Honour existing java.awt.headless configuration

Previously, SpringApplication would set the java.awt.headless system
property even if it had already been set. This commit updates
SpringApplication to honour any existing configuration when setting
the property.

Fixes #1189

spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -138,6 +138,7 @@ import org.springframework.web.context.support.StandardServletEnvironment;
  *
  * @author Phillip Webb
  * @author Dave Syer
+ * @author Andy Wilkinson
  * @see #run(Object, String[])
  * @see #run(Object[], String[])
  * @see #SpringApplication(Object...)
@@ -153,6 +154,8 @@ public class SpringApplication {
 	private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
 	"org.springframework.web.context.ConfigurableWebApplicationContext" };
 
+	private static final String SYSTEM_PROPERTY_JAVA_AWT_HEADLESS = "java.awt.headless";
+
 	private final Log log = LogFactory.getLog(getClass());
 
 	private final Set<Object> sources = new LinkedHashSet<Object>();
@@ -262,7 +265,10 @@ public class SpringApplication {
 		stopWatch.start();
 		ConfigurableApplicationContext context = null;
 
-		System.setProperty("java.awt.headless", Boolean.toString(this.headless));
+		System.setProperty(
+				SYSTEM_PROPERTY_JAVA_AWT_HEADLESS,
+				System.getProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS,
+						Boolean.toString(this.headless)));
 
 		Collection<SpringApplicationRunListener> runListeners = getRunListeners(args);
 		for (SpringApplicationRunListener runListener : runListeners) {

spring-boot/src/test/java/org/springframework/boot/SpringApplicationTests.java

@@ -23,6 +23,7 @@ import java.util.Set;
 import java.util.concurrent.atomic.AtomicReference;
 
 import org.junit.After;
+import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
@@ -81,9 +82,12 @@ import static org.mockito.Mockito.verify;
  *
  * @author Phillip Webb
  * @author Dave Syer
+ * @author Andy Wilkinson
  */
 public class SpringApplicationTests {
 
+	private String headlessProperty;
+
 	@Rule
 	public ExpectedException thrown = ExpectedException.none();
 
@@ -96,6 +100,23 @@ public class SpringApplicationTests {
 		throw new IllegalStateException("Could not obtain Environment");
 	}
 
+	@Before
+	public void storeAndClearHeadlessProperty() {
+		this.headlessProperty = System.getProperty("java.awt.headless");
+		System.clearProperty("java.awt.headless");
+	}
+
+	@After
+	public void reinstateHeadlessProperty() {
+		if (this.headlessProperty == null) {
+			System.clearProperty("java.awt.headless");
+		}
+		else {
+			System.setProperty("java.awt.headless", this.headlessProperty);
+		}
+
+	}
+
 	@After
 	public void close() {
 		if (this.context != null) {
@@ -504,6 +525,15 @@ public class SpringApplicationTests {
 		assertThat(System.getProperty("java.awt.headless"), equalTo("false"));
 	}
 
+	@Test
+	public void headlessSystemPropertyTakesPrecedence() throws Exception {
+		System.setProperty("java.awt.headless", "false");
+		TestSpringApplication application = new TestSpringApplication(ExampleConfig.class);
+		application.setWebEnvironment(false);
+		application.run();
+		assertThat(System.getProperty("java.awt.headless"), equalTo("false"));
+	}
+
 	private boolean hasPropertySource(ConfigurableEnvironment environment,
 			Class<?> propertySourceClass, String name) {
 		for (PropertySource<?> source : environment.getPropertySources()) {