ClassCastException on attempt to mock Querydsl SQLQueryFactory - mockito

Question

Trying to mock SQLQueryFactory of Querydsl for DAO unit testing. Using Mockito's deep stub for the very first time.
Below is the minimal code which fails
#Test
void tryMockQueryDsl() {
SQLQueryFactory sql = Mockito.mock(SQLQueryFactory.class, Mockito.RETURNS_DEEP_STUBS);
Mockito.when(sql.select(ArgumentMatchers.<Expression<?>>any())
.from(ArgumentMatchers.<Expression<?>>any())
.fetchFirst()
).thenReturn(null);
}
with the following exception:
java.lang.ClassCastException: class com.querydsl.sql.ProjectableSQLQuery$MockitoMock$1584151766 cannot be cast to class com.querydsl.sql.SQLQuery (com.querydsl.sql.ProjectableSQLQuery$MockitoMock$1584151766 and com.querydsl.sql.SQLQuery are in unnamed module of loader 'app')
What can be the problem?

Answer 1

Mocks can not be cast. Instead of RETURN_DEEP_STUBS mock each method on its own and return a mocked instance of the expected class.
If you do not want to suppress the warnings you will get for not defining the generic types, you can use the #Mock annotation instead to create the mocks, like described here.
This example doesn't make much sense for a testcase (as it tests nothing), but its a showcase on how to avoid the exception.
#RunWith(MockitoJUnitRunner.class)
public class Test {
#Test
public void tryMockQueryDsl() {
ProjectableSQLQuery projectableQuery = Mockito.mock(ProjectableSQLQuery.class);
// not really needed as this is the default behaviour
Mockito.when(projectableQuery.fetchFirst()).thenReturn(null);
SQLQuery query = Mockito.mock(SQLQuery.class);
Mockito.when(query.from(ArgumentMatchers.<Expression<?>>any())).thenReturn(projectableQuery);
SQLQueryFactory sql = Mockito.mock(SQLQueryFactory.class);
Mockito.when(sql.select(ArgumentMatchers.<Expression<?>>any())).thenReturn(query);
Expression expr = Mockito.mock(Expression.class);
sql.select(expr).from(expr).fetchFirst();
}
}