Java MessageDigest.update(ByteBuffer): the input position moves to its limit

Hash the intended slice

A packet buffer may contain a header followed by a payload. If the protocol authenticates only the payload, position the buffer at the payload boundary before updating the digest. The method consumes the remaining view; a later parser must duplicate or reposition the buffer deliberately.

The fixture compares the buffer update with a fresh digest of the text payload. The matching result verifies this one slice, not any hidden metadata. ByteBuffer layout covers position and limit rules used by binary protocols.

Keep reusable state local

A MessageDigest instance is mutable. Calling digest finishes and resets it, so a second computation on the same instance needs its own input updates. Do not share one instance across independent concurrent requests.

Working program

import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.Arrays;

public class PacketPayloadDigest {
    public static void main(String[] args) throws Exception {
        ByteBuffer packet = ByteBuffer.wrap("header:data-47".getBytes(StandardCharsets.UTF_8));
        packet.position(7);
        MessageDigest digest = MessageDigest.getInstance("SHA-256");
        digest.update(packet);
        byte[] payloadHash = digest.digest();
        byte[] directHash = MessageDigest.getInstance("SHA-256").digest("data-47".getBytes(StandardCharsets.UTF_8));
        System.out.println("remaining=" + packet.remaining());
        System.out.println("same=" + Arrays.equals(payloadHash, directHash));
    }
}

Output

remaining=0
same=true

Cost and ownership

The digest processes O(n) remaining bytes with fixed-size internal state; the fixture's packet and result arrays use space proportional to their sizes. Moving a buffer position is cheap, but silently reusing a consumed buffer changes what later code reads.

Common Mistakes

• Do not assume update(ByteBuffer) leaves position unchanged.
• Do not hash an entire backing array when only the remaining slice is authorized.
• Do not share mutable MessageDigest state across requests.

Read next

Java ByteBuffer byte order: decode the protocol before reading integers, Java SHA-256: hash declared bytes, not an implicit string encoding, Java byte streams: partial reads and bounded copying, hmac sha256 verify.