func LoginMiddleware(ctx *gin.Context) {
	var UserId uint
	UserId = 2 //example
	fmt.Printf("LoginMiddleware UserId: %v\n", UserId)
	ctx.Set("UserId", UserId)
	ctx.Next()
}

在入口函数上添加中间件

group := r.Group("/api/group").Use(LoginMiddleware)
	{
		group.POST("/create", GroupController{}.Create)
	}