Restore proper use of the fork parameter

Previously, disabling forking was not possible if JVM arguments or an
agent are specified, even if the fork attribute is explicitly set to
`false`.

The  fork attribute is now detected as it should and forking is disabled
when the fork attribute is set to false, even if JVM arguments or an
agent is either specified or discovered.

Fixes gh-2220

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

@@ -0,0 +1,32 @@
+<?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-disable-fork</artifactId>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+	</properties>
+	<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>
+							<jvmArguments>-Dfoo=bar</jvmArguments>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+</project>

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

@@ -0,0 +1,29 @@
+/*
+ * 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) {
+		String foo = System.getProperty("foo");
+		if ("bar".equals(foo)) {
+			throw new IllegalStateException("System property foo should not be available. Fork disabled");
+		}
+		System.out.println("I haz been run");
+	}
+
+}

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

@@ -0,0 +1,6 @@
+import static org.junit.Assert.assertTrue
+
+def file = new File(basedir, "build.log")
+assertTrue file.text.contains("I haz been run")
+assertTrue file.text.contains("Fork mode disabled, ignoring JVM argument(s) [-Dfoo=bar]")
+