交代： SqlSession 作為MyBatis工作的主要頂層API彩掐，表示和數(shù)據(jù)庫交互的會(huì)話衙猪，完成必要數(shù)據(jù)庫增刪改查功能冰沙。里面主要有兩個(gè)參數(shù)垂券，一個(gè)是configuration另一個(gè)是executor董饰。其中Configuration就像一個(gè)大管家，承載了所有的配置文件的信息，例如Map<String, MappedStatement> mappedStatements卒暂，Map<String, ResultMap> resultMaps啄栓，``Map<String, ParameterMap> parameterMaps`等；還有一個(gè)是executor也祠，執(zhí)行crud用的昙楚。其中mappedStatements很重要，因?yàn)閤ml文件里的crud標(biāo)簽的sql語句都會(huì)封裝到這個(gè)對(duì)象中诈嘿。

所用版本為：
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

通常我們會(huì)將@MapperScan("xxx")堪旧，寫到SpringBoot的主函數(shù)入口，就是為了能夠掃描到包

@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 {

我們可以看到Import這個(gè)注解奖亚，其實(shí)就是和@Bean這種注解差不多淳梦，可以將括號(hào)里的類，加載到Bean容器中昔字；這里將MapperScannerRegistrar加到了容器中爆袍，這個(gè)類主要負(fù)責(zé)掃描接口，然后偷天換日將本應(yīng)該屬于接口的BeanDefinition的class作郭，換成MapperFactoryBean陨囊，這個(gè)類就是最終就能通過getObject來獲取真正的要執(zhí)行的代理類。

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));
    }
  }
}

獲取MapperScann注解做標(biāo)注的類夹攒，然后獲取他寫的屬性名字蜘醋，例如我這里MapperScan寫了"com.runchuang.liangpi.dao"

4.registerBeanDefinitions

用一個(gè)BeanDefinitionBuilder來構(gòu)造關(guān)于MapperScannerConfigurer的BeanDefinition，獲取掃描的包名咏尝，然后設(shè)置到BeanDefinition的屬性中压语。然后動(dòng)態(tài)注冊(cè)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.接下來我們看看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));
  }

}

由于實(shí)現(xiàn)了BeanDefinitionRegistryPostPostProcesser這個(gè)后置處理器编检，所以在實(shí)例化過程中胎食，做了包掃描注冊(cè)以及jdk反向代理。

6.這時(shí)候會(huì)執(zhí)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);
    }

這個(gè)doScan方法并不是調(diào)用自己本類的doScan方法蒙谓，而是調(diào)用父類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;
  }
}

第一個(gè)是掃描包斥季，將接口的注冊(cè)成BeanDefinitionHolder，但是這個(gè)BeanDefinition肯定不能實(shí)例化啦累驮，因?yàn)槭墙涌诼锖ㄇ悖缓髮⑦@個(gè)Set集合的BeanDefinitions處理然后注冊(cè)，這里就涉及了偷天換日

7.偷天換日

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);
    }
  }

這里將傳進(jìn)來的beanDefinitions的class全部換掉換成MapperFactoryBean谤专，自此躁锡，這個(gè)Bean的實(shí)例化不再是接口，而是MapperFactoryBean置侍，這個(gè)Bean還是一個(gè)FactoryBean映之，會(huì)在獲取真正的bean的時(shí)候拦焚，通過getObject去獲取。

8.MapperFactoryBean

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

在這里調(diào)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);
    }
  }

注意一個(gè)問題這個(gè)mapperProxyFactory.newInstance(sqlSession);就是調(diào)用的動(dòng)態(tài)代理了杠输。

9.動(dòng)態(tài)代理的調(diào)用赎败。

  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.代理類MapperProxy實(shí)現(xiàn)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.執(zhí)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.到底在哪里裝配的Configuration呢？

其實(shí)是在MapperFactoryBean他的父類里蠢甲，DaoSupport僵刮，他實(shí)現(xiàn)了InitializingBean，所以在初始化之后鹦牛，實(shí)現(xiàn)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搞糕；
具體實(shí)現(xiàn)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具體實(shí)現(xiàn)，并且將接口類型穿進(jìn)來了曼追，以便jdk動(dòng)態(tài)代理窍仰。

  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);
        }
      }
    }
  }