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) {
e1.printStackTrace();
return null;
}
try {
vector = vectorChannel.map(MapMode.READ_ONLY,0,vectorChannel.size());
} catch (IOException e) {
e.printStackTrace();
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?
Answer
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.