CommandService: fix the DynamicCommand run hack

With 3927f28, we tried to avoid a
scenario where ClassCastException is thrown when running a command
that implements Module (e.g. DynamicCommand).

Unfortunately, that fix is problematic because the command no longer
actually runs; this breaks e.g. OptionsPlugins launched from the menu.
Instead, let's run it, but return null for the result, and log an
informative debug message. This should be no worse than originally.

Author: ctrueden

src/main/java/org/scijava/command/DefaultCommandService.java

@@ -183,22 +183,16 @@ public <C extends Command> Future<CommandModule> run(
 	public Future<CommandModule> run(final CommandInfo info,
 		final boolean process, final Object... inputs)
 	{
-		validateModuleType(info);
-		@SuppressWarnings({ "rawtypes", "unchecked" })
-		final Future<CommandModule> future =
-			(Future) moduleService.run(info, process, inputs);
-		return future;
+		final Future<Module> future = moduleService.run(info, process, inputs);
+		return validateFuture(future, info);
 	}
 
 	@Override
 	public Future<CommandModule> run(final CommandInfo info,
 		final boolean process, final Map<String, Object> inputMap)
 	{
-		validateModuleType(info);
-		@SuppressWarnings({ "rawtypes", "unchecked" })
-		final Future<CommandModule> future =
-			(Future) moduleService.run(info, process, inputMap);
-		return future;
+		final Future<Module> future = moduleService.run(info, process, inputMap);
+		return validateFuture(future, info);
 	}
 
 	// -- PTService methods --
@@ -352,25 +346,32 @@ private <PT extends SciJavaPlugin> List<PluginInfo<?>> downcast(
 	}
 
 	/**
-	 * A HACK to prevent calling run when the resultant module will not be a
-	 * {@link CommandModule}. This is an API design flaw in CommandService
-	 * currently, but for now work around it rather than breaking backwards API
-	 * compatibility.
+	 * A HACK to avoid {@link ClassCastException} when calling run when the
+	 * resultant module will not be a {@link CommandModule}. This is an API design
+	 * flaw in CommandService currently, but for now we work around it rather than
+	 * breaking backwards API compatibility.
 	 */
-	private void validateModuleType(final CommandInfo info) {
+	private Future<CommandModule> validateFuture(final Future<Module> future,
+		final CommandInfo info)
+	{
 		try {
 			final Class<?> commandClass = info.loadDelegateClass();
 			if (Module.class.isAssignableFrom(commandClass)) {
-				throw new IllegalArgumentException("Cannot use " +
-					"commandService.run(commandClass, ...) " +
-					"with classes that extend Module directly. Please call " +
-					"moduleService.run(commandService.getCommand(commandClass), ...) " +
-					"instead.");
+				log.debug("The command '" + info.getIdentifier() +
+					"' extends Module directly. Due to a design flaw in the " +
+					"CommandService API, the result cannot be coerced to a " +
+					"Future<CommandModule>, so null will be returned instead. " +
+					"If you need the resulting module, please instead call " +
+					"moduleService.run(commandService.getCommand(commandClass), ...).");
+				return null;
 			}
 		}
 		catch (final ClassNotFoundException exc) {
 			throw new IllegalStateException("Command class unavailable: " + //
 				info.getDelegateClassName(), exc);
 		}
+		@SuppressWarnings({ "rawtypes", "unchecked" })
+		final Future<CommandModule> result = (Future) future;
+		return result;
 	}
 }