问Java AsynchronousFileChannel和ByteBuffer

EN

AsynchronousFileChannel asyncRead = AsynchronousFileChannel.open(sourcePath);
AsynchronousFileChannel asyncWrite = AsynchronousFileChannel.open(targetPath, StandardOpenOption.WRITE, StandardOpenOption.CREATE);

CompletionHandler<Integer, ByteBuffer> handlerRead = new CompletionHandler<Integer, ByteBuffer>() {

        @Override
        public void completed(Integer arg0, ByteBuffer arg1) {
            System.out.println("finished read ...");

            // question line
            asyncWrite.write(ByteBuffer.wrap(arg1.array()), 0, null, handlerWrite);
        }

        @Override
        public void failed(Throwable arg0, ByteBuffer arg1) {
            System.out.println("failed to read ...");
        }
    };

asyncRead.read(buffer, 0, buffer, handlerRead);

// no output
asyncWrite.write(arg1, 0, null, handlerWrite);

// works fine
asyncWrite.write(ByteBuffer.wrap(arg1.array()), 0, null, handlerWrite);