如何使用mockMvc检查响应体中的String

I have simple integration test

我有简单的集成测试

    @Test
public void shouldReturnErrorMessageToAdminWhenCreatingUserWithUsedUserName() throws Exception {
    mockMvc.perform(post("/api/users").header("Authorization", base64ForTestUser).contentType(MediaType.APPLICATION_JSON)
            .content("{\"userName\":\"testUserDetails\",\"firstName\":\"xxx\",\"lastName\":\"xxx\",\"password\":\"xxx\"}"))
            .andDo(print())
            .andExpect(status().isBadRequest())
            .andExpect(?);
}