Java – Memory-mapped files in Java


I've been trying to write some very fast Java code that has to do a lot of I/O. I'm using a memory mapped file that returns a ByteBuffer:

public static ByteBuffer byteBufferForFile(String fname){
    FileChannel vectorChannel;
    ByteBuffer vector;
    try {
        vectorChannel = new FileInputStream(fname).getChannel();
    } catch (FileNotFoundException e1) {
        return null;
    try {
        vector =,0,vectorChannel.size());
    } catch (IOException e) {
        return null;
    return vector;

The problem that I'm having is that the ByteBuffer .array() method (which should return a byte[] array) doesn't work for read-only files. I want to write my code so that it will work with both memory buffers constructed in memory and buffers read from the disk. But I don't want to wrap all of my buffers a ByteBuffer.wrap() function because I'm worried that this will slow things down. So I've been writing two versions of everything, one that takes a byte[], the other that takes a ByteBuffer.

Should I just wrap everything? Or should I double-write everything?

Best Solution

Did anyone actually check to see if ByteBuffers created by memory mapping support invoking .array() in the first place, regardless of readonly/readwrite?

From my poking around, as far as I can tell, the answer is NO. A ByteBuffer's ability to return a direct byte[] array via ByteBuffer.array() is goverened by the presence of ByteBuffer.hb (byte[]), which is always set to null when a MappedByteBuffer is created.

Which kinda sucks for me, because I was hoping to do something similar to what the question author wanted to do.