Dealing with System.exit(0) in JUnit tests

Question

I am implementing some tests for an existing Java Swing application, so that I can safely refactor and extend the code without breaking anything. I started with some unit tests in JUnit, since that seems the simplest way to get started, but now my priority is to create some end-to-end tests to exercise the application as a whole.

I am starting the application afresh in each test by putting each test method in a separate test case, and using the fork="yes" option in Ant's junit task. However, some of the use cases I would like to implement as tests involve the user exiting the application, which results in one of the methods calling System.exit(0). This is regarded by JUnit as an error: junit.framework.AssertionFailedError: Forked Java VM exited abnormally.

Is there a way to tell JUnit that exiting with a return code of zero is actually OK?

Answer 1

The library System Rules has a JUnit rule called ExpectedSystemExit. With this rule you are able to test code, that calls System.exit(...):

public class MyTest {
    @Rule
    public final ExpectedSystemExit exit = ExpectedSystemExit.none();

    @Test
    public void systemExitWithArbitraryStatusCode() {
        exit.expectSystemExit();
        /* the code under test, which calls System.exit(...)
         * with an arbitrary status
         */
    }

    @Test
    public void systemExitWithSelectedStatusCode0() {
        exit.expectSystemExitWithStatus(0);
        //the code under test, which calls System.exit(0)
    }
}

System Rules needs at least JUnit 4.9.

Full disclosure: I'm the author of System Rules.

Answer 2

If anybody needs this functionality for JUnit 5, I've written an extension to do this. This is a simple annotation you can use to tell your test case to expect and exit status code or a specific exit status code.

For example, any exit code will do:

public class MyTestCases { 

    @Test
    @ExpectSystemExit
    public void thatSystemExitIsCalled() {
        System.exit(1);
    }
}

If we want to look for a specific code:

public class MyTestCases {

    @Test
    @ExpectSystemExitWithStatus(1)
    public void thatSystemExitIsCalled() {
        System.exit(1);
    }
}