Improve UIService headless mode and fix test

After much hand-wringing, @hinerm and I gave up and decided to call
java.awt.GraphicsEnvironment.isHeadless() to make sure the UIService
reports the correct headless status. Otherwise, testing becomes very
tricky with more edge cases, and more importantly, the
UIService.isHeadless() method does not behave as one might expect.

Author: ctrueden

src/main/java/org/scijava/ui/DefaultUIService.java

@@ -29,6 +29,7 @@
 
 package org.scijava.ui;
 
+import java.awt.GraphicsEnvironment;
 import java.io.File;
 import java.io.FileFilter;
 import java.util.ArrayList;
@@ -191,9 +192,9 @@ public void setHeadless(final boolean headless) {
 
 	@Override
 	public boolean isHeadless() {
-		// NB: We do not use java.awt.GraphicsEnvironment.isHeadless()
-		// because scijava-common eschews java.awt.* classes when possible.
-		return forceHeadless || Boolean.getBoolean("java.awt.headless");
+		return forceHeadless ||
+			Boolean.getBoolean("java.awt.headless") ||
+			GraphicsEnvironment.isHeadless();
 	}
 
 	@Override

src/main/java/org/scijava/ui/UIService.java

@@ -119,7 +119,15 @@ public interface UIService extends SciJavaService {
 	 */
 	void setHeadless(boolean isHeadless);
 
-	/** Gets whether the UI is running in headless mode (no UI). */
+	/**
+	 * Gets whether the UI is running in headless mode (no UI).
+	 * <p>
+	 * More precisely: returns true when {@code java.awt.headless} system
+	 * property is set, and/or {@link java.awt.GraphicsEnvironment.isHeadless()}
+	 * returns true, and/or {@link #setHeadless(boolean)} was called with {@code
+	 * true} to force headless behavior in an otherwise headful environment.
+	 * </p>
+	 */
 	boolean isHeadless();
 
 	/**

src/test/java/org/scijava/ui/UIServiceTest.java

@@ -31,6 +31,7 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
 
 import java.util.List;
@@ -79,19 +80,40 @@ public void testAvailableUIs() {
 
 	@Test
 	public void testHeadlessUI() {
+		// If true here, before we messed with it, the assumption is that we are
+		// in a truly headless environment, not just forced via setHeadless(true).
+		boolean reallyHeadless = uiService.isHeadless();
+
 		final MockUserInterface mockUI = new MockUserInterface();
 		uiService.setDefaultUI(mockUI);
 
 		// test non-headless behavior
-		uiService.setHeadless(false);
-		assertFalse(uiService.isHeadless());
-		assertTrue(uiService.getDefaultUI() instanceof MockUserInterface);
-
-		// test headless behavior
-		uiService.setHeadless(true);
-		assertTrue(uiService.isHeadless());
-		assertTrue("UIService should return HeadlessUI when running \"headless\"",
-			uiService.getDefaultUI() instanceof HeadlessUI);
+		if (reallyHeadless) {
+			// This environment is truly headless, and
+			// we should not be able to override it.
+			uiService.setHeadless(false);
+			assertTrue(uiService.isHeadless());
+			assertTrue("UIService should return HeadlessUI when running \"headless\"",
+				uiService.getDefaultUI() instanceof HeadlessUI);
+		}
+		else {
+			// This environment is not headless! We can test more things.
+			assertSame("UIService default UI override failed",
+				mockUI, uiService.getDefaultUI());
+
+			// This environment isn't headless now;
+			// let's test overriding it to be so.
+			uiService.setHeadless(true);
+			assertTrue(uiService.isHeadless());
+			assertTrue("UIService should return HeadlessUI when running \"headless\"",
+				uiService.getDefaultUI() instanceof HeadlessUI);
+
+			// Now we put it back!
+			uiService.setHeadless(false);
+			assertFalse(uiService.isHeadless());
+			assertSame("UIService default UI override was not restored",
+				mockUI, uiService.getDefaultUI());
+		}
 	}
 
 	private static final class MockUserInterface extends AbstractUserInterface {