Add system properties support using explicit configuration tag

See gh-10741

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/test/java/org/springframework/boot/maven/SystemPropertyFormatterTests.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2012-2017 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.maven;
+
+import org.assertj.core.api.Assertions;
+import org.junit.Test;
+
+import org.springframework.boot.maven.AbstractRunMojo.SystemPropertyFormatter;
+
+/**
+ * Tests for {@link AbstractRunMojo.SystemPropertyFormatter}
+ */
+public class SystemPropertyFormatterTests {
+
+	@Test
+	public void parseEmpty() throws Exception {
+		Assertions.assertThat(SystemPropertyFormatter.format(null, null))
+				.isEqualTo("");
+	}
+
+	@Test
+	public void parseOnlyKey() throws Exception {
+		Assertions.assertThat(SystemPropertyFormatter.format("key1", null))
+				.isEqualTo("-Dkey1");
+	}
+
+	@Test
+	public void parseKeyWithValue() throws Exception {
+		Assertions.assertThat(SystemPropertyFormatter.format("key1", "value1"))
+				.isEqualTo("-Dkey1=value1");
+	}
+
+	@Test
+	public void parseKeyWithEmptyValue() throws Exception {
+		Assertions.assertThat(SystemPropertyFormatter.format("key1", ""))
+				.isEqualTo("-Dkey1");
+	}
+}