The following example shows how we can unit test the validations we added in:

     @Test
     public void createTodo_withValidationError() throws Exception {
       Todo mockTodo = new Todo(CREATED_TODO_ID, "Jack", 
       "Learn Spring MVC", new Date(), false);

       String todo = "{"user":"Jack","desc":"Learn","done":false}";

       when( service.addTodo(
         anyString(), anyString(), isNull(), anyBoolean()))
        .thenReturn(mockTodo);

         MvcResult result = mvc.perform(
         MockMvcRequestBuilders.post("/users/Jack/todos")
        .content(todo)
        .contentType(MediaType.APPLICATION_JSON))
        .andExpect(
           status().is4xxClientError()).andReturn();
     }

Some important points to note are as follows:

• "desc":"Learn": We are using a desc value of length 5. This would cause a validation failure for the @Size(min = 9, message = "Enter atleast 10 Characters.") check.
• .andExpect(status().is4xxClientError()): Checks for validation error status.