stanfordnlp/CoreNLP · warning

Failed to handle | |

Error message

Failed to handle |${s}|

What it means

readAndParseTag constructs an XMLTag from the given string; any Exception thrown by the XMLTag constructor is caught and logged as 'Failed to handle |s|', and the method returns null. The |s| in the message is the offending input string. This means the argument was not a recognizable XML tag, and callers who do not null-check will hit a NullPointerException downstream.

Solutions

1. Null-check the return value of readAndParseTag before use
2. Validate that the input starts with '<' and ends with '>' and contains a tag name before calling
3. Read the logged warning to see which string failed and correct its construction
4. Wrap the call in your own try/catch or Optional handling at the call site

Example fix

// before
XMLTag t = XMLUtils.readAndParseTag(s);
String name = t.name; // NPE when s is not a valid tag
// after
XMLTag t = XMLUtils.readAndParseTag(s);
if (t == null) { throw new IllegalArgumentException("Not a valid XML tag: " + s); }
String name = t.name;

if (s == null || !(s.startsWith("<") && s.endsWith(">") && s.length() > 2))
  throw new IllegalArgumentException("Not an XML tag: " + s);

XMLTag t = XMLUtils.readAndParseTag(s);
if (t == null) throw new IllegalArgumentException("readAndParseTag could not parse: " + s);

XMLTag t = XMLUtils.readAndParseTag(s);
if (t == null) {
  throw new IllegalArgumentException("Invalid XML tag input: " + s);
}

When it happens

Trigger: Calling XMLUtils.readAndParseTag(s) (also reached via XMLUtils.tag(...)) with a string that XMLTag cannot parse — missing angle brackets, malformed attributes, empty/null-like input, or text that is not a single tag.

Common situations: Splitting documents into tokens and feeding non-tag text fragments; hand-built tag strings with unescaped quotes or stray characters; off-by-one slicing that cuts off the closing '>' ; assuming the return is never null.

Related errors

• Attempted to parse empty/null tag
• attempt to get word when sentence and lattice are null!
• org.xml.sax.SAXException
• TIMEX3 should only contain text
• unexpected element

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/a5b9da3440870c8f. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/util/XMLUtils.java:418

      b.append((char) c);
      c = r.read();
    }
    return b.toString();
  }

  /**
   * @return the new XMLTag object, or null if couldn't be created
   */
  public static XMLTag readAndParseTag(Reader r) throws IOException {
    String s = readTag(r);
    if (s == null) {
      return null;
    }
    XMLTag ret = null;
    try {
      ret = new XMLTag(s);
    } catch (Exception e) {
      log.warn("Failed to handle |" + s + "|");
    }
    return ret;
  }

  // Pattern is reentrant, going by the statement "many matchers can share the same pattern"
  // on the Pattern javadoc.  Therefore, this should be safe as a static final variable.
  private static final Pattern xmlEscapingPattern = Pattern.compile("&.+?;");

  public static String unescapeStringForXML(String s) {
    StringBuilder result = new StringBuilder();
    Matcher m = xmlEscapingPattern.matcher(s);
    int end = 0;
    while (m.find()) {
      int start = m.start();
      result.append(s, end, start);
      end = m.end();
      result.append(translate(s.substring(start, end)));
    }

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