How I use more than one codec filter with one acceptor

[elf.cheng]

        I'am using a JSON based Protcol in my server program based on
mina.To
deal with json string,I write  codecs to decode from string or 
encode to string.I used this codecfactory like the following:
         acceptor.getFilterChain().addLast("logger", new
LoggingFilter());
      acceptor.getFilterChain().addLast("codec_txt", new
ProtocolCodecFilter(new
TextLineCodecFactory(Charset.forName("UTF-8"))));
       acceptor.getFilterChain().addLast("codec_json", new
ProtocolCodecFilter(new JsonCodecFactory()));
    
        but the filter "codec_json" dose not work! who can tell me why?

elf.cheng