JetBrains/intellij-community · error · FormatDecode.IllegalFormatException

{2, choice, 1#flag|1<flags} ''{0}'' not allowed in ''{1}''

Error message

{2, choice, 1#flag|1<flags} ''{0}'' not allowed in ''{1}''

What it means

IllegalFormatException from FormatDecode.checkFlags during format-string validation (the 'Format String' language injection and format-string inspections). It fires when a specifier uses flags not permitted for its conversion — the value carries bits outside allowedFlags. Message pluralizes 'flag/flags' by count. This mirrors java.util.Formatter semantics but is raised statically by the inspection.

Source

Thrown at java/java-analysis-impl/src/com/siyeh/ig/format/FormatDecode.java:128

      result.append('0');
    }
    if ((flags & GROUP) != 0) {
      result.append(',');
    }
    if ((flags & PARENTHESES) != 0) {
      result.append('(');
    }
    if ((flags & PREVIOUS) != 0) {
      result.append('<');
    }
    return result.toString();
  }

  private static void checkFlags(int value, int allowedFlags, String specifier) {
    final int result = value & ~allowedFlags;
    if (result != 0) {
      final String flags = flagString(result);
      throw new IllegalFormatException(
        InspectionGadgetsBundle.message("format.string.error.flags.not.allowed", flags, specifier, flags.length()));
    }
  }

  public static Validator[] decodePrefix(String prefix, int argumentCount) {
    return decode(prefix, argumentCount, true, false);
  }

  public static Validator @NotNull [] decode(String formatString, int argumentCount) {
    return decode(formatString, argumentCount, false, false);
  }

  public static Validator @NotNull [] decodeNoVerify(String formatString, int argumentCount) {
    return decode(formatString, argumentCount, false, true);
  }

  /**
   * A single format specifier.

View on GitHub (pinned to be881553f2)

Solutions

1. Remove the disallowed flags named in the message from that specifier
2. Check the conversion character's allowed flags against java.util.Formatter docs (e.g. %n and %% accept none/minimal)
3. Test the string with String.format at runtime to confirm the JDK agrees

Example fix

// before
String.format("%+n")
// after
String.format("%n")

try { FormatDecode.decode(fmt, argc); } catch (IllegalFormatException e) { /* report offending specifier and its disallowed flags */ }

When it happens

Trigger: A format specifier like '%+n' (flags not allowed for %n conversion, allowedFlags=0), '%0s' (zero-pad not allowed for %s), or '%#,e'. checkFlags(value & ~allowedFlags != 0) trips during decode() of the format string.

Common situations: Copy-pasting printf format strings between conversions (e.g. C-style '%05s'); using '+' or ',' with %n or %% where java.util.Formatter forbids them; inspections flagging migrated logging code.

Related errors

• unexpected character ''{0}'' in ''{1}''
• duplicate flag ''{0}'' in ''{1}''
• width (''{0}'') not allowed in ''{1}''
• unnecessary argument position specifier ''{0}'' in ''{1}''
• previous flag ''<'' used but no previous format specifier fo

AI-assisted analysis of JetBrains/intellij-community@be881553f2 (2026-08-14). Data as JSON: /api/errors/4e2447cba1edea68. Report an issue: GitHub.