MyBatis是如何让我们通过接口就能调用到SQL的

 	   MapperFactoryBean: 
       	public T getObject() throws Exception {
       		return getSqlSession().getMapper(this.mapperInterface);
       	}
       
       DefaultSqlSession: 
           public <T> T getMapper(Class<T> type) {
           	//configuration是mybatis的重要配置类, 在初始化的时候, 就会将mapper接口添加到configuration中
           	return configuration.getMapper(type, this);
           }
       
       Configuration: 
       	public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
       		return mapperRegistry.getMapper(type, sqlSession);
       	}
       
       MapperRegistry: 
       	public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
       	    //获取mapper代理工厂
       		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);
       		}
       	}
       
       MapperProxyFactory: 
       	protected T newInstance(MapperProxy<T> mapperProxy) {
       		return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
       	}
       
       	public T newInstance(SqlSession sqlSession) {
       		//返回一个新代理对象
       		final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
       		return newInstance(mapperProxy);
       	}

      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
        	//过滤掉object的方法
          if (Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
          } else {
            return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
          }
        } catch (Throwable t) {
          throw ExceptionUtil.unwrapThrowable(t);
        }
      }

public interface UserMapper {
	List<SysUser> selectAll();
}

public class MyMapperProxy<T> implements InvocationHandler {
	private Class<T> mapperInterface;
	private SqlSession sqlSession;
	
	public MyMapperProxy(Class<T> mapperInterface, SqlSession sqlSession) {
			this.mapperInterface = mapperInterface;
			this.sqlSession = sqlSession;
	}

	@Override
	public Object invoke(Object proxy , Method method , Object[] args)
			throws Throwable {
		//针对不同的 sql 类型，需要调用sqlSession不同的方法
		//接口方法中的参数也有很多情况 ，这里只考虑没有有参数的情况
		List<T> list= sqlSession.selectList(
				mapperInterface.getCanonicalName() + ”.” + method.getName());
		//返回数据也有很多情况，这里不做处理直接返回
		return list;
	}
}

//获取sqlSession
SqlSession sqlSession = getSqlSession();
//获取 UserMapper 接口
MyMapperProxy userMapperProxy = new MyMapperProxy(
		UserMapper.class , sqlSession) ;
UserMapper userMapper = (UserMapper) Proxy.newProxyinstance (
		Thread.currentThread().getContextClassLoader(),
		new Class[ ] {UserMapper.class},
		userMapperProxy) ;
//调 用 selectAll 方 法
List<SysUser> user= userMapper.selectAll();