Allow ExitCodeGenerator to be used on Exceptions

Update exit code support to allow the ExitCodeGenerator interface to
be placed on an Exception. Any uncaught exception implementing the
interface and returning a non `0` status will now trigger a System.exit
with the code.

Fixes gh-4803

spring-boot-samples/spring-boot-sample-simple/src/main/java/sample/simple/ExitException.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2012-2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package sample.simple;
+
+import org.springframework.boot.ExitCodeGenerator;
+
+public class ExitException extends RuntimeException implements ExitCodeGenerator {
+
+	@Override
+	public int getExitCode() {
+		return 10;
+	}
+
+}

spring-boot-samples/spring-boot-sample-simple/src/main/java/sample/simple/SampleSimpleApplication.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,6 +37,9 @@ public class SampleSimpleApplication implements CommandLineRunner {
 	@Override
 	public void run(String... args) {
 		System.out.println(this.helloWorldService.getHelloMessage());
+		if (args.length > 0 && args[0].equals("exitcode")) {
+			throw new ExitException();
+		}
 	}
 
 	public static void main(String[] args) throws Exception {