBean
  public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
    ......

  }

  @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);
    }
  }
}

SqlSessionTemplate是 SqlSession的子类，所以现在二者都有了。

Mapper的扫描分两种方式讨论

1. @MapperScan方式

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

}

可以看到，导入了MapperScannerRegistrar类

public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {
  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    AnnotationAttributes mapperScanAttrs = AnnotationAttributes
        .fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    if (mapperScanAttrs != null) {
      registerBeanDefinitions(importingClassMetadata, mapperScanAttrs, registry,
          generateBaseBeanName(importingClassMetadata, 0));
    }
  }

  void registerBeanDefinitions(AnnotationMetadata annoMeta, AnnotationAttributes annoAttrs,
      BeanDefinitionRegistry registry, String beanName) {

    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
    ......

    registry.registerBeanDefinition(beanName, builder.getBeanDefinition());
  }
}

因为MapperScannerRegistrar实现了ImportBeanDefinitionRegistrar接口，所以会被调用registerBeanDefinitions方法，最后注册MapperScannerConfigurer 咱们先记住MapperScannerConfigurer这个类，去看看@Mapper的方式
2. @Mapper方式 在 MybatisAutoConfiguration中，有这么一段代码

@org.springframework.context.annotation.Configuration
// 如果满足条件，则导入AutoConfiguredMapperScannerRegistrar
@Import(AutoConfiguredMapperScannerRegistrar.class)
// 如果MapperFactoryBean和MapperScannerConfigurer都没注册，则满足条件
@ConditionalOnMissingBean({ MapperFactoryBean.class, MapperScannerConfigurer.class })
public static class MapperScannerRegistrarNotFoundConfiguration implements InitializingBean {

  ......

}

我们在@MapperScan方式看到，是已经注册了MapperScannerConfigurer类的。所以，@MapperScan会覆盖@Mapper 继续看看 AutoConfiguredMapperScannerRegistrar

public static class AutoConfiguredMapperScannerRegistrar
      implements BeanFactoryAware, EnvironmentAware, ImportBeanDefinitionRegistrar {

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

    ......

    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
    builder.addPropertyValue("annotationClass", Mapper.class);
    ......

    registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition());
  }

  ......

}

可以看到，同样是注册了MapperScannerConfigurer 也就是两种注解方式都是通过MapperScannerConfigurer扫描mapper注册的
3. 通用部分 继续追踪MapperScannerConfigurer的调用链

// MapperScannerConfigurer#postProcessBeanDefinitionRegistry
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
  ......

  ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
  ......

  // 注册过滤器(@Mapper和@MapperScan的区别体现在这里)
  scanner.registerFilters();
  // 开始扫描bean
  scanner.scan(
      StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
}

public void registerFilters() {
  boolean acceptAllInterfaces = true;

  // 如果指定了扫描类型（@Mapper走这里）
  // annotationClass在前面的AutoConfiguredMapperScannerRegistrar#registerBeanDefinitions被注入
  // 就是这段builder.addPropertyValue("annotationClass", Mapper.class);
  if (this.annotationClass != null) {
    addIncludeFilter(new AnnotationTypeFilter(this.annotationClass));
    acceptAllInterfaces = false;
  }

  ......

  // 如果没指定扫描类型，则扫描全部（@MapperScan走这里）
  if (acceptAllInterfaces) {
    addIncludeFilter((metadataReader, metadataReaderFactory) -> true);
  }

  // exclude package-info.java
  addExcludeFilter((metadataReader, metadataReaderFactory) -> {
    String className = metadataReader.getClassMetadata().getClassName();
    return className.endsWith("package-info");
  });
}

看完了过滤器的注册，继续回到扫描逻辑scanner.scan

// ClassPathMapperScanner#scan(String... basePackages) -->
// ClassPathMapperScanner#doScan(String... basePackages)
public Set doScan(String... basePackages) {
  // 扫描mapper（此时是原始对象）
  Set beanDefinitions = super.doScan(basePackages);

  if (beanDefinitions.isEmpty()) {
    LOGGER.warn(() -> "No MyBatis mapper was found in '" + Arrays.toString(basePackages)
        + "' package. Please check your configuration.");
  } else {
    // 通过MapperFactoryBean类将mapper对象转换成代理对象MapperProxy
    processBeanDefinitions(beanDefinitions);
  }

  return beanDefinitions;
}

@MapperScan和 @Mapper能一起用（不会报错），但是 @Mapper是没有效果的。

Original: https://www.cnblogs.com/konghuanxi/p/16277509.html
Author: 王谷雨
Title: Mybatis源码解读-SpringBoot中配置加载和Mapper的生成