The code to unit test retrieveTodo is as follows:

     @Test
     public void retrieveTodo() throws Exception {
       Todo mockTodo = new Todo(1, "Jack", "Learn Spring MVC", 
       new Date(), false);
 
       when(service.retrieveTodo(anyInt())).thenReturn(mockTodo);

       MvcResult result = mvc.perform(
       MockMvcRequestBuilders.get("/users/Jack/todos/1")
       .accept(MediaType.APPLICATION_JSON))
       .andExpect(status().isOk()).andReturn();

       String expected = "{id:1,user:Jack,desc:"Learn Spring
       MVC",done:false}";
 
      JSONAssert.assertEquals(expected, 
       result.getResponse().getContentAsString(), false);

     }

A few important things to note are as follows:

• when(service.retrieveTodo(anyInt())).thenReturn(mockTodo): We are mocking the retrieveTodo service method to return the mock todo.
• MvcResult result = ..: We are accepting the result of the request into an MvcResult variable to enable us to perform assertions on the response.
• JSONAssert.assertEquals(expected, result.getResponse().getContentAsString(), false): Asserts whether the result is as expected.