Prevent JVM from exiting with 1 when main thread is only non-daemon

DevTools deliberately throws an uncaught exception on the main thread
as a safe way of causing it to stop processing. This exception is
caught and swallowed by an uncaught exception handler. Unfortunately,
this has the unwanted side-effect of causing the JVM to exit with 1
once all running threads are daemons.

Normally, this isn't a problem. Non-daemon threads, such as those
started by an embedded servlet container, will keep the JVM alive and
restarts of the application context will occur when the user makes to
their application. However, if the user adds DevTools to an
application that doesn't start any non-daemon threads, i.e. it starts,
runs, and then exits, it will exit with 1. This causes both
bootRun in Gradle and spring-boot:run in Maven to report that the
build has failed. While there's no benefit to using DevTools with an
application that behaves in this way, the side-effect of causing the
JVM to exit with 1 is unwanted.

This commit address the problem by updating the uncaught exception
handler to call System.exit(0) if the JVM is going to exit as a
result of the uncaught exception causing the main thread to die. In
other words, if the main thread was the only non-daemon thread, its
death as a result of the uncaught exception will now cause the JVM
to exit with 1 rather than 0. If there are other non-daemon threads
that will keep the JVM alive, the behaviour is unchanged.

Closes gh-5968

spring-boot-devtools/src/test/java/org/springframework/boot/devtools/restart/SilentExitExceptionHandlerTests.java

@@ -16,11 +16,14 @@
 
 package org.springframework.boot.devtools.restart;
 
+import java.util.concurrent.CountDownLatch;
+
 import org.junit.Test;
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.nullValue;
 import static org.junit.Assert.assertThat;
+import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
 /**
@@ -57,6 +60,24 @@ public class SilentExitExceptionHandlerTests {
 		assertThat(testThread.getThrown().getMessage(), equalTo("Expected"));
 	}
 
+	@Test
+	public void preventsNonZeroExitCodeWhenAllOtherThreadsAreDaemonThreads() {
+		try {
+			SilentExitExceptionHandler.exitCurrentThread();
+		}
+		catch (Exception ex) {
+			TestSilentExitExceptionHandler silentExitExceptionHandler = new TestSilentExitExceptionHandler();
+			silentExitExceptionHandler.uncaughtException(Thread.currentThread(), ex);
+			try {
+				assertTrue(silentExitExceptionHandler.nonZeroExitCodePrevented);
+			}
+			finally {
+				silentExitExceptionHandler.cleanUp();
+			}
+		}
+
+	}
+
 	private static abstract class TestThread extends Thread {
 
 		private Throwable thrown;
@@ -81,4 +102,58 @@ public class SilentExitExceptionHandlerTests {
 
 	}
 
+	private static class TestSilentExitExceptionHandler
+			extends SilentExitExceptionHandler {
+
+		private boolean nonZeroExitCodePrevented;
+
+		private final Object monitor = new Object();
+
+		TestSilentExitExceptionHandler() {
+			super(null);
+		}
+
+		@Override
+		protected void preventNonZeroExitCode() {
+			this.nonZeroExitCodePrevented = true;
+		}
+
+		@Override
+		protected Thread[] getAllThreads() {
+			final CountDownLatch threadRunning = new CountDownLatch(1);
+			Thread daemonThread = new Thread(new Runnable() {
+
+				@Override
+				public void run() {
+					synchronized (TestSilentExitExceptionHandler.this.monitor) {
+						threadRunning.countDown();
+						try {
+							TestSilentExitExceptionHandler.this.monitor.wait();
+						}
+						catch (InterruptedException ex) {
+							Thread.currentThread().interrupt();
+						}
+					}
+				}
+
+			});
+			daemonThread.setDaemon(true);
+			daemonThread.start();
+			try {
+				threadRunning.await();
+			}
+			catch (InterruptedException ex) {
+				Thread.currentThread().interrupt();
+			}
+			return new Thread[] { Thread.currentThread(), daemonThread };
+		}
+
+		private void cleanUp() {
+			synchronized (this.monitor) {
+				this.monitor.notifyAll();
+			}
+		}
+
+	}
+
 }