foo.java

  public static void validateHeaderValue(CharSequence seq) {

    int state = 0;
    // Start looping through each of the character
    for (int index = 0; index < seq.length(); index++) {
      state = validateValueChar(seq, state, seq.charAt(index));
    }

    if (state != 0) {
      throw new IllegalArgumentException("a header value must not end with '\\r' or '\\n':" + seq);
    }
  }

  private static final int HIGHEST_INVALID_VALUE_CHAR_MASK = ~15;

  private static int validateValueChar(CharSequence seq, int state, char character) {
    /*
     * State:
     * 0: Previous character was neither CR nor LF
     * 1: The previous character was CR
     * 2: The previous character was LF
     */
    if ((character & HIGHEST_INVALID_VALUE_CHAR_MASK) == 0) {
      // Check the absolutely prohibited characters.
      switch (character) {
        case 0x0: // NULL
          throw new IllegalArgumentException("a header value contains a prohibited character '\0': " + seq);
        case 0x0b: // Vertical tab
          throw new IllegalArgumentException("a header value contains a prohibited character '\\v': " + seq);
        case '\f':
          throw new IllegalArgumentException("a header value contains a prohibited character '\\f': " + seq);
      }
    }

    // Check the CRLF (HT | SP) pattern
    switch (state) {
      case 0:
        switch (character) {
          case '\r':
            return 1;
          case '\n':
            return 2;
        }
        break;
      case 1:
        switch (character) {
          case '\n':
            return 2;
          default:
            throw new IllegalArgumentException("only '\\n' is allowed after '\\r': " + seq);
        }
      case 2:
        switch (character) {
          case '\t':
          case ' ':
            return 0;
          default:
            throw new IllegalArgumentException("only ' ' and '\\t' are allowed after '\\n': " + seq);
        }
    }
    return state;
  }