UIService: fix behavior of isHeadless/setHeadless

When setHeadless is called, it should toggle a flag local to the
context, not override the java.awt.headless system property.
That way, each context can be headless or not independently.

Closes #424.

Author: ctrueden

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

@@ -116,6 +116,14 @@ public final class DefaultUIService extends AbstractService implements
 	/** The default user interface to use, if one is not explicitly specified. */
 	private UserInterface defaultUI;
 
+	/**
+	 * When true, {@link #isHeadless()} will return true regardless of the value
+	 * of the {@code java.awt.headless} system property. When false, {@link
+	 * #isHeadless()} matches the global JVM headless state defined by {@code
+	 * java.awt.headless}.
+	 */
+	private boolean forceHeadless;
+
 	private boolean activationInvocationPending = false;
 
 	// -- UIService methods --
@@ -178,12 +186,14 @@ public boolean isVisible(final String name) {
 
 	@Override
 	public void setHeadless(final boolean headless) {
-		System.setProperty("java.awt.headless", String.valueOf(headless));
+		forceHeadless = headless;
 	}
 
 	@Override
 	public boolean isHeadless() {
-		return Boolean.getBoolean("java.awt.headless");
+		// NB: We do not use java.awt.GraphicsConfiguration.isHeadless()
+		// because scijava-common eschews java.awt.* classes when possible.
+		return forceHeadless || Boolean.getBoolean("java.awt.headless");
 	}
 
 	@Override

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

@@ -101,7 +101,22 @@ public interface UIService extends SciJavaService {
 	/** Gets whether the UI with the given name or class name is visible. */
 	boolean isVisible(String name);
 
-	/** Sets whether the application is running in headless mode (no UI). */
+	/**
+	 * Sets whether the application should run in headless mode (no UI).
+	 * <p>
+	 * Note that if the system itself is headless&mdash;which can be detected via
+	 * the {@code java.awt.headless} system property or by calling
+	 * {@code java.awt.GraphicsEnvironment.isHeadless()}&mdash;then calling
+	 * {@code setHeadless(false)} will have no effect; the system will still be
+	 * headless, and {@link #isHeadless()} will still return true.
+	 * </p>
+	 * <p>
+	 * But if the system itself is <em>not</em> headless, calling
+	 * {@code setHeadless(true)} will force {@link #isHeadless()} to return true,
+	 * instructing the application to behave in a headless manner insofar as it
+	 * can.
+	 * </p>
+	 */
 	void setHeadless(boolean isHeadless);
 
 	/** Gets whether the UI is running in headless mode (no UI). */