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

Question

I have been developing a command-line tool which calls System.exit() (don't want to use exceptions instead of) on certain inputs.

I am familiar with Java: How to test methods that call System.exit()? and its the most elegant approach.

Unfortunately, it is not enough pure, due to I had to add the dependencies to system-rules, junit-interface

Is there any common pattern for dealing with System.exit in specs2 which is more pure than my current approach which don't use specs2?

import org.junit.Rule;
import org.junit.Test;
import org.junit.contrib.java.lang.system.ExpectedSystemExit;

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

    @Test
    public void emptyArgs() {
        exit.expectSystemExit();
        Converter.main(new String[]{});
    }

    @Test
    public void missingOutArgument() {
        exit.expectSystemExitWithStatus(1);
        Converter.main(new String[]{"--in", "src/test/resources/078.xml.gz"});
    }
}

Answer 1

If you really wish to go with a method using System.exit(), the simplest way to test it was actually called is to replace your SecurityManager with one that'll throw an ExitException (subclassing SecurityException) when System.exit() is called:

class SystemExitSpec

import java.security.Permission

import org.specs2.mutable.Specification
import org.specs2.specification.BeforeAfterAll

sealed case class ExitException(status: Int) extends SecurityException("System.exit() is not allowed") {
}

sealed class NoExitSecurityManager extends SecurityManager {
  override def checkPermission(perm: Permission): Unit = {}

  override def checkPermission(perm: Permission, context: Object): Unit = {}

  override def checkExit(status: Int): Unit = {
    super.checkExit(status)
    throw ExitException(status)
  }
}


abstract class SystemExitSpec extends Specification with BeforeAfterAll {

  sequential

  override def beforeAll(): Unit = System.setSecurityManager(new NoExitSecurityManager())

  override def afterAll(): Unit = System.setSecurityManager(null)
}

test ConverterSpec

import org.specs2.execute.Failure

import scala.io.Source

class ConverterSpec extends SystemExitSpec {

"ConverterSpec" should {

    "empty args" >> {
      try {
        Converter.main(Array[String]())
        Failure("shouldn't read this code")
      } catch {
        case e: ExitException =>
          e.status must_== 1
      }
      1 must_== 1
    }
}

Answer 2

First option: use some exception instead of System.exit.

Second option: call application in separate thread and check return codes.

Third option: mock System.exit. There are many possibilities to do that, mentioned one is quite good.

However, there is no specs2-specific pattern to work with System.exit. Personally I'd suggest first or second options.