WriteBeforeConnect.java

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.string.StringEncoder;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.util.LinkedList;
import java.util.Queue;

public class WriteBeforeConnect {

    static Logger log = LoggerFactory.getLogger("main");

    public static void main(String[] args) throws Exception {
        Bootstrap b = new Bootstrap()
                .group(new NioEventLoopGroup())
                .channel(NioSocketChannel.class)
                .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 5000)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) {
                        ch.pipeline().addLast(new StringEncoder())
                                .addLast(new QueueWritesBeforeConnect());
                    }
                });

        Channel ch = b.bind(0).sync().channel();
        ch.connect(new InetSocketAddress("127.0.0.1", 9000));
        ch.writeAndFlush("ABC");
        ch.writeAndFlush("DEF");
        ch.writeAndFlush("GHI");

        ch.closeFuture().sync();
    }

    static class QueueWritesBeforeConnect extends ChannelDuplexHandler {

        static final Logger logger = LoggerFactory.getLogger("queue");

        private int count;
        private Queue<Write> queuedWrites = new LinkedList<Write>();

        static class Write {
            private final Object msg;
            private final ChannelPromise promise;

            Write(Object msg, ChannelPromise promise) {
                this.msg = msg;
                this.promise = promise;
            }

            void run(ChannelHandlerContext ctx) { ctx.write(msg, promise); }

            void fail() { promise.setFailure(new IOException()); }
        }

        @Override
        public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
            logger.debug("[{}] write: {}, queue it", count++, msg);
            queuedWrites.add(new Write(msg, promise));
        }

        @Override
        public void flush(ChannelHandlerContext ctx) throws Exception {
            logger.debug("[{}] flush, ignore", count++);
        }

        @Override
        public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
            logger.debug("[{}] channelRegistered", count++);
            super.channelRegistered(ctx);
        }

        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            logger.debug("[{}] channel active, flush queue", count++);
            Write op;
            while ((op = queuedWrites.poll()) != null) op.run(ctx);
            ctx.flush();
            ctx.pipeline().remove(this);
            super.channelActive(ctx);
        }

        @Override
        public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
            logger.debug("[{}] handler removed", count++);
        }

        @Override
        public void channelUnregistered(ChannelHandlerContext ctx) throws Exception {
            logger.debug("channel unregistered, fail queue");
            Write op;
            while ((op = queuedWrites.poll()) != null) op.fail();
            super.channelUnregistered(ctx);
        }


    }
}

// Normal operation...
//
// 22:04:09.515 [nioEventLoopGroup-2-1] DEBUG queue - [0] channelRegistered
// 22:04:09.521 [nioEventLoopGroup-2-1] DEBUG queue - [1] write: ABC, queue it
// 22:04:09.522 [nioEventLoopGroup-2-1] DEBUG queue - [2] write: DEF, queue it
// 22:04:09.522 [nioEventLoopGroup-2-1] DEBUG queue - [3] write: GHI, queue it
// 22:04:09.522 [nioEventLoopGroup-2-1] DEBUG queue - [4] flush, ignore
// 22:04:09.522 [nioEventLoopGroup-2-1] DEBUG queue - [5] channel active, flush queue
// 22:04:09.532 [nioEventLoopGroup-2-1] DEBUG queue - [6] handler removed

// But occasionally this happens... handler gets called after it was removed.
//
// 22:02:04.850 [nioEventLoopGroup-2-1] DEBUG queue - [0] channelRegistered
// 22:02:04.854 [nioEventLoopGroup-2-1] DEBUG queue - [1] channel active, flush queue
// 22:02:04.854 [nioEventLoopGroup-2-1] DEBUG queue - [2] handler removed
// 22:02:04.854 [nioEventLoopGroup-2-1] DEBUG queue - [3] write: ABC, queue it
// 22:02:04.854 [nioEventLoopGroup-2-1] DEBUG queue - [4] write: DEF, queue it
// 22:02:04.855 [nioEventLoopGroup-2-1] DEBUG queue - [5] write: GHI, queue it
// 22:02:04.855 [nioEventLoopGroup-2-1] DEBUG queue - [6] flush, ignore