adam bien's blog

EntityManager is an interface and can be easily mocked out with https://code.google.com/p/mockito/.

At the same time the EntityManager is also a factory, which creates Query instance, which in turn creates results. Mocking a query involves therefore a three step (=three lines process):

public class RegistrationsTest {

    Registrations cut;

    @Before
    public void init() {
        this.cut = new Registrations();
        this.cut.priceCalculator = mock(VatCalculator.class);

        this.cut.em = mock(EntityManager.class);

		//...
    }

    void mockQuery(String name, List<Registration> results) {

        Query mockedQuery = mock(Query.class);
        when(mockedQuery.getResultList()).thenReturn(results);
        when(this.cut.em.createNamedQuery(name)).thenReturn(mockedQuery);

    }

After this step you can easily return whatever results you like:

    @Test
    public void convertEmptyListToJson() {

        mockQuery(Registration.findAll, Collections.EMPTY_LIST);

        final JsonArray result = this.cut.allAsJson();
        assertNotNull(result);
        assertTrue(result.isEmpty());
    }

If you would like to ignore the parameters, or react to specific query parameters, the method Query::setParameter needs to be mocked as well:

  when(mockedQuery.setParameter(Matchers.anyString(), Matchers.anyObject())).thenReturn(mockedQuery);

See the entire unit test: RegistrationsTest.java. The whole example is available as maven archetype.

Usually complex queries are going to be encapsulated in dedicated controls, so it is easier to mock out the whole control instead.