Dealing with System.exit(0) in JUnit tests

问题:

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?

回答1:

How I deal with that is to install a security manager that throws an exception when System.exit is called. Then there is code that catches the exception and doesn't fail the test.

public class NoExitSecurityManager
    extends java.rmi.RMISecurityManager
{
    private final SecurityManager parent;

    public NoExitSecurityManager(final SecurityManager manager)
    {
        parent = manager;
    }

    public void checkExit(int status)
    {
        throw new AttemptToExitException(status);
    }

    public void checkPermission(Permission perm)
    {
    }
}

And then in the code, something like:

catch(final Throwable ex)
{
    final Throwable cause;

    if(ex.getCause() == null)
    {
        cause = ex;
    }
    else
    {
        cause = ex.getCause();
    }

    if(cause instanceof AttemptToExitException)
    {
        status = ((AttemptToExitException)cause).getStatus();
    }
    else
    {
        throw cause;
    }
}

assertEquals("System.exit must be called with the value of " + expectedStatus, expectedStatus, status);

回答2:

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.

回答3:

Could you abstract out the "system exiting" into a new dependency, so that in your tests you could just have a fake which records the fact that exit has been called (and the value), but use an implementation which calls System.exit in the real application?

回答4:

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);
    }
}