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目录

0.说明

1.@MapperScan

2. MapperScannerRegister

3.ClassPathMapperScanner

4. MapperFactoryBean

0.说明

mybatis构建过程主要包括：

1. 解析mybatis配置文件，构造Configuration配置类对象和SqlSessionFactory；
2. 利用@MapperScan注册BeanDefinition到BeanFactory工厂中；

其中第一步中创建了Mapper接口代理类，并存储到Configuration中；

下面主要介绍第二步，利用@MapperScan注册BeanDefinition的过程；

1.@MapperScan

/**
 * Use this annotation to register MyBatis mapper interfaces when using Java
 * Config. It performs when same work as {@link MapperScannerConfigurer} via
 * {@link MapperScannerRegistrar}.
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
public @interface MapperScan {

  /**
   * Alias for the {@link #basePackages()} attribute. Allows for more concise
   * annotation declarations e.g.:
   * {@code @EnableMyBatisMapperScanner("org.my.pkg")} instead of {@code
   * @EnableMyBatisMapperScanner(basePackages= "org.my.pkg"})}.
   */
  String[] value() default {};

  /**
   * Base packages to scan for MyBatis interfaces. Note that only interfaces
   * with at least one method will be registered; concrete classes will be
   * ignored.
   */
  String[] basePackages() default {};

  /**
   * Type-safe alternative to {@link #basePackages()} for specifying the packages
   * to scan for annotated components. The package of each class specified will be scanned.
   * <p>Consider creating a special no-op marker class or interface in each package
   * that serves no purpose other than being referenced by this attribute.
   */
  Class<?>[] basePackageClasses() default {};

  /**
   * The {@link BeanNameGenerator} class to be used for naming detected components
   * within the Spring container.
   */
  Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;

  /**
   * This property specifies the annotation that the scanner will search for.
   * <p>
   * The scanner will register all interfaces in the base package that also have
   * the specified annotation.
   * <p>
   * Note this can be combined with markerInterface.
   */
  Class<? extends Annotation> annotationClass() default Annotation.class;

  /**
   * This property specifies the parent that the scanner will search for.
   * <p>
   * The scanner will register all interfaces in the base package that also have
   * the specified interface class as a parent.
   * <p>
   * Note this can be combined with annotationClass.
   */
  Class<?> markerInterface() default Class.class;

  /**
   * Specifies which {@code SqlSessionTemplate} to use in the case that there is
   * more than one in the spring context. Usually this is only needed when you
   * have more than one datasource.
   */
  String sqlSessionTemplateRef() default "";

  /**
   * Specifies which {@code SqlSessionFactory} to use in the case that there is
   * more than one in the spring context. Usually this is only needed when you
   * have more than one datasource.
   */
  String sqlSessionFactoryRef() default "";

  /**
   * Specifies a custom MapperFactoryBean to return a mybatis proxy as spring bean.
   *
   */
  Class<? extends MapperFactoryBean> factoryBean() default MapperFactoryBean.class;
}

MapperScan注解属性如下：

1. value、basePackages、basePackageClasses都是用于提供扫描接口类的包名；
2. BeanNameGenerator作为Bean名称生成器，支持自定义；
3. annotationClass、markerInterface作为marker标记，限定需要扫描的接口需要满足的条件（包含特定注解或实现某个接口）；
4. sqlSessionTemplateRef、sqlSessionFactoryRef在有多个数据源的情况下指定使用哪一个数据源；
5. factoryBean，指定FactoryBean实现类，用于生成接口代理类，默认为MapperFactoryBean.class, 支持自定义；

另外MapperScan通过import注解导入了MapperScannerRegistrar类，该接口实现了ImportBeanDefinitionRegistrar接口，用于注册BeanDefinition；

2. MapperScannerRegistrar

MapperScannerRegistrar类结构如下：

ImportBeanDefinitionRegistrar：是spring开放出来的用于注册其他BeanDefinition的接口，供子类具体实现；

MapperScannerRegistrar：实现ImportBeanDefinitionRegistrar接口，同时依赖ClassPathMapperScanner获取候选bean定义, 并对bean定义进行后处理；

ClassPathMapperScanner：继承ClassPathBeanDefinitionScanner，完成指定包下bean定义的注册；

(传送门 Spring源码：ClassPathBeanDefinitionScanner源码分析)

覆盖的ImportBeanDefinitionRegistrar接口中的registerBeanDefinitions方法具体如下，主要完成了ClassPathMapperScanner的构造，并执行doScan方法进行扫描；

MapperScannerRegistrar.registerBeanDefinitions：

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

    AnnotationAttributes annoAttrs = AnnotationAttributes.fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);

    // this check is needed in Spring 3.1
    if (resourceLoader != null) {
      scanner.setResourceLoader(resourceLoader);
    }

    Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");
    if (!Annotation.class.equals(annotationClass)) {
      scanner.setAnnotationClass(annotationClass);
    }

    Class<?> markerInterface = annoAttrs.getClass("markerInterface");
    if (!Class.class.equals(markerInterface)) {
      scanner.setMarkerInterface(markerInterface);
    }

    Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");
    if (!BeanNameGenerator.class.equals(generatorClass)) {
      scanner.setBeanNameGenerator(BeanUtils.instantiateClass(generatorClass));
    }

    Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");
    if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {
      scanner.setMapperFactoryBean(BeanUtils.instantiateClass(mapperFactoryBeanClass));
    }

    scanner.setSqlSessionTemplateBeanName(annoAttrs.getString("sqlSessionTemplateRef"));
    scanner.setSqlSessionFactoryBeanName(annoAttrs.getString("sqlSessionFactoryRef"));

    List<String> basePackages = new ArrayList<String>();
    for (String pkg : annoAttrs.getStringArray("value")) {
      if (StringUtils.hasText(pkg)) {
        basePackages.add(pkg);
      }
    }
    for (String pkg : annoAttrs.getStringArray("basePackages")) {
      if (StringUtils.hasText(pkg)) {
        basePackages.add(pkg);
      }
    }
    for (Class<?> clazz : annoAttrs.getClassArray("basePackageClasses")) {
      basePackages.add(ClassUtils.getPackageName(clazz));
    }
    scanner.registerFilters();
    scanner.doScan(StringUtils.toStringArray(basePackages));
  }

3.ClassPathMapperScanner

ClassPathMapperScanner主要用于注册指定包名下的bean定义，并对注册的bean定义进行后处理，这里主要是设置beanClass为MapperFactoryBean、设置构造函数参数对象、设置autoWireMode=byType（MapperScan注解属性没有指定时，默认将容器中的DefalultSqlSectionFactory绑定到set方法的入参对象中，构造PropertyValue值），其doScan方法如下：

/**
   * Calls the parent search that will search and register all the candidates.
   * Then the registered objects are post processed to set them as
   * MapperFactoryBeans
   */
  @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;
  }

  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
      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);
      }
    }
  }

4. MapperFactoryBean

MapperFactoryBean实现FactoryBean接口，继承SqlSessionDaoSupport，其中SqlSessionDaoSupport有成员变量sqlSession，初始化是在setSqlSessionFactory方法中完成的（setSqlSessionFactory是根据autoWire=byType属性注入进来的）

SqlSessionDaoSupport.setSqlSessionFactory：

public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
    if (!this.externalSqlSession) {
      this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
    }
}

因为MapperFactoryBean是FactoryBean的子类，所以bean实例化时会调用MapperFactoryBean的getObject方法:

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

进而调用sqlSession的getMapper接口，即是sqlSessionTemplete的getMapper方法：

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

这样最终就从Configuration中获取mapper接口的代理对象；而Configuration中的mapper接口代理对象是在mybatis配置解析时构造完成的；