How to write a Compression Filter ?

[Chris S]

Hi all,

i am trying to write a IoFilter for Googles Snappy compressor /
decompressor. I am using the Mina-LZip -Compressor Filter as a base an
rewirited it for Snappy. The usage of Snappy is quite simpel
(Snappy.compress(byte[]):byte[]  / Snappy.uncompress(byte[]):byte[]) but
Mina give me a headache ! I dont know what i am doing wrong, but i am
loosing messages, even if i disable the compression (see the attached
TestCompressor-class)

This is what i do :
Compression

   1. compress the data
   2. write the length of the compressed data as an int-value(4 byte)
   3. write the data

Decompression

   1. check if i there is enough data for the length field (4byte)
   2. read the length of the appended data
   3. check if there is enough data to read the compressed data
   4. read and decompress the compressed data


Here is my Code

public class CompressionFilter extends WriteRequestFilter {
@Override
public void messageReceived(NextFilter nextFilter, IoSession session, Object
message) throws Exception {
if (!(message instanceof IoBuffer)) {
nextFilter.messageReceived(session, message);
return;
}
IoBuffer inBuffer = (IoBuffer) message;
boolean hasMoreData;
do {
hasMoreData = decode(inBuffer, nextFilter, session);
}
while (hasMoreData);
}
private boolean decode(IoBuffer inBuffer, NextFilter nextFilter, IoSession
session) {
int position = inBuffer.position();
final int remaining = inBuffer.remaining();
if (remaining < 4) {
inBuffer.position(position);
return false;
}
final int size = inBuffer.getInt();
if (size > remaining - 4) {
inBuffer.position(position);
return false;
}
byte[] inBytes = new byte[size];
inBuffer.get(inBytes);
final byte[] uncompress = uncompress(inBytes);
IoBuffer outBuffer = IoBuffer.wrap(uncompress);
nextFilter.messageReceived(session, outBuffer);
return true;
}
private byte[] uncompress(byte[] compressed) {
// return Snappy.uncompress(compressed);
return compressed;
}
@Override
protected Object doFilterWrite(NextFilter nextFilter, IoSession session,
WriteRequest writeRequest) throws IOException {
IoBuffer inBuffer = (IoBuffer) writeRequest.getMessage();
if (!inBuffer.hasRemaining()) {
// Ignore empty buffers
return null;
} else {
byte[] inBytes = new byte[inBuffer.remaining()];
inBuffer.get(inBytes).flip();
byte[] outBytes = compress(inBytes);
IoBuffer outBuf = IoBuffer.allocate(outBytes.length + 4);
outBuf.putInt(outBytes.length);
outBuf.put(outBytes);
outBuf.flip();
return outBuf;
}
}
private byte[] compress(byte[] uncompressed) {
// return Snappy.compress(inBytes);
return uncompressed;
}
}