How to test OS-specific method with JUnit?

Question

I would like to test the following method with JUnit:

private static boolean systemIsWindows() {
    String os = System.getProperty("os.name").toLowerCase();
    return os.startsWith("win");
}

Frankly, the only thing I've come up with is to basically copy to same logic to the test. This would, of course, protect against the method being inadvertently broken, but sounds somehow counter-intuitive.

What would be a better way to test this method?

Answer 1

In your Unit tests, you can change the value of the property:

System.setProperty("os.name", "Linux")

After that, you can then test/call your systemIsWindows() method to check that what it returns using asserts.

To make it easier to set a System property and to unset that property on completion of the test (thereby facilitating test isolation, self containment) you could use either of the following JUnit add-ons:

• JUnit4: JUnit System Rules
• JUnit5: JUnit Extensions

For example:

@Test
@SystemProperty(name = "os.name", value = "Windows")
public void aTest() {
    assertThat(systemIsWindows(), is(true));
}


@Test
@SystemProperty(name = "os.name", value = "MacOs")
public void aTest() {
    assertThat(systemIsWindows(), is(false));
}