How Can I Test Methods Invoking System.exit() in JUnit?

Managing System.exit() Calls in Unit Tests

Testing methods that invoke System.exit() poses a challenge in JUnit environments. When System.exit() is called, it terminates the Java Virtual Machine (JVM), including the test process.

Common Approaches:

• Substitute System.exit(): Consider using a testing library that allows for stubbing or mocking System.exit(). This approach ensures the JVM continues running for testing purposes.
• Prevent JVM Termination: Implement a custom security manager that prevents System.exit() from terminating the JVM. This involves catching the SecurityException thrown by attempting to exit.
• JUnit 4.9 System Rules: For JUnit versions 4.9 and above, utilize System Rules to specifically handle System.exit() within unit tests. You can verify that System.exit() is called and even assert the exit status.

Example Using NoExitSecurityManager:

The following Java code demonstrates using a custom security manager to prevent JVM termination during testing:

public class NoExitTestCase extends TestCase {

    private static class NoExitSecurityManager extends SecurityManager {

        @Override
        public void checkExit(int status) {
            super.checkExit(status);
            throw new ExitException(status);
        }
    }

    @Override
    protected void setUp() throws Exception {
        super.setUp();
        System.setSecurityManager(new NoExitSecurityManager());
    }

    @Override
    protected void tearDown() throws Exception {
        System.setSecurityManager(null);
        super.tearDown();
    }

    public void testNoExit() throws Exception {
        System.out.println("Printing works");
    }

    public void testExit() throws Exception {
        try {
            System.exit(42);
        } catch (ExitException e) {
            assertEquals("Exit status", 42, e.status);
        }
    }
}

System Rules for JUnit 4.9 :

JUnit 4.9 and above offer system rules specifically designed to handle System.exit(). The following example uses the ExpectedSystemExit rule to verify that System.exit() is called and the exit status:

public class MyTest {

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

    @Test
    public void noSystemExit() {
        //passes
    }

    @Test
    public void systemExitWithArbitraryStatusCode() {
        exit.expectSystemExit();
        System.exit(0);
    }

    @Test
    public void systemExitWithSelectedStatusCode0() {
        exit.expectSystemExitWithStatus(0);
        System.exit(0);
    }
}

2023 JVM Update:

Note that for Java 21 onwards, you must set the system property -Djava.security.manager=allow to prevent System.exit() from terminating the JVM during testing.

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