Question

I am getting IOException: Map Failed when trying to write a large byte array. I use the method below to write a byte array to a file

private static void write(byte[] data) throws Exception {
        File file = new File("C:/temp/file.json");
        int length = data.length;
        RandomAccessFile raf = new RandomAccessFile(file, "rw");
        FileChannel fc = raf.getChannel();
        MappedByteBuffer buffer = fc.map(FileChannel.MapMode.READ_WRITE, 0, length);
        for (int i = 0; i < length; i++) {
            buffer.put(data[i]);
        }
}

The byte array is about 270mb.
Can anyone explain what I am doing wrong?
Thanks.

Answer 1

I am not sure why the map failed, but I wouldn’t do it the way you have done.

FileOutputStream out = new FileOutputStream(filename);
out.write(data);
out.close();

to do it progressively you can use

FileOutputStream out = new FileOutputStream(filename);
for(int i = 0; i < data.length; i += 8192)
    out.write(data, i, Math.min(data.length-i, 8192));
out.close();

The map could fail if you have a 32-bit JVM and you call this method repeatedly. e.g. you run out of virtual memory.