How to mock persisting and Entity with Mockito and jUnit

Question

I'm trying to find a way to test my entity using Mockito;

This is the simple test method:

@Mock
private EntityManager em;

@Test
public void persistArticleWithValidArticleSetsArticleId() {
    Article article = new Article();
    em.persist(article);
    assertThat(article.getId(), is(not(0L)));
}

How do I best mock the behaviour that the EntityManager changes the Id from 0L to i.e. 1L? Possibly with the least obstructions in readability.

Edit: Some extra information; Outside test-scope the EntityManager is produced by an application-container

Answer 1

public class AssignIdToArticleAnswer implements Answer<Void> {

    private final Long id;

    public AssignIdToArticleAnswer(Long id) {
        this.id = id;
    }

    @Override
    public Void answer(InvocationOnMock invocation) throws Throwable {
        Article article = (Article) invocation.getArguments()[0];
        article.setId(id);
        return null;
    }
}

And then

doAnswer(new AssignIdToArticleAnswer(1L)).when(em).persist(any(Article.class));

Answer 2

You could use a Mockito Answer for this.

doAnswer(new Answer<Object>(){
     @Override
     public Object answer(InvocationOnMock invocation){
        Article article = (Article) invocation.getArguments()[0];
        article.setId(1L);
        return null;
     }
  }).when(em).persist(any(Article.class));

This tells Mockito that when the persist method is called, the first argument should have its setId method invoked.

But if you do this, I don't understand what the purpose of the test would be. You'd really just be testing that the Mockito Answer mechanism works, not that the code of Article or of EntityManager works correctly.

Answer 3

similar answer as above, but with lambdas

   doAnswer((InvocationOnMock invocation) -> {
        Article article = (Article) invocation.getArguments()[0];
        article.setId(1L);
        return null;
    }).when(em).persist(any(Article.class));