Mock MVC - Add Request Parameter to test

If anyone came to this question looking for ways to add multiple parameters at the same time (my case), you can use .params with a MultivalueMap instead of adding each .param :

LinkedMultiValueMap<String, String> requestParams = new LinkedMultiValueMap<>()
requestParams.add("id", "1");
requestParams.add("name", "john");
requestParams.add("age", "30");

mockMvc.perform(get("my/endpoint").params(requestParams)).andExpect(status().isOk())

When i analyzed your code. I have also faced the same problem but my problem is if i give value for both first and last name means it is working fine. but when i give only one value means it says 400. anyway use the .andDo(print()) method to find out the error

public void testGetUserByName() throws Exception {
    String firstName = "Jack";
    String lastName = "s";       
    this.userClientObject = client.createClient();
    mockMvc.perform(get("/byName")
            .sessionAttr("userClientObject", this.userClientObject)
            .param("firstName", firstName)
            .param("lastName", lastName)               
    ).andDo(print())
     .andExpect(status().isOk())
            .andExpect(content().contentType("application/json"))
            .andExpect(jsonPath("$[0].id").exists())
            .andExpect(jsonPath("$[0].fn").value("Marge"));
}

If your problem is org.springframework.web.bind.missingservletrequestparameterexception you have to change your code to

@RequestMapping(value = "/byName", method = RequestMethod.GET)
    @ResponseStatus(HttpStatus.OK)
    public
    @ResponseBody
    String getUserByName(
        @RequestParam( value="firstName",required = false) String firstName,
        @RequestParam(value="lastName",required = false) String lastName, 
        @ModelAttribute("userClientObject") UserClient userClient)
    {

        return client.getUserByName(userClient, firstName, lastName);
    }