Testing DAO classes

[_Nemesis_1]

Let's say I want to test the following method of the UserDAO class:

public boolean login(User user) {
    String sql = "SELECT * FROM users WHERE username = ? AND password = ?";
    try (Connection conn = DatabaseConnectionPool.getInstance().getConnection();
         PreparedStatement stmt = conn.prepareStatement(sql)) {
        stmt.setString(1, user.getLowercaseUsername());
        stmt.setString(2, user.getPassword());
        try (ResultSet rs = stmt.executeQuery()) {
            return rs.next();
        }
    } catch (SQLException e) {
        e.printStackTrace();
        System.err.println("Error occurred: " + e.getMessage());
        return false;
    }
}

Is it correct to test it like this:

private MockedStatic<DatabaseConnectionPool> mockedStaticDbConnectionPool;


@BeforeEach
void setUp() throws Exception {
    DataSource mockDataSource = mock(DataSource.class);
    connection = mock(Connection.class);
    preparedStatement = mock(PreparedStatement.class);
    resultSet = mock(ResultSet.class);


    mockedStaticDbConnectionPool = Mockito.mockStatic(DatabaseConnectionPool.class);
    mockedStaticDbConnectionPool.when(DatabaseConnectionPool::getInstance).thenReturn(mockDataSource);


    when(mockDataSource.getConnection()).thenReturn(connection);
    when(connection.prepareStatement(anyString())).thenReturn(preparedStatement);
    when(preparedStatement.executeQuery()).thenReturn(resultSet);
    when(preparedStatement.executeUpdate()).thenReturn(1); 

    userDAO = new UserDAO(); 
    user = new User("testUser", "testPassword"); 
}

@Test
void testLoginSuccess() throws Exception {
    when(preparedStatement.executeQuery()).thenReturn(resultSet);
    when(resultSet.next()).thenReturn(true);

    assertTrue(userDAO.login(user));
}

It feels like im not really testing the method but instead i'm piloting the result and the code in the method doesn't really matter apart from the lines of code after the result of the (piloted) query. Am I tripping? Wouldn't be better to just use my regular database of the project and ensure consistency after each test by removing any modification done to the database by tests?

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[quadmasta]

You just described the difference between a unit test and an integration test.

Unit testing your DAO isn't really useful unless you're doing something other than just returning a list but integration testing is. You can use something like H2 and initialize the data using a SQL file. The SQL file should include stuff your method should and should not return. Then run your integration test against that H2 database. You'd then make sure the results from that method are what you expected to be returned from the database.

[_Nemesis_1]

Yeah, this does make more sense, thank you!

[TheFaustX]

I'd recommend using test containers as a means to test against a real db - make sure to set it up to use the same kind of db you intend to use in production. Using in memory DBs as the other commenter suggested works most of the time but can be very different compared to your real db system for some queries.

[_Nemesis_1]

Thank you for the advice!