mybatis中Mapper接口如何实例化的(为什么mybatis的mapper没有实现类)

public class UserMapperTest {
	@Test
	public void testQueryUserById() throws IOException {
		String resource ="mybatis/mybatis-config.xml";
		InputStream inputStream = Resources.getResourceAsStream(resource);
		SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
		SqlSession sqlSession = sqlSessionFactory.openSession(true);
		UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
		User user = userMapper.queryUserById(1l);
		System.out.println(user);
	}
}

接上篇mybatis内部运行原理
跟踪sqlSession.getMapper(UserMapper.class)方法:;来到接口SqlSession的实现类SqlSessionTemplate

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

定位到configuration.getMapper(type, this),来到类Configuration

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

定位到mapperRegistry.getMapper(type, sqlSession)来到类MapperRegistry

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);
    }
  }

在这个方法里，首先获得一个MapperProxyFactory的对象mapperProxyFactory，然后调用该对象的newInstance方法创建我们需要的bean，定位到newInstance方法,来到类
MapperProxyFactory

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

在这里，首先构造出一个MapperProxy对象，然后调用自身的newInstance（重载的另一个方法）方法。
Proxy.newProxyInstance()这大家应该不陌生,它就是生成一个代理类.

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

newProxyInstance的第三个参数即为代理执行器的handler了，它传入的是一个mapperProxy对象
再查看mapperProxy的代码：

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;

  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }
}

看这段代码:

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

那么我们执行sqlSession.getMapper(UserMapper.class)这句代码最终返回的应该就是一个由jdk动态代理生成的代理类userMapper.
当执行userMapper.queryUserById()方法时，最终执行的也就是mapperMethod.execute(sqlSession,args)的代码
所以标题的答案就是它采用了：Java动态代理实现接口