Enable fork more when devtools is present

This commit improves the run goal to automatically fork the process when
devtools is present and log a warning when fork has been disabled via
configuration since devtools will not work on a non-forked process.

We don't want devtools to kick in for integration tests so the logic has
been placed in `RunMojo` requiring a couple of protected methods to
override.

Closes gh-5137

spring-boot-tools/spring-boot-maven-plugin/src/it/run-devtools/pom.xml

@@ -0,0 +1,33 @@
+<?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>run-devtools</artifactId>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+	</properties>
+	<!-- Devtools cannot be added here but the project has the class
+	     that the code is looking for -->
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>@project.groupId@</groupId>
+				<artifactId>@project.artifactId@</artifactId>
+				<version>@project.version@</version>
+				<executions>
+					<execution>
+						<phase>package</phase>
+						<goals>
+							<goal>run</goal>
+						</goals>
+						<configuration>
+							<fork>false</fork>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+</project>

spring-boot-tools/spring-boot-maven-plugin/src/it/run-devtools/src/main/java/org/springframework/boot/devtools/Restarter.java

@@ -0,0 +1,25 @@
+/*
+ * 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.springframework.boot.devtools.restart;
+
+/**
+ * Only meant to make sure that the plugin considers that devtools
+ * is present.
+ */
+public class Restarter {
+
+}

spring-boot-tools/spring-boot-maven-plugin/src/it/run-devtools/src/main/java/org/test/SampleApplication.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2012-2014 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;
+
+public class SampleApplication {
+
+	public static void main(String[] args) {
+		System.out.println("I haz been run");
+	}
+
+}

spring-boot-tools/spring-boot-maven-plugin/src/it/run-devtools/verify.groovy

@@ -0,0 +1,6 @@
+import static org.junit.Assert.assertTrue
+
+def file = new File(basedir, "build.log")
+assertTrue 'Devtools should have been detected', file.text.contains('Fork mode disabled, devtools will be disabled')
+assertTrue 'Application should have ran', file.text.contains("I haz been run")
+