Java FileChannel.transferTo: verify the byte count on every pass

Last updated: 5 Oct 20265 min read

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FileChannel.transferTo requests a byte range be copied to another channel. A return smaller than the requested count is permitted, so a complete copy needs explicit progress accounting.

Operational contract

The method creates a new destination, captures the source size, and advances by the actual transferred count. It fails on zero progress to avoid an infinite loop. The size is only an observation: if another process changes the source during transfer, the result may not represent one stable version. For reliable publication, copy from an immutable source into a staged destination and then apply an accepted move contract. A failed transfer can leave a partial destination, so the owner must remove or quarantine it.

Failure case

A 47-megabyte archive is copied to a new review location. A transfer call moves only part of the requested range. Without the loop, the review location has a plausible filename but missing tail bytes. The loop checks every returned count; it does not claim that this operation verifies content identity or durability.

Java code

import java.io.IOException;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;

public class ArchiveChannelTransfer {
    public static long copy(Path source, Path destination) throws IOException {
        try (FileChannel input = FileChannel.open(source, StandardOpenOption.READ);
             FileChannel output = FileChannel.open(destination,
                 StandardOpenOption.CREATE_NEW, StandardOpenOption.WRITE)) {
            long expected = input.size();
            long copied = 0;
            while (copied < expected) {
                long count = input.transferTo(copied, expected - copied, output);
                if (count <= 0) throw new IOException("Transfer stopped before expected size");
                copied += count;
            }
            return copied;
        }
    }
}

Performance and ownership cost

The method performs O(B) data movement for B bytes and uses O(1) application buffer space. An operating system may optimize some transfers, but that is platform-dependent. File creation and failure cleanup remain separate costs.

Common Mistakes

Do not assume one transferTo call copies its full count.
Do not trust a size sampled before concurrent mutation as a snapshot.
Do not leave a partial destination available as a valid archive.

Connected lessons

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file channel boundaries

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