Add support for environment variables

See gh-12800

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

@@ -0,0 +1,38 @@
+<?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-envargs</artifactId>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+		<maven.compiler.source>@java.version@</maven.compiler.source>
+		<maven.compiler.target>@java.version@</maven.compiler.target>
+	</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>
+							<environmentVariables>
+								<ENV1>5000</ENV1>
+								<ENV2>Some Text</ENV2>
+								<ENV3/>
+								<ENV4></ENV4>
+							</environmentVariables>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+</project>

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

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2012-2018 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) {
+		assertEnvValue("ENV1", "5000");
+		assertEnvValue("ENV2", "Some Text");
+		assertEnvValue("ENV3", "");
+		assertEnvValue("ENV4", "");
+
+		System.out.println("I haz been run");
+
+	}
+
+
+	static void assertEnvValue(String envKey, String expectedValue) {
+		String actual = System.getenv(envKey);
+		if (!expectedValue.equals(actual)) {
+			throw new IllegalStateException("env property [" + envKey + "] mismatch (got [" + actual + "], expected [" + expectedValue + "]");
+		}
+	}
+
+}

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

@@ -0,0 +1,3 @@
+def file = new File(basedir, "build.log")
+return file.text.contains("I haz been run")
+