android netty java.lang.ClassCastExcepti?400报错

//--创建客户端channel的辅助类,发起connection请求
        bootstrap = new ClientBootstrap(
                new NioClientSocketChannelFactory(
                        Executors.newCachedThreadPool(),
                        Executors.newCachedThreadPool()));


        // Set up the pipeline factory.
        bootstrap.setPipelineFactory(new ChannelPipelineFactory() {
            public ChannelPipeline getPipeline()
            throws Exception {
                ChannelPipeline pipeline = Channels.pipeline();
                pipeline.addLast("decoder", new StringDecoder());
                pipeline.addLast("encoder", new StringEncoder());
                // and then business logic.
                pipeline.addLast("handler", new ClientHandler());
                return pipeline;
            }
        });
        
        bootstrap.setOption("tcpNoDelay", true);  
        bootstrap.setOption("keepAlive", true);
        
        //--创建无连接传输channel的辅助类(UDP)
        channelFuture = bootstrap.connect(new InetSocketAddress(ip, port));
        channelFuture.awaitUninterruptibly();
        
        //--获取通信端口，用来发送数据。
        channel = channelFuture.awaitUninterruptibly().getChannel();