在上一篇 Mybatis:Mapper接口編程原理分析（一）中炕横，Mapper 接口最后向MapperRegistry 注冊。MapperRegistry 它是用來注冊 Mapper 接口和獲取 Mapper 接口代理類實(shí)例的工具類，完成這兩個工作是通過 getMapper 方法和 addMapper 方法。
它的源碼如下：

//注冊Mapper接口與獲取生成代理類實(shí)例的工具類
public class MapperRegistry {
    
  //全局配置文件對象
  private final Configuration config;
  
  //一個HashMap Key是mapper的類型對象, Value是MapperProxyFactory對象
  //這個MapperProxyFactory是創(chuàng)建Mapper代理對象的工廠 
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }
   //獲取生成的代理對象
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //通過Mapper的接口類型 去Map當(dāng)中查找 如果為空就拋異常
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      //否則創(chuàng)建一個當(dāng)前接口的代理對象 并且傳入sqlSession
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }
  
  public <T> boolean hasMapper(Class<T> type) {
    return knownMappers.containsKey(type);
  }
  //注冊Mapper接口
  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<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

  /**
   * @since 3.2.2
   */
  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }

  /**
   * @since 3.2.2
   */
  //注冊Mapper接口
  public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }

  /**
   * @since 3.2.2
   */
  //通過包名掃描下面所有接口
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }
}

從源碼可知荆几，Mapper 接口只會被加載一次粗井，然后緩存在 HashMap 中唧取，其中 key 是 mapper 接口類對象航夺，value 是 mapper 接口對應(yīng)的 代理工廠。需注意的是腥光，每個 mapper 接口對應(yīng)一個代理工廠关顷。