回顾下写法：

public static void main(String[] args) {
//前三步都相同
 InputStream inputStream =
Resources.getResourceAsStream("sqlMapConfig.xml");
 SqlSessionFactory factory = new
SqlSessionFactoryBuilder().build(inputStream);
 SqlSession sqlSession = factory.openSession();
 
 //这里不再调用sqlSession，而是获得了接口对象，调用接口中的方法。
 UserMapper mapper = sqlSession.getMapper(UserMapper.class);
 List<User> list = mapper.getUserByName("tom");
}

思考一个问题，通常的Mapper接口我们都没有实现方法却可以直接使用，是为什么？答案很简单，动态代理。
开始之前介绍一下Mybatis初始化时对接口的处理：MapperRegistry是Configuration的一个属性，它内部维护了一个HashMap用于存放mapper接口的工厂类，每一个接口对应一个工厂类。mappers中可以配置接口的包接口，或者某个具体的接口类。

<mappers>
 <mapper class="com.lagou.mapper.UserMapper"/>
 <package name="com.lagou.mapper"/>
</mappers>

当解析mappers标签时，它会判断解析到的是mapper配置文件时，会再对应配置文件中的增删查改标签一一封装成MappedStatement对象，存入mappedStatement。当判断解析到接口时，会建此接口对应的MapperProxyFactory对象，存入HashMap中，key=接口的字节码对象，value=此接口对应的MapperProxyFactory对象。

源码剖析-getMapper

DefaultSqlSession#getMapper

//DefaultSqlSession#getMapper

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

//Configuration#getMapper
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

//MapperRegistry#getMapper
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//从MapperRegistry中的HashMap中拿MapperProxyFactory
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
//通过动态代理工厂生成实例
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

//mapperProxyFactory#newInstance
  public T newInstance(SqlSession sqlSession) {
// 创建JDK动态代理的Handler类
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
//调用重载方法
    return newInstance(mapperProxy);
  }

MapperProxy类，实现了InvocationHandler接口

public class MapperProxy<T> implements InvocationHandler, Serializable {

//省略部分源码

  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;
}

//构造方法，传入了SqlSessio，说明每个session中的代理对象
  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

//省略部分源码
}

源码剖析-invoke

在动态代理返回了示例后，我们就可以调用mapper类中的方法了，但代理对象调用方法，执行是MapperProxy中的invoke方法中。
MapperProxy#invoke

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
//如果是Object 定义的方法直接调用
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
// 获取 MapperMethod 对象
    final MapperMethod mapperMethod = cachedMapperMethod(method);
//重点在这，MapperMethod最终调用了执行方法
    return mapperMethod.execute(sqlSession, args);
  }

进入execute方法

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
//判断mapper中的方法类型，最终调用的还是SqlSession中的方法
    switch (command.getType()) {
      case INSERT: {
//转换参数
        Object param = method.convertArgsToSqlCommandParam(args);
//执行  INSERT 操作 ，转换 rowCount
        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;
      }
      case SELECT:
//无返回，并且有 ResultHandler 方法参数，则将查询结果提交给ResultHandler 进行处理
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
//转换参数
          Object param = method.convertArgsToSqlCommandParam(args);
//查询单条
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        throw new BindingException("Unknown execution method for: " + command.getName());
    }
//返回结果为null，并且返回类型为基本类型，则抛出 BindingException异常
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
//返回结果
    return result;
  }