你还需要一次嘲弄...

connectionMock.createStatement()

...将返回null，除非您为其设置了预期值。
例如，添加...

@Mock
private Statement statement;

...

when(connectionMock.createStatement()).thenReturn(statement);
when(statement.executeQuery(sql)).thenAnswer(...);

更新

若要回答下面的注解，应返回结果集，而不是字符串。例如...

@Mock
private ResultSet resultSet;

...

when(statement.executeQuery(sql)).thenReturn(resultSet);
when(resultSet.getString(1)).thenReturn("X");

... call the class under the test...

// Add verification that "next" was called before "getString"...
// (not 100% necessary, but makes it a more thorough test)
InOrder order = inOrder(resultSet);
order.verify(resultSet).next();
order.verify(resultSet).getString(1);

更新#2
删除错误的内容

你可以试着用RETURNS_DEEP_STUBS;
在您的程式码中，它会看起来像这样：

public Connection connectionMock;

    @Before
    public void setUp(){
        connectionMock = Mockito.mock(Connection.class, RETURNS_DEEP_STUBS);
    }

    ...

实际上，你可以有一个更好的用法关于Mockito：

@RunWith(MockitoJUnitRunner.class)
 public class ExampleTest {
     @Mock
     public Connection connectionMock;     
 }