JUnit @Rule lifecycle interaction with @Before

问题:

I have some JUnit tests that use the TemporaryFolder @Rule. They use the TemporaryFolder in a @Before method to perform some setup:

@Rule
public TemporaryFolder folder = new TemporaryFolder();

@Before
public void init() {
  folder.newFile("my-file.txt");
}

@Test
public void myTest() { ... }

Most of the time this works perfectly. However, when using the SpringJUnit4ClassRunner I find that in some cases the init() method is invoked before the Statement inside my TemporaryFolder instance is applied. Because of this, the temporary folder location is unset (i.e: null) when folder is used within init() and my file ends up in the working directory, not /tmp.

So in some cases @Before methods are executed ahead of the rules, however, I can't establish a definite pattern. I occasionally see a similar problem with some of my own rule implementations.

Is there any way that I can ensure that my rule statements are applied before any setup methods?

回答1:

In JUnit 4.10, BlockJUnit4ClassRunner (the superclass of SpringJUnit4ClassRunner) appears to take care to construct Statement chains in such a way that rules run before any @Before methods. From JUnit 4.10:

protected Statement methodBlock(FrameworkMethod method) {
    // ...
    Statement statement= methodInvoker(method, test);
    statement= possiblyExpectingExceptions(method, test, statement);
    statement= withPotentialTimeout(method, test, statement);
    statement= withBefores(method, test, statement);
    statement= withAfters(method, test, statement);
    statement= withRules(method, test, statement);
    return statement;
}

JUnit 4.7 appears to stitch Statement chains together in a different order:

Statement statement= methodInvoker(method, test);
statement= possiblyExpectingExceptions(method, test, statement);
statement= withPotentialTimeout(method, test, statement);
statement= withRules(method, test, statement);
statement= withBefores(method, test, statement);
statement= withAfters(method, test, statement);
return statement;

spring-test-3.0.5's parent POM seems to indicate that it depends on JUnit 4.7. I wonder if getting it to use a newer JUnit would help?

回答2:

For what it's worth, I've used the following as a quick workaround:

@Rule
public TemporaryFolder tmpFolder = new TemporaryFolder() {
    @Override
    protected void before() throws Throwable {
        if (getRoot() == null) {
            super.before();
        }
    }

    @Override
    public File newFile(String fileName) throws IOException {
        try {
            before();
        }
        catch (Throwable t) {
            throw new RuntimeException(t.getMessage(), t);
        }

        return super.newFile(fileName);
    }

    @Override
    public File newFolder(String folderName) {
        try {
            before();
        }
        catch (Throwable t) {
            throw new RuntimeException(t.getMessage(), t);
        }

        return super.newFolder(folderName);
    }
};

This ensures that the TemporaryFolder is initialized properly, regardless of whether the @Before methods are run before or after the rules.