Only check for the "L" and ";" if we're in an array.

I can't help but over-optimize. We're parsing the result of class.getName(),
it will be a bare class name if not in the array.

Also: Stop going out of our way to not throw an exception if the input
is malformed, as that's the desired result for bad input.

core/src/main/java/com/threerings/io/ObjectInputStream.java

@@ -278,35 +278,42 @@ public class ObjectInputStream extends DataInputStream
     protected void validateClassPrefix (short code, String cname)
         throws IOException
     {
-        for (int start = 0, end = cname.length(); start < end; ++start) {
-            char c = cname.charAt(start);
-            if (c == '[') continue; // continue the loop to step past array markers
-
-            // if it's a single character do a quick check:
-            if (start == end - 1) {
-                switch (c) {
+        if (cname.charAt(0) == '[') {
+            // array handling. It's ok to throw if the cname is in an invalid format...
+            for (int start = 1, last = cname.length() - 1; true; ++start) {
+                switch (cname.charAt(start)) {
+                case '[': continue; // advance `start`
                 case 'I': case 'Z': case 'B': case 'S': case 'C': case 'J': case 'F': case 'D':
-                  return; // valid
-                }
-            } else {
-                // This is annoying: Something else is pre-stripping the L if not in an array?
-                if (c == 'L' && cname.endsWith(";")) {
-                  ++start;
-                  --end;
-                }
-                String className = cname.substring(start, end);
-                for (String prefix : _allowedPrefixes) {
-                    if (className.startsWith(prefix)) return; // valid
+                    if (start == last) return; // valid
+                    break;
+
+                case 'L':
+                    if (cname.charAt(last) == ';' &&
+                        isValidClassPrefix(cname.substring(start + 1, last))) return; // valid
+                    break;
                 }
+                break; // once we see any non-[ character, we're done
             }
-            break; // if we didn't see a [, we're done processing
-        }
+
+        } else if (isValidClassPrefix(cname)) return;
 
         log.warning("Blocked deserialization of non-whitelisted class",
             "code", code, "class", cname);
         throw new IOException("Deserialization of class not allowed: " + cname);
     }
 
+    /**
+     * Validate a class name against the prefix whitelist, after array information has been
+     * removed and primitive arrays handled elsewhere.
+     */
+    protected boolean isValidClassPrefix (String className)
+    {
+        for (String prefix : _allowedPrefixes) {
+            if (className.startsWith(prefix)) return true;
+        }
+        return false;
+    }
+
     /**
      * Reads an object from the input stream that was previously written with {@link
      * ObjectOutputStream#writeBareObject(Object)}.