交代： SqlSession 作为MyBatis工作的主要顶层API，表示和数据库交互的会话，完成必要数据库增删改查功能。里面主要有两个参数，一个是configuration另一个是executor。其中Configuration就像一个大管家，承载了所有的配置文件的信息，例如Map<String, MappedStatement> mappedStatements，Map<String, ResultMap> resultMaps，``Map<String, ParameterMap> parameterMaps`等；还有一个是executor，执行crud用的。其中mappedStatements很重要，因为xml文件里的crud标签的sql语句都会封装到这个对象中。

所用版本为：
Group ID: org.mybatis Artifact ID: mybatis-spring Version: 2.0.2
Group ID: org.mybatis Artifact ID: mybatis Version: 3.5.2

1.文件入口

通常我们会将@MapperScan("xxx")，写到SpringBoot的主函数入口，就是为了能够扫描到包

@SpringBootApplication
@MapperScan("com.runchuang.liangpi.dao")
public class LiangpiApplication {
    public static void main(String[] args) {
        SpringApplication.run(LiangpiApplication.class, args);
    }
}

2.MapperScan注解

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

我们可以看到Import这个注解，其实就是和@Bean这种注解差不多，可以将括号里的类，加载到Bean容器中；这里将MapperScannerRegistrar加到了容器中，这个类主要负责扫描接口，然后偷天换日将本应该属于接口的BeanDefinition的class，换成MapperFactoryBean，这个类就是最终就能通过getObject来获取真正的要执行的代理类。

3.MapperScannerRegistrar

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

获取MapperScann注解做标注的类，然后获取他写的属性名字，例如我这里MapperScan写了"com.runchuang.liangpi.dao"

4.registerBeanDefinitions

用一个BeanDefinitionBuilder来构造关于MapperScannerConfigurer的BeanDefinition，获取扫描的包名，然后设置到BeanDefinition的属性中。然后动态注册到Bean的容器中。

 void registerBeanDefinitions(AnnotationAttributes annoAttrs, BeanDefinitionRegistry registry, String beanName) {
    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
    ...
    builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(basePackages));
    registry.registerBeanDefinition(beanName, builder.getBeanDefinition());
  }

5.接下来我们看看MapperConfigurer

public class MapperScannerConfigurer
    implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
 @Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
      processPropertyPlaceHolders();
    }
    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);

    ....    
    
    scanner.scan(
        StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

}

由于实现了BeanDefinitionRegistryPostPostProcesser这个后置处理器，所以在实例化过程中，做了包扫描注册以及jdk反向代理。

6.这时候会执行scan方法

    public int scan(String... basePackages) {
        int beanCountAtScanStart = this.registry.getBeanDefinitionCount();

        doScan(basePackages);

        // Register annotation config processors, if necessary.
        if (this.includeAnnotationConfig) {
            AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
        }

        return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
    }

这个doScan方法并不是调用自己本类的doScan方法，而是调用父类ClassPathMapperScanner方法

public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {`
  @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;
  }
}

第一个是扫描包，将接口的注册成BeanDefinitionHolder，但是这个BeanDefinition肯定不能实例化啦，因为是接口嘛，然后将这个Set集合的BeanDefinitions处理然后注册，这里就涉及了偷天换日

7.偷天换日

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    GenericBeanDefinition definition;
    for (BeanDefinitionHolder holder : beanDefinitions) {
      definition = (GenericBeanDefinition) holder.getBeanDefinition();
      String beanClassName = definition.getBeanClassName();
      // the mapper interface is the original class of the bean
      // but, the actual class of the bean is MapperFactoryBean
      definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName); // issue #59
      definition.setBeanClass(this.mapperFactoryBeanClass);
      ....省略...
      if (!explicitFactoryUsed) {
              definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
      }
      definition.setLazyInit(lazyInitialization);
    }
  }

这里将传进来的beanDefinitions的class全部换掉换成MapperFactoryBean，自此，这个Bean的实例化不再是接口，而是MapperFactoryBean，这个Bean还是一个FactoryBean，会在获取真正的bean的时候，通过getObject去获取。

8.MapperFactoryBean

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

在这里调用的是MapperRegistry的getMapper方法

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

注意一个问题这个mapperProxyFactory.newInstance(sqlSession);就是调用的动态代理了。

9.动态代理的调用。

  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

10.代理类MapperProxy实现了InvocationHandler

public class MapperProxy<T> implements InvocationHandler, Serializable {


  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }
}

11.执行方法

public class MapperMethod {

  private final SqlCommand command;
  private final MethodSignature method;

  public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, mapperInterface, method);
  }

  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }

    ……
 

12.到底在哪里装配的Configuration呢？

其实是在MapperFactoryBean他的父类里，DaoSupport，他实现了InitializingBean，所以在初始化之后，实现了afterPropertiesSet方法

    @Override
    public final void afterPropertiesSet() throws IllegalArgumentException, BeanInitializationException {
        // Let abstract subclasses check their configuration.
        checkDaoConfig();

        // Let concrete implementations initialize themselves.
        try {
            initDao();
        }
        catch (Exception ex) {
            throw new BeanInitializationException("Initialization of DAO failed", ex);
        }
    }

主要在checkDaoConfig();配置了Configuration；
具体实现在MapperFactoryBean里

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {

  private Class<T> mapperInterface;

  private boolean addToConfig = true;

  public MapperFactoryBean() {
    // intentionally empty
  }

  public MapperFactoryBean(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  protected void checkDaoConfig() {
    super.checkDaoConfig();

    notNull(this.mapperInterface, "Property 'mapperInterface' is required");

    Configuration configuration = getSqlSession().getConfiguration();
    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
      try {
        configuration.addMapper(this.mapperInterface);
      } 
    }
  }

addMapper具体实现，并且将接口类型穿进来了，以便jdk动态代理。

  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        knownMappers.put(type, new MapperProxyFactory<>(type));        
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }