count members with jsonpath?

Is it possible to count the number of members using JsonPath?

Using spring mvc test I'm testing a controller that generates

{"foo": "oof", "bar": "rab"}

with

standaloneSetup(new FooController(fooService)).build()
			.perform(get("/something").accept(MediaType.APPLICATION_JSON)).andExpect(status().isOk())
			.andExpect(jsonPath("$.foo").value("oof"))
			.andExpect(jsonPath("$.bar").value("rab"));

I'd like to make sure that no other members are present in the generated json. Hopefully by counting them using jsonPath. Is it possible? Alternate solutions are welcome too.

To test size of array: jsonPath("$", hasSize(4))

To count members of object: jsonPath("$.*", hasSize(4))

---

I.e. to test that API returns an array of 4 items:

accepted value: [1,2,3,4]

mockMvc.perform(get(API_URL))
       .andExpect(jsonPath("$", hasSize(4)));

---

to test that API returns an object containing 2 members:

accepted value: {"foo": "oof", "bar": "rab"}

mockMvc.perform(get(API_URL))
       .andExpect(jsonPath("$.*", hasSize(2)));

---

I'm using Hamcrest version 1.3 and Spring Test 3.2.5.RELEASE

hasSize(int) javadoc

Note: You need to include hamcrest-library dependency and import static org.hamcrest.Matchers.*; for hasSize() to work.

You can also use the methods inside the jsonpath, so instead of

mockMvc.perform(get(API_URL))
   .andExpect(jsonPath("$.*", hasSize(2)));

you can do

mockMvc.perform(get(API_URL))
   .andExpect(jsonPath("$.length()", is(2)));

We can use JsonPath functions like size() or length(), like this:

@Test
public void givenJson_whenGetLengthWithJsonPath_thenGetLength() {
    String jsonString = "{'username':'jhon.user','email':'[email protected]','age':'28'}";

    int length = JsonPath
        .parse(jsonString)
        .read("$.length()");

    assertThat(length).isEqualTo(3);
}

Or simply parsing to net.minidev.json.JSONObject and get the size:

@Test
public void givenJson_whenParseObject_thenGetSize() {
    String jsonString = "{'username':'jhon.user','email':'[email protected]','age':'28'}";

    JSONObject jsonObject = (JSONObject) JSONValue.parse(jsonString);

    assertThat(jsonObject)
        .size()
        .isEqualTo(3);
}

Indeed, the second approach looks to perform better than the first one. I made a JMH performance test and I get the following results:

| Benchmark                                       | Mode  | Cnt | Score       | Error        | Units |
|-------------------------------------------------|-------|-----|-------------|--------------|-------|
| JsonPathBenchmark.benchmarkJSONObjectParse      | thrpt | 5   | 3241471.044 | ±1718855.506 | ops/s |
| JsonPathBenchmark.benchmarkJsonPathObjectLength | thrpt | 5   | 1680492.243 | ±132492.697  | ops/s |

The example code can be found here.

Been dealing with this myself today. It doesn't seem like this is implemented in the available assertions. However, there is a method to pass in an org.hamcrest.Matcher object. With that you can do something like the following:

final int count = 4; // expected count

jsonPath("$").value(new BaseMatcher() {
    @Override
    public boolean matches(Object obj) {
        return obj instanceof JSONObject && ((JSONObject) obj).size() == count;
    }

    @Override
    public void describeTo(Description description) {
        // nothing for now
    }
})

if you don't have com.jayway.jsonassert.JsonAssert on your classpath (which was the case with me), testing in the following way may be a possible workaround:

assertEquals(expectedLength, ((net.minidev.json.JSONArray)parsedContent.read("$")).size());

[note: i assumed that the content of the json is always an array]