jwtk/jjwt · error · java.lang.IllegalArgumentException

Negative skip length

Error message

Negative skip length: ${n}

What it means

Argument-validation guard in BaseNCodecInputStream.skip: thrown immediately when the requested skip count n is negative. It is a programming-error signal from the caller (e.g. a miscomputed remaining count or an inverted subtraction), not a decoding failure.

Solutions

1. Clamp or validate the skip amount at the call site (e.g. Math.max(0, n)) before invoking skip
2. Fix the arithmetic that produced the negative value (often an underflow in a remaining-bytes computation)
3. If a negative value can legitimately occur from external input, guard it and skip nothing instead of calling skip

When it happens

Trigger: Thrown at impl/src/main/java/io/jsonwebtoken/impl/io/BaseNCodecInputStream.java:218 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of jwtk/jjwt@fb71496164 (2026-09-09). Data as JSON: /api/errors/7355d86d989059b2. Report an issue: GitHub.

Appendix: source

Thrown at impl/src/main/java/io/jsonwebtoken/impl/io/BaseNCodecInputStream.java:218

     *
     * @throws IOException if this method is invoked
     * @since 1.7
     */
    @Override
    public synchronized void reset() throws IOException {
        throw new IOException("mark/reset not supported");
    }

    /**
     * {@inheritDoc}
     *
     * @throws IllegalArgumentException if the provided skip length is negative
     * @since 1.7
     */
    @Override
    public long skip(final long n) throws IOException {
        if (n < 0) {
            throw new IllegalArgumentException("Negative skip length: " + n);
        }

        // skip in chunks of 512 bytes
        final byte[] b = new byte[512];
        long todo = n;

        while (todo > 0) {
            int len = (int) Math.min(b.length, todo);
            len = this.read(b, 0, len);
            if (len == BaseNCodec.EOF) {
                break;
            }
            todo -= len;
        }

        return n - todo;
    }
}

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