If the extension throws an exception then there's not much a @Test method can do since the test runner will never reach the @Test method. In this case, I think, you have to test the extension outside of its use in the normal test flow i.e. let the extension be the SUT.
For the extension provided in your question, the test might be something like this:
@Test
public void willRejectATestMethodHavingANameStartingWithAnUpperCaseLetter() throws NoSuchMethodException {
ExtensionContext extensionContext = Mockito.mock(ExtensionContext.class);
Method method = Testable.class.getMethod("MethodNameStartingWithUpperCase");
Mockito.when(extensionContext.getRequiredTestMethod()).thenReturn(method);
DisallowUppercaseLetterAtBeginning sut = new DisallowUppercaseLetterAtBeginning();
RuntimeException actual =
assertThrows(RuntimeException.class, () -> sut.beforeEach(extensionContext));
assertThat(actual.getMessage(), is("test method names should start with lowercase."));
}
@Test
public void willAllowTestMethodHavingANameStartingWithAnLowerCaseLetter() throws NoSuchMethodException {
ExtensionContext extensionContext = Mockito.mock(ExtensionContext.class);
Method method = Testable.class.getMethod("methodNameStartingWithLowerCase");
Mockito.when(extensionContext.getRequiredTestMethod()).thenReturn(method);
DisallowUppercaseLetterAtBeginning sut = new DisallowUppercaseLetterAtBeginning();
sut.beforeEach(extensionContext);
// no exception - good enough
}
public class Testable {
public void MethodNameStartingWithUpperCase() {
}
public void methodNameStartingWithLowerCase() {
}
}
However, your question suggests that the above extension is only an example so, more generally; if your extension has a side effect (e.g. sets something in an addressable context, populates a System property etc) then your @Test method could assert that this side effect is present. For example:
public class SystemPropertyExtension implements BeforeEachCallback {
@Override
public void beforeEach(ExtensionContext context) {
System.setProperty("foo", "bar");
}
}
@ExtendWith(SystemPropertyExtension.class)
public class SystemPropertyExtensionTest {
@Test
public void willSetTheSystemProperty() {
assertThat(System.getProperty("foo"), is("bar"));
}
}
This approach has the benefit of side stepping the potentially awkward setup steps of: creating the ExtensionContext and populating it with the state required by your test but it may come at the cost of limiting the test coverage since you can really only test one outcome. And, of course, it is only feasible if the extension has a side effect which can be evaulated in a test case which uses the extension.
So, in practice, I suspect you might need a combination of these approaches; for some extensions the extension can be the SUT and for others the extension can be tested by asserting against its side effect(s).
