系列文章
Dubbo分析Serialize層
Dubbo分析之Transport層
Dubbo分析之Exchange 層
Dubbo分析之Protocol層
前言
緊接着上文Dubbo分析之Transport層,本文繼續介紹Exchange層,此層官方介紹爲信息交換層:封裝請求響應模式,同步轉異步,以 Request, Response 爲中心,擴展接口爲 Exchanger, ExchangeChannel, ExchangeClient, ExchangeServer;下面分別進行介紹
Exchanger分析
Exchanger是此層的核心接口類,提供了connect()和bind()接口,分別返回ExchangeClient和ExchangeServer;dubbo提供了此接口的默認實現類HeaderExchanger,代碼如下:
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))));
}
}
在實現類中在connect和bind中分別實例化了HeaderExchangeClient和HeaderExchangeServer,傳入的參數是Transporters,可以認爲這裏就是Transport層的入口類;這裏的ExchangeClient/ExchangeServer其實就是對Client/Server的包裝,同時傳入了自己的ChannelHandler;ChannelHandler已經在Transport層介紹過了,提供了連接建立,連接端口,發送請求,接受請求等接口;已默認使用的Netty爲例,這裏就是對NettyClient和NettyServer的包裝,同時傳入DecodeHandler,在NettyHandler中被調用;
ExchangeClient分析
ExchangeClient本身也繼承於Client,同時也繼承於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負責將上層的data包裝成Request,然後發送給Transport層;具體的邏輯在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;
}
創建了一個Request,在構造器中同時會產生一個RequestId;設置了協議版本,是否雙向通信,最後設置了真實的業務數據;接下來實例化了一個DefaultFuture類,此類實現了同步轉異步的方式,channel調用send發送請求之後,不需要等待結果,直接將DefaultFuture返回給上層,上層可以通過調用DefaultFuture的get方法來獲取響應,get方法會阻塞等待獲取服務器的響應纔會返回;Client接收消息在handler裏面,比如Netty在NettyHandler裏面messageReceived方法介紹響應消息,NettyHandler最終會調用上面傳入的DecodeHandler,DecodeHandler會先判斷一下是否已經解碼,如果解碼就直接調用HeaderExchangeHandler,默認已經設置了編碼解碼器,所以會直接調用HeaderExchangeHandler裏面的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);
}
}
服務端和客戶端都會使用此方法,這裏是客戶端接受的是Response,直接調用handleResponse方法:
static void handleResponse(Channel channel, Response response) throws RemotingException {
if (response != null && !response.isHeartbeat()) {
DefaultFuture.received(channel, response);
}
}
接收到響應之後,再去告訴DefaultFuture已經收到響應,DefaultFuture本身存放了requestId對應DefaultFuture的一個ConcurrentHashMap;具體怎麼映射過去,Response也包含一個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);
}
}
通過responseId獲取了之前請求時創建的DefaultFuture,然後再更新DefaultFuture內部的response對象,更新完之後在調用Condition的signal方法,用戶喚起通過DefaultFuture的get方法獲取響應的阻塞線程:
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();
}
可以發現阻塞要麼被獲取被signal方法喚醒,要麼等待超時;以上大致是客戶端發送獲取響應的流程,下面看看服務器端流程
ExchangeServer分析
ExchangeServer繼承於Server,同時提供了兩個包裝服務端Channel的方法
public interface ExchangeServer extends Server {
Collection<ExchangeChannel> getExchangeChannels();
ExchangeChannel getExchangeChannel(InetSocketAddress remoteAddress);
}
服務器端主要用於接收Request消息,然後處理消息,最後把響應發送給客戶端,相關接收消息已經在上面介紹過了,同樣是在HeaderExchangeHandler裏面的received方法中,只不過這裏的消息類型爲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;
}
首先創建了一個Response,並且指定responseId爲requestId,方便在客戶端定位到具體的DefaultFuture;然後調用handler的reply方法處理消息,返回結果,如何處理的將在後面的protocol層介紹,大致就是通過Request的信息,反射調用Server端的服務,然後返回結果,然後將結果放入Response對象中,通過channel將消息發送客戶端;
總結
本文介紹了Exchange層的大體流程,圍繞Exchanger,ExchangeClient和ExchangeServer展開;請求封裝成Request,響應封裝成Response,客戶端通過異步的方式接收服務器請求;
示例代碼地址
https://github.com/ksfzhaohui...
https://gitee.com/OutOfMemory...