Let's start with writing a unit test to retrieve all todos—http://localhost:8080/users/Jack/todos. The following code is an example response:

[
 {
 "id":1,
 "user":"Jack",
 "desc":"Learn Spring MVC",
 "targetDate":"2019-01-08T12:42:42.337+0000",
 "done":false
 },
 {
 "id":2,
 "user":"Jack",
 "desc":"Learn Struts",
 "targetDate":"2019-01-08T12:42:42.337+0000",
 "done":false
 }
]

The REST API uses the service.retrieveTodos method to retrieve the Todo details for a user. In a unit test, we want to mock the service, and configure a mock response. We can do this using Mockito:

List < Todo > mockList = Arrays.asList(
    new Todo(1, "Jack", "Learn Spring MVC", new Date(), false),
    new Todo(2, "Jack", "Learn Struts", new Date(), false)
);
when(service.retrieveTodos(anyString()))
    .thenReturn(mockList);

When the retrieveTodos method is called, it will return a mock hardcoded response.

Here's the complete unit test:

@Test
public void retrieveTodos() throws Exception {
    //GIVEN
    List < Todo > mockList = Arrays.asList(
        new Todo(1, "Jack", "Learn Spring MVC", new Date(), false),
        new Todo(2, "Jack", "Learn Struts", new Date(), false)
    );

    when(service.retrieveTodos(anyString())).thenReturn(mockList);

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

    //THEN
    String expected = "[" + "{id:1,user:Jack,desc:"Learn Spring MVC",done:false}" + "," +
        "{id:2,user:Jack,desc:"Learn Struts",done:false}" + "]";
    JSONAssert.assertEquals(expected, result.getResponse().getContentAsString(), false);
}

JSONAssert is a very useful framework that is used to perform asserts on JSON. It compares the response text with the expected value:

JSONAssert.assertEquals(expected, result.getResponse().getContentAsString(), false)

The last parameter, false, indicates the use of a non-strict mode. In a non-strict mode, JSONAssert only compares the elements that are specified in the expected JSON string. If it is changed to true, then the expected string should exactly match the result.