Getting httpServletRequest attribute with MockMvc

You could use

mvc.perform(post("/api/v1/...")
    .with(request -> {
      request.addHeader(HEADER_USERNAME_KEY, approver);
      request.setAttribute("attrName", "attrValue");
      return request;
    })
    .contentType(MediaType.APPLICATION_JSON)...

You add a request attribute by calling requestAttr ^^

mockMvc.perform(get("/api/test").requestAttr("userId", "testUserId")...