前言
之前已经写了三篇关于DUBBO的了，本文将是DUBBO系列的最后最后一篇，这篇写完之后也就对DUBBO的整个初始化和调用流程都有了较为详细的了解了，让我们赶紧来做个Ending吧。
源码解析
消费者调用流程涉及到消费者端和生产者端的交互，所以将分为三个部分来讲解，分别是
-消费者发起调用请求
-生产者响应调用请求
-消费者获取调用结果
消费者发起调用请求
之前文章中讲过消费者初始化时最后返回的是一个InvokerInvocationHandler的代理对象，根据动态代理的原理，DUBBO接口的方法调用都会由invoke方法代理，我们来看一下其实现
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
String methodName = method.getName();
Class<?>[] parameterTypes = method.getParameterTypes();
if (method.getDeclaringClass() == Object.class) {
return method.invoke(invoker, args);
}
if ("toString".equals(methodName) && parameterTypes.length == 0) {
return invoker.toString();
}
if ("hashCode".equals(methodName) && parameterTypes.length == 0) {
return invoker.hashCode();
}
if ("equals".equals(methodName) && parameterTypes.length == 1) {
return invoker.equals(args[0]);
}
return invoker.invoke(new RpcInvocation(method, args)).recreate();
}
正常情况下的方法调用会走invoker.invoke(new RpcInvocation(method, args)).recreate()这个分支，首先来看new RpcInvocation(method, args)
public RpcInvocation(Method method, Object[] arguments) {
this(method.getName(), method.getParameterTypes(), arguments, null, null);
}
public RpcInvocation(String methodName, Class<?>[] parameterTypes, Object[] arguments, Map<String, String> attachments, Invoker<?> invoker) {
this.methodName = methodName;
this.parameterTypes = parameterTypes == null ? new Class<?>[0] : parameterTypes;
this.arguments = arguments == null ? new Object[0] : arguments;
this.attachments = attachments == null ? new HashMap<String, String>() : attachments;
this.invoker = invoker;
}
非常简单的一个初始化赋值操作，就不做过多讲解了，接着回头看invoker.invoke(new RpcInvocation(method, args))方法，这里的invoker之前也说过了，是一个通过SPI机制生成的对象，以默认设置的参数failover为例，这里的invoker就是一个MockClusterInvoker对象中包含了一个FailoverClusterInvoker对象引用的类似链式的对象，那么我们来详细看看MockClusterInvoker的invoke方法
public Result invoke(Invocation invocation) throws RpcException {
Result result = null;
//获取mock属性的值，没有配置，默认false
String value = directory.getUrl().getMethodParameter(invocation.getMethodName(), Constants.MOCK_KEY, Boolean.FALSE.toString()).trim();
if (value.length() == 0 || value.equalsIgnoreCase("false")){
//no mock
result = this.invoker.invoke(invocation);
} else if (value.startsWith("force")) {
if (logger.isWarnEnabled()) {
logger.info("force-mock: " + invocation.getMethodName() + " force-mock enabled , url : " + directory.getUrl());
}
//force:direct mock
result = doMockInvoke(invocation, null);
} else {
//fail-mock
try {
result = this.invoker.invoke(invocation);
}catch (RpcException e) {
if (e.isBiz()) {
throw e;
} else {
if (logger.isWarnEnabled()) {
logger.info("fail-mock: " + invocation.getMethodName() + " fail-mock enabled , url : " + directory.getUrl(), e);
}
result = doMockInvoke(invocation, e);
}
}
}
return result;
}
当没有配置mock值时，value值得到的是默认值false，会去执行result = this.invoker.invoke(invocation)，this.invoker刚才提到过了是一个FailoverClusterInvoker类型的对象，但该对象并没有实现invoke方法，实际上该方法是继承自父类AbstractClusterInvoker的，来看一下
public Result invoke(final Invocation invocation) throws RpcException {
checkWheatherDestoried();
LoadBalance loadbalance;
List<Invoker<T>> invokers = list(invocation);
if (invokers != null && invokers.size() > 0) {
loadbalance = ExtensionLoader.getExtensionLoader(LoadBalance.class).getExtension(invokers.get(0).getUrl()
.getMethodParameter(invocation.getMethodName(),Constants.LOADBALANCE_KEY, Constants.DEFAULT_LOADBALANCE));
} else {
loadbalance = ExtensionLoader.getExtensionLoader(LoadBalance.class).getExtension(Constants.DEFAULT_LOADBALANCE);
}
//异步操作默认添加invocation id
RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation);
return doInvoke(invocation, invokers, loadbalance);
}
这里的list(invocation)方法根据invocation中的参数来获取所有的invoker列表，就不深入讲了，接着来看loadbalance对象的生成，loadbalance对象根据SPI机制生成，具体实现由loadbalance参数决定，也就是具体的负载均衡策略，DUBBO提供的实现有random、roundrobin、leastactive、consistenthash四种，其中没有根据服务端负载进行调节的策略。其中默认实现为random，生成的loadbalance就是一个RandomLoadBalance的对象。本次只分析同步的接口调用方式，跳过RpcUtils.attachInvocationIdIfAsync，接着看doInvoke(invocation, invokers, loadbalance)方法，该方法实现在FailoverClusterInvoker中
public Result doInvoke(Invocation invocation, final List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
List<Invoker<T>> copyinvokers = invokers;
//检查invokers是否为空
checkInvokers(copyinvokers, invocation);
//获取重试次数
int len = getUrl().getMethodParameter(invocation.getMethodName(), Constants.RETRIES_KEY, Constants.DEFAULT_RETRIES) + 1;
if (len <= 0) {
len = 1;
}
// retry loop.
RpcException le = null; // last exception.
List<Invoker<T>> invoked = new ArrayList<Invoker<T>>(copyinvokers.size()); // invoked invokers.
Set<String> providers = new HashSet<String>(len);
for (int i = 0; i < len; i++) {
//重试时，进行重新选择，避免重试时invoker列表已发生变化.
//注意：如果列表发生了变化，那么invoked判断会失效，因为invoker示例已经改变
if (i > 0) {
checkWheatherDestoried();
//获得InvokerWrapper的List
copyinvokers = list(invocation);
//重新检查一下
checkInvokers(copyinvokers, invocation);
}
Invoker<T> invoker = select(loadbalance, invocation, copyinvokers, invoked);
invoked.add(invoker);
RpcContext.getContext().setInvokers((List)invoked);
try {
Result result = invoker.invoke(invocation);
if (le != null && logger.isWarnEnabled()) {
logger.warn("");
}
return result;
} catch (RpcException e) {
if (e.isBiz()) { // biz exception.
throw e;
}
le = e;
} catch (Throwable e) {
le = new RpcException(e.getMessage(), e);
} finally {
providers.add(invoker.getUrl().getAddress());
}
}
throw new RpcException
/**
* 略去部分代码
*/
}
这里select(loadbalance, invocation, copyinvokers, invoked)方法根据传入的loadbalance对象挑选出一个执行用的invoker，里面调用链较深，在此不做详细分析。最终将通过invoker.invoke(invocation)进行调用并返回一个Result类型的对象，也就是最终的执行结果，这里的invoker对象是InvokerWrapper的实例，该实例引用了一个ListenerInvokerWrapper的实例，接着又链式引用了AbstractInvoker的实例，因此最终执行的invoke方法在AbstractInvoker中，来看一下
public Result invoke(Invocation inv) throws RpcException {
if(destroyed) {
throw new RpcException("略");
}
RpcInvocation invocation = (RpcInvocation) inv;
invocation.setInvoker(this);
if (attachment != null && attachment.size() > 0) {
invocation.addAttachmentsIfAbsent(attachment);
}
Map<String, String> context = RpcContext.getContext().getAttachments();
if (context != null) {
invocation.addAttachmentsIfAbsent(context);
}
if (getUrl().getMethodParameter(invocation.getMethodName(), Constants.ASYNC_KEY, false)){
invocation.setAttachment(Constants.ASYNC_KEY, Boolean.TRUE.toString());
}
//异步操作默认添加invocation id
RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation);
try {
return doInvoke(invocation);
} catch (InvocationTargetException e) { // biz exception
/**
* 略去部分代码
*/
}
}
这里的关键方法是doInvoke(invocation)，其实现在具体的Invoker实现类中，这里我们采用的是默认的dubbo协议，所以实现类为DubboInvoker，来看看其doInvoke方法
@Override
protected Result doInvoke(final Invocation invocation) throws Throwable {
RpcInvocation inv = (RpcInvocation) invocation;
final String methodName = RpcUtils.getMethodName(invocation);
inv.setAttachment(Constants.PATH_KEY, getUrl().getPath());
inv.setAttachment(Constants.VERSION_KEY, version);
ExchangeClient currentClient;
//消费者初始化时与服务端建立的连接
if (clients.length == 1) {
currentClient = clients[0];
} else {
currentClient = clients[index.getAndIncrement() % clients.length];
}
try {
boolean isAsync = RpcUtils.isAsync(getUrl(), invocation);
boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);
int timeout = getUrl().getMethodParameter(methodName, Constants.TIMEOUT_KEY,Constants.DEFAULT_TIMEOUT);
if (isOneway) {
boolean isSent = getUrl().getMethodParameter(methodName, Constants.SENT_KEY, false);
currentClient.send(inv, isSent);
RpcContext.getContext().setFuture(null);
return new RpcResult();
} else if (isAsync) {
ResponseFuture future = currentClient.request(inv, timeout) ;
RpcContext.getContext().setFuture(new FutureAdapter<Object>(future));
return new RpcResult();
} else {
RpcContext.getContext().setFuture(null);
return (Result) currentClient.request(inv, timeout).get();
}
/**
* 略去部分代码
*/
}
这里的isOneway和isAsync两个标志位分别区分单向调用(不在乎调用结果)和异步调用，这里我们分析同步调用的流程，这里的currentClient是一个ReferenceCountExchangeClient类型的对象
public ResponseFuture request(Object request) throws RemotingException {
return client.request(request);
}
这里的client是一个HeaderExchangeClient类型的对象，
public ResponseFuture request(Object request) throws RemotingException {
return channel.request(request);
}
这里的channel是一个HeaderExchangeChannel类型的对象，继续跟进去
public ResponseFuture request(Object request) throws RemotingException {
return request(request, channel.getUrl().getPositiveParameter(Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT));
}
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("2.0.0");
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;
}
这里的request方法自己又进行了一次内部调用，可以看到具体实现时创建了一个DefaultFuture对象并且通过channel.send(req)方法发送请求到生产者端，这里不做具体深入了。接着我们跳回DubboInvoker类doInvoke方法中的currentClient.request(inv, timeout).get()，这里是不是和jdk中future的用法很像，事实上这里也确实是通过get方法的调用将线程阻塞在这里等待结果，从而将异步调用转化为同步。为了证实这个想法，我们来看看DefaultFuture的get方法
public Object get() throws RemotingException {
return get(timeout);
}
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();
}
从done.await(timeout, TimeUnit.MILLISECONDS)可以看到这里不仅是等待isDone()这个状态位，同时还有超时时间的限制。isDone()判断的是什么，来看一下
public boolean isDone() {
return response != null;
}
判断response对象是否为空，那么后面的流程其实不难猜，生产者处理完结果会来填充response。
生产者响应调用请求
生产者开启了端口监听，消息的解码由Netty处理，解码后交由NettyHandler的messageReceived方法进行业务处理，来看一下
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler);
try {
handler.received(channel, e.getMessage());
} finally {
NettyChannel.removeChannelIfDisconnected(ctx.getChannel());
}
}
先来看一下NettyChannel.getOrAddChannel
static NettyChannel getOrAddChannel(org.jboss.netty.channel.Channel ch, URL url, ChannelHandler handler) {
if (ch == null) {
return null;
}
NettyChannel ret = channelMap.get(ch);
if (ret == null) {
NettyChannel nc = new NettyChannel(ch, url, handler);
if (ch.isConnected()) {
ret = channelMap.putIfAbsent(ch, nc);
}
if (ret == null) {
ret = nc;
}
}
return ret;
}
主要是从channelMap中获取对应的NettyChannel，接着回到NettyHandler的messageReceived方法来看handler.received(channel, e.getMessage())，这里的handler是一个NettyServer的实例，但它本身没有实现received方法，该方法要追溯到它的父类的父类的父类（真的就是这么长的继承关系。。。）AbstractPeer中，来看一下
public void received(Channel ch, Object msg) throws RemotingException {
if (closed) {
return;
}
handler.received(ch, msg);
}
这里的handler是MultiMessageHandler对象的实例，来看一下其received方法的实现
@Override
public void received(Channel channel, Object message) throws RemotingException {
if (message instanceof MultiMessage) {
MultiMessage list = (MultiMessage)message;
for(Object obj : list) {
handler.received(channel, obj);
}
} else {
handler.received(channel, message);
}
}
这里的handler又是HeartbeatHandler类的实例
public void received(Channel channel, Object message) throws RemotingException {
setReadTimestamp(channel);
if (isHeartbeatRequest(message)) {
Request req = (Request) message;
if (req.isTwoWay()) {
Response res = new Response(req.getId(), req.getVersion());
res.setEvent(Response.HEARTBEAT_EVENT);
channel.send(res);
if (logger.isInfoEnabled()) {
int heartbeat = channel.getUrl().getParameter(Constants.HEARTBEAT_KEY, 0);
if(logger.isDebugEnabled()) {
logger.debug("Received heartbeat from remote channel " + channel.getRemoteAddress()
+ ", cause: The channel has no data-transmission exceeds a heartbeat period"
+ (heartbeat > 0 ? ": " + heartbeat + "ms" : ""));
}
}
}
return;
}
if (isHeartbeatResponse(message)) {
if (logger.isDebugEnabled()) {
logger.debug(
new StringBuilder(32)
.append("Receive heartbeat response in thread ")
.append(Thread.currentThread().getName())
.toString());
}
return;
}
handler.received(channel, message);
}
因为不是心跳类的消息，所以执行handler.received(channel, message)继续这个调用链，这里的handler是AllChannelHandler类型的
public void received(Channel channel, Object message) throws RemotingException {
ExecutorService cexecutor = getExecutorService();
try {
cexecutor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
} catch (Throwable t) {
throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
}
}
这里终于结束了调用链，转而启动了一个线程池来执行任务，那我们来看看具体的任务线程ChannelEventRunnable中到底需要执行什么任务
public void run() {
switch (state) {
case CONNECTED:
try{
handler.connected(channel);
}catch (Exception e) {
logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel, e);
}
break;
case DISCONNECTED:
try{
handler.disconnected(channel);
}catch (Exception e) {
logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel, e);
}
break;
case SENT:
try{
handler.sent(channel,message);
}catch (Exception e) {
logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel
+ ", message is "+ message,e);
}
break;
case RECEIVED:
try{
handler.received(channel, message);
}catch (Exception e) {
logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel
+ ", message is "+ message,e);
}
break;
case CAUGHT:
try{
handler.caught(channel, exception);
}catch (Exception e) {
logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is "+ channel
+ ", message is: " + message + ", exception is " + exception,e);
}
break;
default:
logger.warn("unknown state: " + state + ", message is " + message);
}
}
这里传入的是RECEIVED状态，执行对应分支又是调用handler.received(channel, message)，好吧继续。。。
这里的handler是DecodeHandler的实例，继续跟下去
public void received(Channel channel, Object message) throws RemotingException {
if (message instanceof Decodeable) {
decode(message);
}
if (message instanceof Request) {
decode(((Request)message).getData());
}
if (message instanceof Response) {
decode( ((Response)message).getResult());
}
handler.received(channel, message);
}
调用链还在继续，这次的handler是HeaderExchangeHandler类型
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;
//判断是心跳还是正常请求
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);
}
}
正常同步请求会开始执行handleRequest(exchangeChannel, request)处理请求，并通过channel.send(response)回复结果，来重点看一下handleRequest方法
Response handleRequest(ExchangeChannel channel, Request req) throws RemotingException {
Response res = new Response(req.getId(), req.getVersion());
//处理异常的请求
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;
}
可以看出正常请求将由handler.reply(channel, msg)处理，这里的handler是DubboProtocol中的一个ExchangeHandlerAdapter实现，其reply方法如下
public Object reply(ExchangeChannel channel, Object message) throws RemotingException {
if (message instanceof Invocation) {
Invocation inv = (Invocation) message;
//通过方法名获取Invoker
Invoker<?> invoker = getInvoker(channel, inv);
//如果是callback 需要处理高版本调用低版本的问题
if (Boolean.TRUE.toString().equals(inv.getAttachments().get(IS_CALLBACK_SERVICE_INVOKE))){
String methodsStr = invoker.getUrl().getParameters().get("methods");
boolean hasMethod = false;
if (methodsStr == null || methodsStr.indexOf(",") == -1){
hasMethod = inv.getMethodName().equals(methodsStr);
} else {
String[] methods = methodsStr.split(",");
for (String method : methods){
if (inv.getMethodName().equals(method)){
hasMethod = true;
break;
}
}
}
if (!hasMethod){
logger.warn(new IllegalStateException("The methodName "+inv.getMethodName()+" not found in callback service interface ,invoke will be ignored. please update the api interface. url is:" + invoker.getUrl()) +" ,invocation is :"+inv );
return null;
}
}
RpcContext.getContext().setRemoteAddress(channel.getRemoteAddress());
return invoker.invoke(inv);
}
throw new RemotingException(channel, "Unsupported request: " + message == null ? null : (message.getClass().getName() + ": " + message) + ", channel: consumer: " + channel.getRemoteAddress() + " --> provider: " + channel.getLocalAddress());
}
这里一共做了两件事，先通过getInvoker(channel, inv)获取具体的invoker，再通过invoker.invoke(inv)执行获取结果，先来看一下getInvoker(channel, inv)
Invoker<?> getInvoker(Channel channel, Invocation inv) throws RemotingException{
boolean isCallBackServiceInvoke = false;
boolean isStubServiceInvoke = false;
int port = channel.getLocalAddress().getPort();
String path = inv.getAttachments().get(Constants.PATH_KEY);
//如果是客户端的回调服务.
isStubServiceInvoke = Boolean.TRUE.toString().equals(inv.getAttachments().get(Constants.STUB_EVENT_KEY));
if (isStubServiceInvoke){
port = channel.getRemoteAddress().getPort();
}
//callback
isCallBackServiceInvoke = isClientSide(channel) && !isStubServiceInvoke;
if(isCallBackServiceInvoke){
path = inv.getAttachments().get(Constants.PATH_KEY)+"."+inv.getAttachments().get(Constants.CALLBACK_SERVICE_KEY);
inv.getAttachments().put(IS_CALLBACK_SERVICE_INVOKE, Boolean.TRUE.toString());
}
String serviceKey = serviceKey(port, path, inv.getAttachments().get(Constants.VERSION_KEY), inv.getAttachments().get(Constants.GROUP_KEY));
DubboExporter<?> exporter = (DubboExporter<?>) exporterMap.get(serviceKey);
if (exporter == null)
throw new RemotingException(channel, "Not found exported service: " + serviceKey + " in " + exporterMap.keySet() + ", may be version or group mismatch " + ", channel: consumer: " + channel.getRemoteAddress() + " --> provider: " + channel.getLocalAddress() + ", message:" + inv);
return exporter.getInvoker();
}
这里又看到了熟悉的exporterMap，之前讲生产者初始化的时候就说过这个map中放入了封装过的Invoker对象exporter，现在又把它取了出了并通过getInvoker()方法获得封装在其中的Invoker对象。
接着来看invoker.invoke(inv)方法，其实现首先在InvokerWrapper类中
public Result invoke(Invocation invocation) throws RpcException {
return invoker.invoke(invocation);
}
然后会调用到AbstractProxyInvoker中的invoke方法
public Result invoke(Invocation invocation) throws RpcException {
try {
return new RpcResult(doInvoke(proxy, invocation.getMethodName(), invocation.getParameterTypes(), invocation.getArguments()));
} catch (InvocationTargetException e) {
return new RpcResult(e.getTargetException());
} catch (Throwable e) {
throw new RpcException("Failed to invoke remote proxy method " + invocation.getMethodName() + " to " + getUrl() + ", cause: " + e.getMessage(), e);
}
}
这里doInvoke方法的实现在JavassistProxyFactory中getInvoker方法中
public <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) {
// TODO Wrapper类不能正确处理带$的类名
final Wrapper wrapper = Wrapper.getWrapper(proxy.getClass().getName().indexOf('$') < 0 ? proxy.getClass() : type);
return new AbstractProxyInvoker<T>(proxy, type, url) {
@Override
protected Object doInvoke(T proxy, String methodName,
Class<?>[] parameterTypes,
Object[] arguments) throws Throwable {
return wrapper.invokeMethod(proxy, methodName, parameterTypes, arguments);
}
};
}
这里根据传入的 proxy对象的类信息创建对它的包装对象Wrapper
并调用其invokeMethod方法，通过传入的参数来调用proxy对象的对应方法，返回调用结果，也就是执行具体的业务。
完成handleRequest(exchangeChannel, request)方法的解析后，回到HeaderExchangeHandler类中接着来看一下channel.send(response)，这里的channel传入的是NettyChannel类型的对象，send方法的实现在其父类的父类AbstractPeer中，来看一下
public void send(Object message) throws RemotingException {
send(message, url.getParameter(Constants.SENT_KEY, false));
}
其具体实现又在NettyChannel中
public void send(Object message, boolean sent) throws RemotingException {
super.send(message, sent);
boolean success = true;
int timeout = 0;
try {
ChannelFuture future = channel.write(message);
if (sent) {
timeout = getUrl().getPositiveParameter(Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);
success = future.await(timeout);
}
Throwable cause = future.getCause();
if (cause != null) {
throw cause;
}
} catch (Throwable e) {
throw new RemotingException(this, "Failed to send message " + message + " to " + getRemoteAddress() + ", cause: " + e.getMessage(), e);
}
if(! success) {
throw new RemotingException(this, "Failed to send message " + message + " to " + getRemoteAddress()
+ "in timeout(" + timeout + "ms) limit");
}
}
可以看到业务处理结果最后通过ChannelFuture对象进行了发送，到此生产者端的任务就完成了。
消费者获取调用结果
这里消费者端通过NETTY从生产者端获取数据的流程和之前的如出一辙，调用链直到HeaderExchangeHandler之前都是一样的，我们先来回顾一下HeaderExchangeHandler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;
//判断是心跳还是正常请求
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);
}
}
之前走的是Request分支，这次因为是响应消息走的是Response分支，那么来看一下handleResponse(channel, (Response) message)的具体实现
static void handleResponse(Channel channel, Response response) throws RemotingException {
if (response != null && !response.isHeartbeat()) {
DefaultFuture.received(channel, response);
}
}
继续跟进去看received方法
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());
}
}
继续看doReceived干了什么
private void doReceived(Response res) {
lock.lock();
try {
response = res;
if (done != null) {
done.signal();
}
} finally {
lock.unlock();
}
if (callback != null) {
invokeCallback(callback);
}
}
看到这里把执行结果赋值给response，正好应证了我们之前的猜想，消费者的同步阻塞也就可以继续执行下去了，这也算是非常经典的异步转同步的实现方案了吧。
总结
本文把消费者端和生产者端交互的大概流程进行了讲解，流程主要分为三个部分，分别是：消费者发起调用请求、生产者响应调用请求和消费者获取调用结果，概括一下就是消费者通过生成的代理对象调用invoke方法通过Netty的通道去请求生产者的exporter进行执行，并且通过future的方式将异步的交互转为了同步响应。
