如何模拟要修改的根文件夹中的文件

Question

For some reason, I have a path (//what/ever) stored in a private variable in a class where the file is modified. So, when I test it, it tries to modify the file, but, since the file is not there, an exception is thrown.

Of course, for unit testing, I don't want to create that file in the filesystem.

Is there a way to mock that very file and force the machine to write in //my/folder instead of //what/ever?

Answer 1

Mocking the file itself might be difficult. Unless, you abstract away your file handling into some interface that you manage and can inject a mock implementation during the test run. Another approach would be to expose the file path such that you can change it as part of your test setup code.

Yet another approach involves JUnit 4. In JUnit 4, you have the ability to manage temporary files and folders in your test runs. It will cleanup any new files or folders you create.

It can be done like so:

public class MyFileTest {
    @Rule
    public TemporaryFolder testFolder = new TemporaryFolder();

    @Test
    public void testSomething() throws IOException {
        File tempFile = testFolder.newFile("myfile");

I think something like this would allow you to create the actual file so your code can modify it, hence not blowing up. You might want to consider exactly the scope of your test in terms of:

• What exactly do you want to test?
• Is it that the right contents are written to the file?
• Is it that the file itself is modified?
• etc.

Answering some of these questions might move you in the right direction of exactly what kind of test you should write and what refactoring you may need to do to facilitate the test requirements.