Added MainRunner. This removes the need for public methods in JUnitCore

MainRunner installs a SecurityManager which traps the System.exit(), thereby
removing the need to have runMainAndExit and runMain public in JUnitCore.
MainRunner is only used in the tests of course.

src/main/java/org/junit/internal/JUnitSystem.java

@@ -3,6 +3,5 @@
 import java.io.PrintStream;
 
 public interface JUnitSystem {
-	void exit(int i);
 	PrintStream out();
 }

src/main/java/org/junit/internal/RealSystem.java

@@ -3,11 +3,6 @@
 import java.io.PrintStream;
 
 public class RealSystem implements JUnitSystem {
-
-	public void exit(int code) {
-		System.exit(code);
-	}
-
 	public PrintStream out() {
 		return System.out;
 	}

src/main/java/org/junit/runner/JUnitCore.java

@@ -39,12 +39,13 @@ public static void main(String... args) {
 	}
 
 	/**
-	 * Do not use. Testing purposes only.
-	 * @param system 
+	 * Runs main and exits
+	 * @param system
+	 * @args args from main()
 	 */
-	public static void runMainAndExit(JUnitSystem system, String... args) {
+	private static void runMainAndExit(JUnitSystem system, String... args) {
 		Result result= new JUnitCore().runMain(system, args);
-		system.exit(result.wasSuccessful() ? 0 : 1);
+		System.exit(result.wasSuccessful() ? 0 : 1);
 	}
 
 	/**
@@ -70,10 +71,10 @@ public static Result runClasses(Class<?>... classes) {
 	}
 
 	/**
-	 * Do not use. Testing purposes only.
 	 * @param system 
+	 * @args args from main()
 	 */
-	public Result runMain(JUnitSystem system, String... args) {
+	private Result runMain(JUnitSystem system, String... args) {
 		system.out().println("JUnit version " + Version.id());
 		List<Class<?>> classes= new ArrayList<Class<?>>();
 		List<Failure> missingClasses= new ArrayList<Failure>();

src/test/java/org/junit/tests/TestSystem.java

@@ -8,18 +8,13 @@
 
 public class TestSystem implements JUnitSystem {
 	private PrintStream out;
-	public int fCode;
 	private ByteArrayOutputStream fOutContents;
 
 	public TestSystem() {
 		fOutContents= new ByteArrayOutputStream();
 		out= new PrintStream(fOutContents);
 	}
 
-	public void exit(int code) {
-		fCode= code;
-	}
-
 	public PrintStream out() {
 		return out;
 	}

src/test/java/org/junit/tests/running/core/CommandLineTest.java

@@ -9,7 +9,6 @@
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
-import org.junit.internal.RealSystem;
 import org.junit.runner.JUnitCore;
 
 public class CommandLineTest {
@@ -34,8 +33,12 @@ static public class Example {
 	}
 
 	@Test public void runATest() {
-		testWasRun= false; // todo create a TestSystem instead
-		new JUnitCore().runMain(new RealSystem(), new String[]{"org.junit.tests.running.core.CommandLineTest$Example"});
+		testWasRun= false;
+		new MainRunner().runWithCheckForSystemExit(new Runnable() {
+			public void run() {
+				JUnitCore.main("org.junit.tests.running.core.CommandLineTest$Example");
+			}
+		});
 		assertTrue(testWasRun);
 	}
 

src/test/java/org/junit/tests/running/core/JUnitCoreReturnsCorrectExitCodeTest.java

@@ -4,7 +4,6 @@
 import static org.junit.Assert.fail;
 import org.junit.Test;
 import org.junit.runner.JUnitCore;
-import org.junit.tests.TestSystem;
 
 public class JUnitCoreReturnsCorrectExitCodeTest {
 
@@ -31,9 +30,12 @@ static public class Succeed {
 		runClass(getClass().getName() + "$Succeed", 0);
 	}
 
-	private void runClass(String className, int returnCode) {
-		TestSystem system= new TestSystem();
-		JUnitCore.runMainAndExit(system, className); 
-		assertEquals(returnCode, system.fCode);
+	private void runClass(final String className, int returnCode) {
+		Integer exitValue= new MainRunner().runWithCheckForSystemExit(new Runnable() {
+			public void run() {
+				JUnitCore.main(className); 
+			}
+		});
+		assertEquals(Integer.valueOf(returnCode), exitValue);
 	}
 }

src/test/java/org/junit/tests/running/core/MainRunner.java

@@ -0,0 +1,69 @@
+package org.junit.tests.running.core;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.security.Permission;
+
+public class MainRunner {
+
+	private static class ExitException extends SecurityException {
+		private static final long serialVersionUID= -9104651568237766642L;
+
+		private final int status;
+
+		public ExitException(int status) {
+			super("");
+			this.status= status;
+		}
+
+		public int getStatus() {
+			return status;
+		}
+	}
+
+	private static class NoExitSecurityManager extends SecurityManager {
+		@Override
+		public void checkPermission(Permission perm) {
+			// allow anything.
+		}
+
+		@Override
+		public void checkPermission(Permission perm, Object context) {
+			// allow anything.
+		}
+
+		@Override
+		public void checkExit(int status) {
+			super.checkExit(status);
+			throw new ExitException(status);
+		}
+	}
+
+	/**
+	 * Execute runnable.run(), preventing System.exit(). If System.exit() is called
+	 * in runnable.run(), the value is returned. If System.exit()
+	 * is not called, null is returned.
+	 * 
+	 * @param runnable
+	 * @return null if System.exit() is not called, Integer.valueof(status) if not
+	 */
+	public Integer runWithCheckForSystemExit(Runnable runnable) {
+		SecurityManager oldSecurityManager = System.getSecurityManager();
+		System.setSecurityManager(new NoExitSecurityManager());
+		PrintStream oldPrintStream = System.out;
+
+		System.setOut(new PrintStream(new ByteArrayOutputStream()));
+		try {
+			runnable.run();
+			System.out.println("System.exit() not called, return null");
+			return null;
+		} catch (ExitException e) {
+			System.out.println("System.exit() called, value=" + e.getStatus());
+			return e.getStatus();
+		} finally {
+			System.setSecurityManager(oldSecurityManager);
+			System.setOut(oldPrintStream);
+		}
+	}
+
+}