apache/flink · error · UTFDataFormatException

malformed input: partial character at end

Error message

malformed input: partial character at end

What it means

Thrown while decoding a modified-UTF-8 string in readString (called by readUTF/readLongUTF). A two-byte sequence (lead byte 110xxxxx, cases 12/13) was started but advancing count by 2 overshoots the declared utflen, meaning the declared byte length was too short to hold the trailing character. This signals truncated or corrupted serialized string data.

Source

Thrown at flink-core/src/main/java/org/apache/flink/core/memory/DataInputDeserializer.java:307

            switch (c >> 4) {
                case 0:
                case 1:
                case 2:
                case 3:
                case 4:
                case 5:
                case 6:
                case 7:
                    /* 0xxxxxxx */
                    count++;
                    chararr[chararrCount++] = (char) c;
                    break;
                case 12:
                case 13:
                    /* 110x xxxx 10xx xxxx */
                    count += 2;
                    if (count > utflen) {
                        throw new UTFDataFormatException(
                                "malformed input: partial character at end");
                    }
                    char2 = (int) bytearr[count - 1];
                    if ((char2 & 0xC0) != 0x80) {
                        throw new UTFDataFormatException("malformed input around byte " + count);
                    }
                    chararr[chararrCount++] = (char) (((c & 0x1F) << 6) | (char2 & 0x3F));
                    break;
                case 14:
                    /* 1110 xxxx 10xx xxxx 10xx xxxx */
                    count += 3;
                    if (count > utflen) {
                        throw new UTFDataFormatException(
                                "malformed input: partial character at end");
                    }
                    char2 = (int) bytearr[count - 2];
                    char3 = (int) bytearr[count - 1];
                    if (((char2 & 0xC0) != 0x80) || ((char3 & 0xC0) != 0x80)) {

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Ensure the same serializer (writeUTF/readUTF or writeLongUTF/readLongUTF pair) is used on both sides.
2. Verify the buffer was fully populated before decoding (available() >= utflen + lengthPrefixSize).
3. If data is untrusted or persisted across versions, wrap readUTF in try/catch UTFDataFormatException and treat as schema/state corruption.

Example fix

// before
String s = input.readUTF();

// after
try {
    String s = input.readUTF();
} catch (UTFDataFormatException e) {
    throw new IOException("Corrupted serialized string: " + e.getMessage(), e);
}

int declared = input.available();
// readUTF reads an unsigned short length prefix, then that many bytes
if (declared < 2) {
    throw new EOFException("Not enough bytes for UTF length prefix");
}

try {
    String s = input.readUTF();
} catch (UTFDataFormatException e) {
    // corrupted/truncated serialized string: surface as state/schema corruption
    throw new IOException("Corrupted UTF string: " + e.getMessage(), e);
}

When it happens

Trigger: readUTF() or readLongUTF() on a buffer whose UTF length prefix declares fewer bytes than the multi-byte character at that position requires; e.g. utflen ends mid-character after a 0xC0-0xDF lead byte.

Common situations: Serializer mismatch between the writer and reader; truncated checkpoint/state/network data; a byte buffer that was sliced before the full string was written; hand-crafted byte arrays that don't follow Java modified UTF-8.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• malformed input around byte {count}
• malformed input around byte {count-1}
• Error reconstructing URI
• Failed to deserialize an element from the source. If you are
• Reached the end of the collection. This could be caused by i

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/ad8e62beaa19fac4. Report an issue: GitHub.