Auto-detect fork value in stop goal

So far, one has to set the "fork" value to both the start and stop
goals. Since they have the same name, sharing them in a global
configuration element does the trick. However, the plugin also supports
auto-detection of the fork value according to other parameters:
typically if an agent or jvm arguments are set, forking will be
automatically enabled. This is a problem since the stop goal is not aware
of that.

This commit transmits the value in a property attached to the
`MavenProject`. That way, the stop goal can retrieve that value and
apply the same defaults. This has the side effect that specifying the
fork value isn't necessary anymore.

Closes gh-6747

spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-automatic-fork/invoker.properties

@@ -0,0 +1 @@
+invoker.goals=clean verify

spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-automatic-fork/pom.xml

@@ -0,0 +1,58 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>org.springframework.boot.maven.it</groupId>
+	<artifactId>start-stop-automatic-fork</artifactId>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+	</properties>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>org.codehaus.mojo</groupId>
+				<artifactId>build-helper-maven-plugin</artifactId>
+				<executions>
+					<execution>
+						<id>reserve-jmx-port</id>
+						<goals>
+							<goal>reserve-network-port</goal>
+						</goals>
+						<phase>process-resources</phase>
+						<configuration>
+							<portNames>
+								<portName>jmx.port</portName>
+							</portNames>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+			<plugin>
+				<groupId>@project.groupId@</groupId>
+				<artifactId>@project.artifactId@</artifactId>
+				<version>@project.version@</version>
+				<executions>
+					<execution>
+						<id>pre-integration-test</id>
+						<goals>
+							<goal>start</goal>
+						</goals>
+						<configuration>
+							<jvmArguments>-Dfoo=bar</jvmArguments>
+						</configuration>
+					</execution>
+					<execution>
+						<id>post-integration-test</id>
+						<goals>
+							<goal>stop</goal>
+						</goals>
+					</execution>
+				</executions>
+				<configuration>
+					<jmxPort>${jmx.port}</jmxPort>
+				</configuration>
+			</plugin>
+		</plugins>
+	</build>
+</project>

spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-automatic-fork/src/main/java/org/test/SampleApplication.java

@@ -0,0 +1,82 @@
+/*
+ * 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 org.test;
+
+import java.lang.management.ManagementFactory;
+
+import javax.management.MBeanServer;
+import javax.management.ObjectName;
+
+/**
+ * This sample app simulates the JMX Mbean that is exposed by the Spring Boot application.
+ */
+public class SampleApplication {
+
+	private static final Object lock = new Object();
+
+	public static void main(String[] args) throws Exception {
+		MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
+		ObjectName name = new ObjectName(
+				"org.springframework.boot:type=Admin,name=SpringApplication");
+		SpringApplicationAdmin mbean = new SpringApplicationAdmin();
+		mbs.registerMBean(mbean, name);
+
+		// Flag the app as ready
+		mbean.ready = true;
+
+		int waitAttempts = 0;
+		while (!mbean.shutdownInvoked) {
+			if (waitAttempts > 30) {
+				throw new IllegalStateException(
+						"Shutdown should have been invoked by now");
+			}
+			synchronized (lock) {
+				lock.wait(250);
+			}
+			waitAttempts++;
+		}
+	}
+
+	public interface SpringApplicationAdminMXBean {
+
+		boolean isReady();
+
+		void shutdown();
+
+	}
+
+	static class SpringApplicationAdmin implements SpringApplicationAdminMXBean {
+
+		private boolean ready;
+
+		private boolean shutdownInvoked;
+
+		@Override
+		public boolean isReady() {
+			System.out.println("isReady: " + this.ready);
+			return this.ready;
+		}
+
+		@Override
+		public void shutdown() {
+			this.shutdownInvoked = true;
+			System.out.println("Shutdown requested");
+		}
+
+	}
+
+}

spring-boot-tools/spring-boot-maven-plugin/src/it/start-stop-automatic-fork/verify.groovy

@@ -0,0 +1,5 @@
+import static org.junit.Assert.assertTrue
+
+def file = new File(basedir, "build.log")
+assertTrue 'Start should have waited for application to be ready', file.text.contains("isReady: true")
+assertTrue 'Shutdown should have been invoked', file.text.contains("Shutdown requested")