mockMvc – Test Error Message

You can use the method status.reason().

For example:

     @Test
     public void loginWithBadCredentials() {
        this.mockMvc.perform(
                post("/rest/login")
                        .contentType(MediaType.APPLICATION_JSON)
                        .content("{\"username\": \"baduser\", \"password\": \"invalidPassword\"}")
                )
                .andDo(MockMvcResultHandlers.print())
                .andExpect(status().isUnauthorized())
                .andExpect(status().reason(containsString("Bad credentials")))
                .andExpect(unauthenticated());
    }


    MockHttpServletResponse:
                  Status = 401
           Error message = Authentication Failed: Bad credentials
            Content type = null
                    Body = 
           Forwarded URL = null
          Redirected URL = null
                 Cookies = []