server和client都是以下方法得到的，Exchanger这个接口只有这么一个实现，将来可能其他更加复杂获得server和cliet方式，以下这种是目前唯一的
public class HeaderExchanger implements Exchanger {
public static final String NAME = "header";
public ExchangeClient connect(URL url, ExchangeHandler handler) throws RemotingException {
return new HeaderExchangeClient(Transporters.connect(url, new DecodeHandler(new HeaderExchangeHandler(handler))), true);
}
public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
}
}
connect和 bind得到的最终的server和client，Transporters.connect调用方法是以下两个：
public static Client connect(URL url, ChannelHandler... handlers) throws RemotingException {
if (url == null) {
throw new IllegalArgumentException("url == null");
}
ChannelHandler handler;
if (handlers == null || handlers.length == 0) {
handler = new ChannelHandlerAdapter();
} else if (handlers.length == 1) {
handler = handlers[0];
} else {
handler = new ChannelHandlerDispatcher(handlers);
}
return getTransporter().connect(url, handler);
}
public static Transporter getTransporter() {
return ExtensionLoader.getExtensionLoader(Transporter.class).getAdaptiveExtension();
}
通过spi的自适应扩展作为生产实例的中间工厂，这个工厂根据url参数得到不同的transport，如果url里面指定netty4，那么就可以得到netty4的client
切换不同transport怎么做到的？那就是通过自适应扩展加url自由切换。
回到最上面的，通过bind已经拿到最终的nettyserver，继续包裹了一层HeaderExchangeServer，这里面主要处理心跳、channel、future的封装，以及屏蔽不同类型的server（netty、netty4等等）。
return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
继续看这里，这个handler很长，对于dubbo来说，最里面这个handler是通过CreateServer方法中的server = Exchangers.bind(url, requestHandler)这个传入进来的，
这个requestHandler就是dubboprotocl里面内部的，这个handler只有reply方法，作用就是执行doinvoe，也就是真正履行provider义务的地方，也只会在request来的时候才会被调用。
HeaderExchangeHandler用来处理request、response的，decodehandler用来解码。这handler到这里是不是已经结束了？显然不是,netty4server初始化的时候：
public NettyServer(URL url, ChannelHandler handler) throws RemotingException {
super(url, ChannelHandlers.wrap(handler, ExecutorUtil.setThreadName(url, SERVER_THREAD_POOL_NAME)));
}
protected ChannelHandler wrapInternal(ChannelHandler handler, URL url) {
return new MultiMessageHandler(new HeartbeatHandler(ExtensionLoader.getExtensionLoader(Dispatcher.class)
.getAdaptiveExtension().dispatch(handler, url)));
}
日志里面经常看到的 “DubboServerHandler”就是这个SERVER_THREAD_POOL_NAME。handler链上又有下面这几个：
MultiMessageHandler：针对multimessage类型消息，在received做拦截。
HeartbeatHandler：针对心跳在received做拦截。
通过dispatch出来的默认的allchannelhandler：对所有io事件做处理，大部分任务都扔到线程池里面做异步处理，防止阻塞netty线程。这个业务线程池的类型、个数也是url指定。
所以这个handler链路上面从外到内：MultiMessageHandler HeartbeatHandler Allchannelhandler DecodeHandler HeadExchangeHandler dubboprotocol里面自带的带有reply方法的handler。
对于netty来说，消息在到达这些handler处理以前，已经被netty的编解码handler处理了，所以DecodeHandler可有可无
HeadExchangeHandler看起来意义不大，其实这个是用来阻断传递到dubbo里面的handler的，它是最后一道防线，用来决定要不要丢给dubbo做reply、doinvoke操作的，最重要的方法：
public void received(Channel channel, Object message) throws RemotingException {
channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());
ExchangeChannel exchangeChannel = HeaderExchangeChannel.getOrAddChannel(channel);
try {
if (message instanceof Request) {
// handle request.
Request request = (Request) message;
if (request.isEvent()) {
handlerEvent(channel, request);
} else {
if (request.isTwoWay()) {
Response response = handleRequest(exchangeChannel, request);
channel.send(response);
} else {
handler.received(exchangeChannel, request.getData());
}
}
} else if (message instanceof Response) {
handleResponse(channel, (Response) message);
} else if (message instanceof String) {
if (isClientSide(channel)) {
Exception e = new Exception("Dubbo client can not supported string message: " + message + " in channel: " + channel + ", url: " + channel.getUrl());
logger.error(e.getMessage(), e);
} else {
String echo = handler.telnet(channel, (String) message);
if (echo != null && echo.length() > 0) {
channel.send(echo);
}
}
} else {
handler.received(exchangeChannel, message);
}
} finally {
HeaderExchangeChannel.removeChannelIfDisconnected(channel);
}
}
如果two-way、对方给我request，需要返回response，那么handleRequest会调用dubboprotocol的reply处理这个request
如果对方给我的response，那么调用handleResponse(channel, (Response) message);
static void handleResponse(Channel channel, Response response) throws RemotingException {
if (response != null && !response.isHeartbeat()) {
DefaultFuture.received(channel, response);
}
}
public static void received(Channel channel, Response response) {
try {
DefaultFuture future = FUTURES.remove(response.getId());
if (future != null) {
future.doReceived(response);
} else {
logger.warn("The timeout response finally returned at "
+ (new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS").format(new Date()))
+ ", response " + response
+ (channel == null ? "" : ", channel: " + channel.getLocalAddress()
+ " -> " + channel.getRemoteAddress()));
}
} finally {
CHANNELS.remove(response.getId());
}
}
收到response以后，在这里找到对应的future，通过future唤醒之前阻塞在发送request以后的业务线程：
private void doReceived(Response res) {
lock.lock();
try {
response = res;
if (done != null) {
done.signal();
}
} finally {
lock.unlock();
}
if (callback != null) {
invokeCallback(callback);
}
}