eclipse-vertx/vert.x · error · IllegalArgumentException

unterminated escape sequence at end of string: ${s}

Error message

unterminated escape sequence at end of string: ${s}

What it means

URI decoding failure in decodeAndTransformURIComponent: a percent sign appears as the last character of the string, so the escape sequence has no following hex digits to decode. The full offending string is included; the input at fault is the URI component being decoded.

Source

Thrown at vertx-core/src/main/java/io/vertx/core/internal/net/RFC3986.java:81

  private static int indexOfPercentOrPlus(String s) {
    for (int i = 0, size = s.length(); i < size; i++) {
      final char c = s.charAt(i);
      if (c == '%' || c == '+') {
        return i;
      }
    }
    return -1;
  }

  private static String decodeAndTransformURIComponent(String s, int i, boolean plus) {
    final byte[] buf = s.getBytes(StandardCharsets.UTF_8);
    int pos = i;  // position in `buf'.
    for (int size = s.length(); i < size; i++) {
      char c = s.charAt(i);
      if (c == '%') {
        if (i == size - 1) {
          throw new IllegalArgumentException("unterminated escape"
            + " sequence at end of string: " + s);
        }
        c = s.charAt(++i);
        if (c == '%') {
          buf[pos++] = '%';  // "%%" -> "%"
          break;
        }
        if (i >= size - 1) {
          throw new IllegalArgumentException("partial escape"
            + " sequence at end of string: " + s);
        }
        c = decodeHexNibble(c);
        final char c2 = decodeHexNibble(s.charAt(++i));
        if (c == Character.MAX_VALUE || c2 == Character.MAX_VALUE) {
          throw new IllegalArgumentException(
            "invalid escape sequence `%" + s.charAt(i - 1)
              + s.charAt(i) + "' at index " + (i - 2)
              + " of: " + s);

View on GitHub (pinned to fb308bd8c3)

Solutions

1. Fix the truncated percent-encoding or encode lone '%' as '%25'
2. Validate/normalize URIs from untrusted sources before decoding

When it happens

Trigger: Thrown at vertx-core/src/main/java/io/vertx/core/internal/net/RFC3986.java:81 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of eclipse-vertx/vert.x@fb308bd8c3 (2026-09-06). Data as JSON: /api/errors/85ac77a4fa9df9fd. Report an issue: GitHub.