skylot/jadx · error · JadxRuntimeException

Unsigned byte value is too big:

Error message

Unsigned byte value is too big: 

What it means

Thrown by ClsSet.writeUnsignedByte() when the value being written is negative or >= 255. This is a defensive programming check during .clst serialization. The unsigned-byte format slot can only hold values 0–254. Any caller passing a value outside that range is a bug in the writing code.

Source

Thrown at jadx-core/src/main/java/jadx/core/clsp/ClsSet.java:482

	}

	private static String readString(DataInputStream in, int len) throws IOException {
		byte[] bytes = new byte[len];
		int count = in.read(bytes);
		while (count != len) {
			int res = in.read(bytes, count, len - count);
			if (res == -1) {
				throw new IOException("String read error");
			} else {
				count += res;
			}
		}
		return new String(bytes, STRING_CHARSET);
	}

	private static void writeUnsignedByte(DataOutputStream out, int value) throws IOException {
		if (value < 0 || value >= 0xFF) {
			throw new JadxRuntimeException("Unsigned byte value is too big: " + value);
		}
		out.writeByte(value);
	}

	private static int readUnsignedByte(DataInputStream in) throws IOException {
		return ((int) in.readByte()) & 0xFF;
	}

	public int getClassesCount() {
		return classes.length;
	}

	public void addToMap(Map<String, ClspClass> nameMap) {
		for (ClspClass cls : classes) {
			nameMap.put(cls.getName(), cls);
		}
	}

View on GitHub (pinned to e738a26571)

Solutions

1. Check the caller of writeUnsignedByte — the value must be in [0, 254]. Add logging before the call to identify which field overflows.
2. If the value legitimately exceeds 254, switch the encoding to a wider type (e.g., writeShort) and update the corresponding reader.
3. Ensure string lengths are validated upstream in writeString before reaching this method.

// If you call writeUnsignedByte directly, validate first
private static void safeWriteUnsignedByte(DataOutputStream out, int value) throws IOException {
    if (value < 0 || value >= 0xFF) {
        throw new IllegalArgumentException(
            "Value out of unsigned-byte range [0, 254]: " + value);
    }
    out.writeByte(value);
}

try {
    writeUnsignedByte(out, value);
} catch (JadxRuntimeException e) {
    logger.error("Value {} exceeds unsigned-byte format limit", value, e);
    // Switch to wider encoding or truncate the data set
    throw e;
}

When it happens

Trigger: Internal programming error where a string length, type list count, or other small value exceeds the single-byte capacity. Could be triggered indirectly by writeString() if the string is exactly 255 bytes (though writeString checks >= 255 first).

Common situations: Modifying ClsSet serialization logic and accidentally passing a value >= 255 or < 0 to writeUnsignedByte. Extremely long names or large counts overflowing the format.

Related errors

• String is too long:
• Unsupported Arg Type:
• Cannot save type:
• Wrong jadx class set header
• Wrong jadx class set version, got:

AI-assisted analysis of skylot/jadx@e738a26571 (2026-08-14). Data as JSON: /api/errors/a6a3f1cb5b58a59c. Report an issue: GitHub.