NIO Simultaneous read and write operation
Hi
I encountered a problem with filechannel. When i had updated the file using a filechannel, changes are not reflected in the buffer, therefore the data is missing after reading the buffer.
So a new buffer from the file needs to be created for every update done and this is very slow since the OS needs to remap the file again. Is there any efficient mechanism that will update the buffer when a write operation is done to the filechannel. Alternatives are also appreciated.
Many thanks.
Example below.
RandomAccessFile rFileAccess =new RandomAccessFile(gl_pathname,"rw");
FileChannel fc = rFileAccess.getChannel();
MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size());
Charset charset = Charset.forName("ISO-8859-1");
CharsetDecoder decoder = charset.newDecoder();
charBuffer = decoder.decode(mbb);
//Write to the same file @ gl_pathname
ByteBuffer bb = ByteBuffer.wrap("This is not the length of the message put in. The actual length can be very much longer than what is in this string. Thank you.".getBytes());
fc.write(bb, fc.size());
String sb =new String("");
while(charBuffer.hasRemaining()){
char c = charBuffer.get();
sb += c;
}
[1598 byte] By [
kajiwaraa] at [2007-11-26 14:55:10]
I'm not pro at all with NIO, but shouldn't you use the decoder after writing to the file channel ?
Something like:
RandomAccessFile rFileAccess = new RandomAccessFile(gl_pathname, "rw");
FileChannel fc = rFileAccess.getChannel();
MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size());
Charset charset = Charset.forName("ISO-8859-1");
CharsetDecoder decoder = charset.newDecoder();
//Write to the same file @ gl_pathname
ByteBuffer bb = ByteBuffer.wrap("This is not the length of the message put in. The actual length can be very much longer than what is in this string. Thank you.".getBytes());
fc.write(bb, fc.size());
charBuffer = decoder.decode(mbb);
String sb = new String("");
while(charBuffer.hasRemaining()){
char c = charBuffer.get();
sb += c;
}
No, it is that you have to get your buffer positions and limits and flips and rewinds organized. mb.put(bb) just puts the entire contents of bb into mb starting from the current position, and if bb won't fit between position and limit you'll get an overflow. You probably need to reset or rewind the buffer so position = zero and limit = capacity. If you don't want to clobber the entire buffer mb you'll have to use a different put method.
OTOH why do you have this other 'bb' in the first place? You should be using the mapped byte buffer for that directly, e.g. decoding straight into it.
ejpa at 2007-7-8 8:43:40 >
Its not about changing the contents that is already present in the mbb, but it
about appending new data to it and the file will grow in size.
The problem is that mbb is not updated after a append using fc.write(), so i will
only have a image of the file prior to writing it. If using mappedbytebuffer.put(),
the new data and mbb data will be greater than its capacity and will return a
bufferoverflowexception and also creating a new mbb takes too many
resources. because a file could easily be up to 10MB or even more in most
cases.
