How to test methods that call System.exit()?

java

test-engineering

java-8

best-practices

byNikita Barsukov·Oct 27, 2024

// With the power of JUnit's ExpectedSystemExit rule, we can trap the exit like a ghostbuster!
import org.junit.Rule;
import org.junit.Test;
import org.junit.contrib.java.lang.system.ExpectedSystemExit;

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

    @Test
    public void doesItReallyExit() {
        exit.expectSystemExit();
        System.exit(42); // Ah, the answer to life, universe, everything!
    }
}

Trap the exit code and prevent pretty much "murder" of JVM during your tests!

Exit alternatives

If using ExpectedSystemExit feels wrong like eating pineapple on pizza, here are a few alternatives:

The aggressive pacifist: Unchecked exceptions

    public void terminateWithGrace(int status) {
        throw new RuntimeException("Roses are red, my love for testing is endless, exiting with status " + status);
    }

No JVM termination, just hurt feelings through exception handling.

The protective parent: Custom SecurityManager

    public class OverprotectedSecurityManager extends SecurityManager {
        @Override
        public void checkExit(int status) {
            super.checkExit(status);
            throw new SecurityException("Young man, where do you think you're going?!");
        }
    }

Treats termination like it's past curfew time! SecurityException: Caught red-handed!

The lamb whisperer: System Lambda

import static com.github.stefanbirkner.systemlambda.SystemLambda.catchSystemExit;
public class CodeWhisperer {
    @Test
    public void exitsLikeANinja() {
        int status = catchSystemExit(() -> {
            System.exit(0); // Code ninja, vanishing!
        });
        assertEquals(0, status); // No trace left behind.
    }
}