Java: Writing with FileChannel to a file make the file shrink?

I tried to write specific bytes to certain position of a file using FileChannel. But actually the file shrink to the last position where I write change. I do it like this:

    Path path = Paths.get("I://music - Copy.mp3");

    System.out.println(Files.size(path)/1024 + "KB");

    try (FileChannel chan = new FileOutputStream(path.toFile()).getChannel()) {
        chan.position(1024 * 1024);

        ByteBuffer b = ByteBuffer.allocate(1024);
        chan.write(b);

        System.out.println("Write 1KB of data");
    }

    System.out.println(Files.size(path)/1024 + "KB");

and this is the output I get:

3670KB
Write 1KB of data
1025KB

Can anybody tell me where it goes wrong??

Solution

FileOutputStream truncates the file to zero length when not in append mode. It does not overwrite a file's contents so much as it throws away the contents and starts over. You can verify this by calling chan.size() after creating the channel, which will give you 0. ^[1]

FileChannels can be advanced past the end of a file and told to write there; this causes the file size to increase to position + bytes_written (emphasis mine):

Setting the position to a value that is greater than the current size is legal but does not change the size of the entity. [..] A later attempt to write bytes at such a position will cause the entity to grow to accommodate the new bytes; the values of any bytes between the previous end-of-file and the newly-written bytes are unspecified.

So while it looks like the FileChannel is cutting off your file after writing, it is the FileOutputStream truncating to 0 length, and then the FileChannel expanding it again.

To prevent this from happening, avoid using a FileOutputStream to create a channel. You have a Path, so you can call Files.newByteChannel or FileChannel.open :

Path path = Paths.get("I://music - Copy.mp3");

System.out.println(Files.size(path)/1024 + "KB");

// pick either:
try (FileChannel chan = FileChannel.open(path, StandardOpenOption.WRITE)) {
try (SeekableByteChannel chan = Files.newByteChannel(path, StandardOpenOption.WRITE)) {
    chan.position(1024 * 1024);

    ByteBuffer b = ByteBuffer.allocate(1024);
    chan.write(b);

    System.out.println("Write 1KB of data");
}

System.out.println(Files.size(path)/1024 + "KB");

[1] Note that programs outside the JVM, such as a file explorer, may not indicate this until you flush or close the stream.