Make the HeadlessUI a real boy

The HeadlessUI is now a SciJava UserInterface plugin, just like
any other user interface. This is useful because while we want it
to always be chosen when headless mode is active, it should also
be used in non-headless mode when nothing else is available.
This will help prevent NPEs etc. in UIService method calls.

Consequently, this commit adds some defensive checks to
DefaultUIService. These are needed now with the new HeadlessUI,
because that UI returns null for methods like dialogPrompt --
something that no other UIs did previously. In other words: we
relaxed the postconditions of some UserInterface methods to allow
for null returns, and therefore the DefaultUIService needs to
behave accordingly, handling those nulls.

Author: ctrueden

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

@@ -189,7 +189,7 @@ public boolean isHeadless() {
 
 	@Override
 	public UserInterface getDefaultUI() {
-		if (isHeadless()) return HeadlessUI.getInstance();
+		if (isHeadless()) return uiMap().get(HeadlessUI.NAME);
 		if (defaultUI != null) return defaultUI;
 		return uiList().isEmpty() ? null : uiList().get(0);
 	}
@@ -302,32 +302,29 @@ public DialogPrompt.Result showDialog(final String message,
 		if (ui == null) return null;
 		final DialogPrompt dialogPrompt =
 			ui.dialogPrompt(message, title, messageType, optionType);
-		return dialogPrompt.prompt();
+		return dialogPrompt == null ? null : dialogPrompt.prompt();
 	}
 
 	@Override
 	public File chooseFile(final File file, final String style) {
 		final UserInterface ui = getDefaultUI();
-		if (ui == null) return null;
-		return ui.chooseFile(file, style);
+		return ui == null ? null : ui.chooseFile(file, style);
 	}
 
 	@Override
 	public File
 		chooseFile(final String title, final File file, final String style)
 	{
 		final UserInterface ui = getDefaultUI();
-		if (ui == null) return null;
-		return ui.chooseFile(title, file, style);
+		return ui == null ? null : ui.chooseFile(title, file, style);
 	}
 
 	@Override
 	public void showContextMenu(final String menuRoot, final Display<?> display,
 		final int x, final int y)
 	{
 		final UserInterface ui = getDefaultUI();
-		if (ui == null) return;
-		ui.showContextMenu(menuRoot, display, x, y);
+		if (ui != null) ui.showContextMenu(menuRoot, display, x, y);
 	}
 
 	@Override

src/main/java/org/scijava/ui/headlessUI/HeadlessUI.java

@@ -31,42 +31,28 @@
 
 package org.scijava.ui.headlessUI;
 
-import static org.scijava.Priority.LAST_PRIORITY;
-
 import java.io.File;
 
-import org.scijava.Context;
+import org.scijava.Priority;
 import org.scijava.display.Display;
-import org.scijava.plugin.PluginInfo;
-import org.scijava.ui.ApplicationFrame;
-import org.scijava.ui.Desktop;
+import org.scijava.plugin.AbstractRichPlugin;
+import org.scijava.plugin.Plugin;
 import org.scijava.ui.DialogPrompt;
-import org.scijava.ui.StatusBar;
-import org.scijava.ui.SystemClipboard;
-import org.scijava.ui.ToolBar;
 import org.scijava.ui.UserInterface;
-import org.scijava.ui.console.ConsolePane;
 import org.scijava.ui.viewer.DisplayWindow;
 
 /**
  * A "null object" UI implementation that can be returned when a UIService is
  * running headless
  *
  * @author Richard Domander (Royal Veterinary College, London)
+ * @author Curtis Rueden
  */
-public class HeadlessUI implements UserInterface {
-
-	private static HeadlessUI instance;
+@Plugin(type = UserInterface.class, name = HeadlessUI.NAME,
+	priority = Priority.VERY_LOW_PRIORITY)
+public class HeadlessUI extends AbstractRichPlugin implements UserInterface {
 
-	private HeadlessUI() {}
-
-	public static HeadlessUI getInstance() {
-		if (instance == null) {
-			instance = new HeadlessUI();
-		}
-
-		return instance;
-	}
+	public static final String NAME = "headless";
 
 	@Override
 	public void show() {}
@@ -76,45 +62,12 @@ public boolean isVisible() {
 		return false;
 	}
 
-	@Override
-	public void show(final Object o) {}
-
 	@Override
 	public void show(final String name, final Object o) {}
 
 	@Override
 	public void show(final Display<?> display) {}
 
-	@Override
-	public Desktop getDesktop() {
-		return null;
-	}
-
-	@Override
-	public ApplicationFrame getApplicationFrame() {
-		return null;
-	}
-
-	@Override
-	public ToolBar getToolBar() {
-		return null;
-	}
-
-	@Override
-	public StatusBar getStatusBar() {
-		return null;
-	}
-
-	@Override
-	public ConsolePane<?> getConsolePane() {
-		return null;
-	}
-
-	@Override
-	public SystemClipboard getSystemClipboard() {
-		return null;
-	}
-
 	@Override
 	public DisplayWindow createDisplayWindow(final Display<?> display) {
 		return null;
@@ -128,11 +81,6 @@ public DialogPrompt dialogPrompt(final String message, final String title,
 		return null;
 	}
 
-	@Override
-	public File chooseFile(final File file, final String style) {
-		return null;
-	}
-
 	@Override
 	public File chooseFile(final String title, final File file,
 		final String style)
@@ -156,37 +104,6 @@ public boolean requiresEDT() {
 		return false;
 	}
 
-	/** Returns null since this is a contextless null object */
-	@Override
-	public Context context() {
-		return null;
-	}
-
-	/** Returns null since this is a contextless null object */
-	@Override
-	public Context getContext() {
-		return null;
-	}
-
-	@Override
-	public void setContext(final Context context) {}
-
-	@Override
-	public PluginInfo<?> getInfo() {
-		return null;
-	}
-
-	@Override
-	public void setInfo(final PluginInfo<?> info) {}
-
 	@Override
 	public void dispose() {}
-
-	@Override
-	public double getPriority() {
-		return LAST_PRIORITY;
-	}
-
-	@Override
-	public void setPriority(final double priority) {}
 }