前言
紧接着上文Dubbo分析之Transport层，本文继续介绍Exchange层，此层官方介绍为信息交换层：封装请求响应模式，同步转异步，以 Request, Response 为中心，扩展接口为 Exchanger, ExchangeChannel, ExchangeClient, ExchangeServer；下面分别进行介绍
Exchanger分析
Exchanger是此层的核心接口类，提供了connect()和bind()接口，分别返回ExchangeClient和ExchangeServer；dubbo提供了此接口的默认实现类HeaderExchanger，代码如下：
public class HeaderExchanger implements Exchanger {
public static final String NAME = "header";
@Override
public ExchangeClient connect(URL url, ExchangeHandler handler) throws RemotingException {
return new HeaderExchangeClient(Transporters.connect(url, new DecodeHandler(new HeaderExchangeHandler(handler))), true);
}
@Override
public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
}
}
在实现类中在connect和bind中分别实例化了HeaderExchangeClient和HeaderExchangeServer，传入的参数是Transporters，可以认为这里就是Transport层的入口类；这里的ExchangeClient/ExchangeServer其实就是对Client/Server的包装，同时传入了自己的ChannelHandler；ChannelHandler已经在Transport层介绍过了，提供了连接建立，连接端口，发送请求，接受请求等接口；已默认使用的Netty为例，这里就是对NettyClient和NettyServer的包装，同时传入DecodeHandler，在NettyHandler中被调用；
ExchangeClient分析
ExchangeClient本身也继承于Client，同时也继承于ExchangeChannel：
public interface ExchangeClient extends Client, ExchangeChannel {
}
public interface ExchangeChannel extends Channel {
ResponseFuture request(Object request) throws RemotingException;
ResponseFuture request(Object request, int timeout) throws RemotingException;
ExchangeHandler getExchangeHandler();
@Override
void close(int timeout);
}
ExchangeChannel负责将上层的data包装成Request，然后发送给Transport层；具体的逻辑在HeaderExchangeChannel中：
public ResponseFuture request(Object request, int timeout) throws RemotingException {
if (closed) {
throw new RemotingException(this.getLocalAddress(), null, "Failed to send request " + request + ", cause: The channel " + this + " is closed!");
}
// create request.
Request req = new Request();
req.setVersion(Version.getProtocolVersion());
req.setTwoWay(true);
req.setData(request);
DefaultFuture future = new DefaultFuture(channel, req, timeout);
try {
channel.send(req);
} catch (RemotingException e) {
future.cancel();
throw e;
}
return future;
}
创建了一个Request，在构造器中同时会产生一个RequestId；设置了协议版本，是否双向通信，最后设置了真实的业务数据；接下来实例化了一个DefaultFuture类，此类实现了同步转异步的方式，channel调用send发送请求之后，不需要等待结果，直接将DefaultFuture返回给上层，上层可以通过调用DefaultFuture的get方法来获取响应，get方法会阻塞等待获取服务器的响应才会返回；Client接收消息在handler里面，比如Netty在NettyHandler里面messageReceived方法介绍响应消息，NettyHandler最终会调用上面传入的DecodeHandler，DecodeHandler会先判断一下是否已经解码，如果解码就直接调用HeaderExchangeHandler，默认已经设置了编码解码器，所以会直接调用HeaderExchangeHandler里面的received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);
}
}
服务端和客户端都会使用此方法，这里是客户端接受的是Response，直接调用handleResponse方法：
static void handleResponse(Channel channel, Response response) throws RemotingException {
if (response != null && !response.isHeartbeat()) {
DefaultFuture.received(channel, response);
}
}
接收到响应之后，再去告诉DefaultFuture已经收到响应，DefaultFuture本身存放了requestId对应DefaultFuture的一个ConcurrentHashMap；具体怎么映射过去，Response也包含一个responseId，此responseId和requestId是相同的；
private final Lock lock = new ReentrantLock();
private final Condition done = lock.newCondition();
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());
}
}
private void doReceived(Response res) {
lock.lock();
try {
response = res;
if (done != null) {
done.signal();
}
} finally {
lock.unlock();
}
if (callback != null) {
invokeCallback(callback);
}
}
通过responseId获取了之前请求时创建的DefaultFuture，然后再更新DefaultFuture内部的response对象，更新完之后在调用Condition的signal方法，用户唤起通过DefaultFuture的get方法获取响应的阻塞线程：
public Object get(int timeout) throws RemotingException {
if (timeout <= 0) {
timeout = Constants.DEFAULT_TIMEOUT;
}
if (!isDone()) {
long start = System.currentTimeMillis();
lock.lock();
try {
while (!isDone()) {
done.await(timeout, TimeUnit.MILLISECONDS);
if (isDone() || System.currentTimeMillis() - start > timeout) {
break;
}
}
} catch (InterruptedException e) {
throw new RuntimeException(e);
} finally {
lock.unlock();
}
if (!isDone()) {
throw new TimeoutException(sent > 0, channel, getTimeoutMessage(false));
}
}
return returnFromResponse();
}
可以发现阻塞要么被获取被signal方法唤醒，要么等待超时；以上大致是客户端发送获取响应的流程，下面看看服务器端流程
ExchangeServer分析
ExchangeServer继承于Server，同时提供了两个包装服务端Channel的方法
public interface ExchangeServer extends Server {
Collection getExchangeChannels();
ExchangeChannel getExchangeChannel(InetSocketAddress remoteAddress);
}
服务器端主要用于接收Request消息，然后处理消息，最后把响应发送给客户端，相关接收消息已经在上面介绍过了，同样是在HeaderExchangeHandler里面的received方法中，只不过这里的消息类型为Request；
Response handleRequest(ExchangeChannel channel, Request req) throws RemotingException {
Response res = new Response(req.getId(), req.getVersion());
if (req.isBroken()) {
Object data = req.getData();
String msg;
if (data == null) msg = null;
else if (data instanceof Throwable) msg = StringUtils.toString((Throwable) data);
else msg = data.toString();
res.setErrorMessage("Fail to decode request due to: " + msg);
res.setStatus(Response.BAD_REQUEST);
return res;
}
// find handler by message class.
Object msg = req.getData();
try {
// handle data.
Object result = handler.reply(channel, msg);
res.setStatus(Response.OK);
res.setResult(result);
} catch (Throwable e) {
res.setStatus(Response.SERVICE_ERROR);
res.setErrorMessage(StringUtils.toString(e));
}
return res;
}
首先创建了一个Response，并且指定responseId为requestId，方便在客户端定位到具体的DefaultFuture；然后调用handler的reply方法处理消息，返回结果，如何处理的将在后面的protocol层介绍，大致就是通过Request的信息，反射调用Server端的服务，然后返回结果，然后将结果放入Response对象中，通过channel将消息发送客户端；
总结
本文介绍了Exchange层的大体流程，围绕Exchanger，ExchangeClient和ExchangeServer展开；请求封装成Request，响应封装成Response，客户端通过异步的方式接收服务器请求；