Mockito: How to test a class's void method? - mockito

Question

Unit test noob here.
I have three classes: Db1Dao, Db2Dao, ExecuteClass where Db1Dao, Db2Dao are database access objects for two different databases. My goal is to fetch some data from db1 using Db1Dao and run executeClass.execute() to "put" the processed data into db2 using Db2Dao.
My ExecuteClass looks like this:
class ExecuteClass {
private Db1Dao db1Dao;
private Db2Dao db2Dao;
public void execute() {
...
List<String> listOfString = getExternalData(someParam);
List<Metadata> metadatum = db1Dao.get(someInputs);
... I do something to generate a list of new class `A` based on listOfString & metadatum ...
try {
db2Dao.put(listOfA);
} catch (PutException e){
...
}
}
public List<String> getExternalData(SomeClass someParam){
... do something
return listOfString;
}
}
Now I want to test:
Given a specific listOfString (returned by getExternalData) and a specific metadatum (returned by db1Dao.get):
Will I get the desired listOfA?
Am I able to call db2Dao.put and its input parameter is listOfA?
Particularly, I have hard-coded sample listOfString and metadatum and desired listOfA (and they will be passed via an object MockData, see the following code) but I don't know how to write the test using Mockito. The following is a test class I wrote but it does not work:
class TestClass extends BaseTest {
#Mock
private Db1Dao db1Dao;
#Mock
private Db2Dao db2Dao;
private ExecuteClass executeClass;
#BeforeEach
public void setUp() {
MockitoAnnotations.initMocks(this);
executeClass = new ExecuteClass(db1Dao, db2Dao);
}
#ParameterizedTest
#MethodSource("MockDataProvider")
public void executeClassTest(final MockData mockData) throws PutException {
Mockito.when(db1Dao.get(Mockito.any(), ...))
.thenReturn(mockData.getMetadatum());
ExecuteClass executeClassSpy = Mockito.spy(executeClass);
Mockito.when(executeClassSpy.getExternalData(Mockito.any()))
.thenReturn(mockData.getListOfString());
executeClassSpy.execute();
// executeClass.execute(); not working neither...
List<A> listOfA = mockData.getDesiredListOfA();
Mockito.verify(db2Dao).put(listOfA);
}
}
Could anyone please let me know? Thank you in advance!!

Answer 1

You should not create a spy of the same class you want to test. Instead, try to write a unit test for the smallest amount of code (e.g. a public method) and mock every external operator (in your case Db1Dao and Db2Dao).
If testing a public method involves calling another public method of the same class, make sure to mock everything inside the other public method (in your case getExternalData). Otherwise, this other public method might be a good candidate for an extra class to have clear separation of concerns.
So, remove the ExecuteClass executeClassSpy = Mockito.spy(executeClass); and make sure you setup everything with Mockito what's called within getExternalData.
To now actually, verify that Db2Dao was called with the correct parameter, either use your current approach with verifying the payload. But here it's important to 100% create the same data structure you get while executing your application code.
Another solution would be to use Mockito's #Captor. This allows you to capture the value of why verifying the invocation of a mock. Later on, you can also write assertions on the captured value:
#Captor
private ArgumentCaptor<ClassOfListOfA> argumentCaptor;
#Test
public void yourTest() {
Mockito.verify(db2Dao).put(argumentCaptor.capture());
assertEquals("StringValue", argumentCaptur.getValue().getWhateverGetterYouHave);
}

Answer 2

The following code worked for me.
I partially accepted #rieckpil's answer. I used #Captor which is very handy.
The reason I had to mock getExternalData() is because its implementation is still a "TODO".
class TestClass extends BaseTest {
#Mock
private Db1Dao db1Dao;
#Mock
private Db2Dao db2Dao;
#Captor
private ArgumentCaptor<List<A>> argumentCaptor;
private ExecuteClass executeClass;
#BeforeEach
public void setUp() {
MockitoAnnotations.initMocks(this);
executeClass = new ExecuteClass(db1Dao, db2Dao);
}
#ParameterizedTest
#MethodSource("MockDataProvider")
public void executeClassTest(final MockData mockData) throws PutException {
Mockito.when(db1Dao.get(Mockito.any(), ...))
.thenReturn(mockData.getMetadatum());
ExecuteClass executeClassSpy = Mockito.spy(executeClass);
Mockito.when(executeClassSpy.getExternalData(Mockito.any()))
.thenReturn(mockData.getListOfString());
executeClassSpy.execute();
List<A> listOfA = mockData.getDesiredListOfA();
Mockito.verify(db2Dao).put(argumentCaptor.capture());
assertEquals(listOfA, argumentCaptor.getValue());
}
}