GUACAMOLE-1976: Add OPTIONAL token modifier.

guacamole-ext/src/main/java/org/apache/guacamole/token/TokenFilter.java

@@ -24,6 +24,8 @@
 import java.util.Map;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Filtering object which replaces tokens of the form "${TOKEN_NAME}" with
@@ -33,6 +35,11 @@
  */
 public class TokenFilter {
 
+    /**
+     * The logger for this class.
+     */
+    private static final Logger LOGGER = LoggerFactory.getLogger(TokenFilter.class);
+
     /**
      * Regular expression which matches individual tokens, with additional
      * capturing groups for convenient retrieval of leading text, the possible
@@ -225,15 +232,25 @@ private String filter(String input, boolean strict)
                 // strict mode is enabled
                 if (tokenValue == null) {
 
+                    // Token marked as optional, so just skip it and update
+                    // last match.
+                    if (modifier != null && modifier.equals("OPTIONAL")) {
+                        LOGGER.warn("The token \"{}\" has no value and has been "
+                                  + "marked as optional, so it will be treated "
+                                  + "as a blank value instead of a literal.",
+                                    tokenName);
+                        endOfLastMatch = tokenMatcher.end();
+                        continue;
+                    }
+
                     // Fail outright if strict mode is enabled
                     if (strict)
                         throw new GuacamoleTokenUndefinedException("Token "
                                 + "has no defined value.", tokenName);
 
                     // If strict mode is NOT enabled, simply interpret as
                     // a literal
-                    String notToken = tokenMatcher.group(TOKEN_GROUP);
-                    output.append(notToken);
+                    output.append(tokenMatcher.group(TOKEN_GROUP));
 
                 }