Question

I’m using MemoryMapped buffer to read a file. Initially I’m getting the channel size and using the same size I”m mapping the file on memory and here the initial position is 0 as I want to map the file from the beginning. Now another 400KB of data is added to that file, now I want to map that 400kb alone. But something is wrong in my code, I’m not able to figure it out and I’m getting this

260java.io.IOException: Channel not open for writing - cannot extend file to required size
at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:812)
at trailreader.main(trailreader.java:55

So here’s my code

    BufferedWriter bw;      

    FileInputStream fileinput = null;

    try {
        fileinput = new FileInputStream("simple.csv");
    } catch (FileNotFoundException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    }


    FileChannel channel = fileinput.getChannel();




    MappedByteBuffer ByteBuffer;

    try {
        ByteBuffer = fileinput.getChannel().map(FileChannel.MapMode.READ_ONLY, 0, channel.size());
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    /*
    * Add some 400 bytes to simple.csv. outside of this program...
    */

                 //following line throw exception.
    try {
        ByteBuffer = fileinput.getChannel().map(FileChannel.MapMode.READ_ONLY, channel.size(), 400);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

So in my code I’m trying to reread the additional data that has been added but its not working, I know the prob is channel.size(), but I’m not able to rectify it.

Answer 1

channel.size() is always the current end of file. You are attempting to map 400 bytes past it. It isn’t there. You need something like:

ByteBuffer = fileinput.getChannel().map(FileChannel.MapMode.READ_ONLY, channel.size()-400, 400);