Only warn about duplicate script engines

Once we have a Python script engine based on real Python (started in
https://github.com/scijava/scripting-cpyton), we will want to avoid
erroring out when there are *two* implementations for the same name,
and instead let the implementation with the higher priority win.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

src/main/java/org/scijava/script/DefaultScriptService.java

@@ -290,7 +290,7 @@ private HashMap<String, Class<?>> aliasMap() {
 	private synchronized void initScriptLanguageIndex() {
 		if (scriptLanguageIndex != null) return; // already initialized
 
-		final ScriptLanguageIndex index = new ScriptLanguageIndex();
+		final ScriptLanguageIndex index = new ScriptLanguageIndex(log);
 
 		// add ScriptLanguage plugins
 		for (final ScriptLanguage language : getInstances()) {

src/main/java/org/scijava/script/ScriptLanguageIndex.java

@@ -39,6 +39,7 @@
 
 import javax.script.ScriptEngineFactory;
 
+import org.scijava.log.LogService;
 import org.scijava.util.FileUtils;
 
 /**
@@ -57,14 +58,32 @@ public class ScriptLanguageIndex extends HashSet<ScriptLanguage> {
 	private final Map<String, ScriptLanguage> byName =
 		new HashMap<String, ScriptLanguage>();
 
+	private final LogService log;
+
+	@Deprecated
+	public ScriptLanguageIndex() {
+		this(null);
+	}
+
+	/**
+	 * Instantiates an index of the available script languages.
+	 * 
+	 * @param logService the log service for errors and warnings
+	 */
+	public ScriptLanguageIndex(final LogService logService) {
+		log = logService;
+	}
+
 	public boolean add(final ScriptEngineFactory factory, final boolean gently) {
 		final String duplicateName = checkDuplicate(factory);
 		if (duplicateName != null) {
 			if (gently) return false;
-			throw new IllegalArgumentException("Duplicate scripting language '" +
-				duplicateName + "': existing=" +
-				byName.get(duplicateName).getClass().getName() + ", new=" +
-				factory.getClass().getName());
+			if (log != null) {
+				log.warn("Duplicate scripting language '" +
+					duplicateName + "': existing=" +
+					byName.get(duplicateName).getClass().getName() +
+					", new=" + factory.getClass().getName());
+			}
 		}
 
 		final ScriptLanguage language = wrap(factory);