Dubbo分析之Protocol层

系列文章

Dubbo分析Serialize层
Dubbo分析之Transport层
Dubbo分析之Exchange 层
Dubbo分析之Protocol层

前言

紧接着上文Dubbo分析之Exchange层，继续分析protocol远程调用层，官方介绍：封装RPC调用，以Invocation, Result为中心，扩展接口为Protocol, Invoker, Exporter；

Protocol接口类分析

Protocol可以说是Dubbo的核心层了，在此基础上可以扩展很多主流的服务，比如：redis，Memcached，rmi，WebService，http(tomcat，jetty)等等；下面看一下接口类源码：

public interface Protocol {
    /**
     * 暴露远程服务：

     * 1. 协议在接收请求时，应记录请求来源方地址信息：RpcContext.getContext().setRemoteAddress();

     * 2. export()必须是幂等的，也就是暴露同一个URL的Invoker两次，和暴露一次没有区别。

     * 3. export()传入的Invoker由框架实现并传入，协议不需要关心。

     * 
     * @param  服务的类型
     * @param invoker 服务的执行体
     * @return exporter 暴露服务的引用，用于取消暴露
     * @throws RpcException 当暴露服务出错时抛出，比如端口已占用
     */
     Exporter export(Invoker invoker) throws RpcException;
  
    /**
     * 引用远程服务：

     * 1. 当用户调用refer()所返回的Invoker对象的invoke()方法时，协议需相应执行同URL远端export()传入的Invoker对象的invoke()方法。

     * 2. refer()返回的Invoker由协议实现，协议通常需要在此Invoker中发送远程请求。

     * 3. 当url中有设置check=false时，连接失败不能抛出异常，需内部自动恢复。

     * 
     * @param  服务的类型
     * @param type 服务的类型
     * @param url 远程服务的URL地址
     * @return invoker 服务的本地代理
     * @throws RpcException 当连接服务提供方失败时抛出
     */
     Invoker refer(Class type, URL url) throws RpcException;
  
}

主要定义了2个接口，一个是暴露远程服务，另一个是引用远程服务，其实就是服务端和客户端；dubbo提供了对多种服务的扩展，可以查看META-INF/dubbo/internal/com.alibaba.dubbo.rpc.Protocol：

filter=com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper
listener=com.alibaba.dubbo.rpc.protocol.ProtocolListenerWrapper
mock=com.alibaba.dubbo.rpc.support.MockProtocol
dubbo=com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocol
injvm=com.alibaba.dubbo.rpc.protocol.injvm.InjvmProtocol
rmi=com.alibaba.dubbo.rpc.protocol.rmi.RmiProtocol
hessian=com.alibaba.dubbo.rpc.protocol.hessian.HessianProtocol
com.alibaba.dubbo.rpc.protocol.http.HttpProtocol
com.alibaba.dubbo.rpc.protocol.webservice.WebServiceProtocol
thrift=com.alibaba.dubbo.rpc.protocol.thrift.ThriftProtocol
memcached=com.alibaba.dubbo.rpc.protocol.memcached.MemcachedProtocol
redis=com.alibaba.dubbo.rpc.protocol.redis.RedisProtocol
rest=com.alibaba.dubbo.rpc.protocol.rest.RestProtocol
registry=com.alibaba.dubbo.registry.integration.RegistryProtocol
qos=com.alibaba.dubbo.qos.protocol.QosProtocolWrapper

dubbo协议是默认提供的协议，其他扩展的协议包括：hessian，http(tomcat，jetty)，injvm，memcached，redis，rest，rmi，thrift，webservice；以上扩展的协议有些仅仅是作为引用远程服务存在(客户端)，比如redis，memcached，通过特定的命令对缓存进行操作；当然也可以扩展自己的协议，分别实现接口类Protocol, Invoker, Exporter；之前分别介绍的serialize层，transport层以及exchange层主要是在使用默认的DubboProtocol才依赖这几个底层，其他扩展协议直接依赖第三方扩展包；
下面重点分析一下DubboProtocol类，首先看一下refer实现方法：

public  Invoker refer(Class serviceType, URL url) throws RpcException {
    optimizeSerialization(url);
    // create rpc invoker.
    DubboInvoker invoker = new DubboInvoker(serviceType, url, getClients(url), invokers);
    invokers.add(invoker);
    return invoker;
}

在客户端定一个的每个dubbo:reference，都会在此处实例化一个对应的DubboInvoker；在方法内部首先对序列化优化进行处理，主要是对Kryo,FST等序列化方式进行优化，此方法不仅在客户端，同时服务器端也存在；接下来就是创建了一个DubboInvoker，同时创建与服务器端的连接：

private ExchangeClient[] getClients(URL url) {
        // whether to share connection
        boolean service_share_connect = false;
        int connections = url.getParameter(Constants.CONNECTIONS_KEY, 0);
        // if not configured, connection is shared, otherwise, one connection for one service
        if (connections == 0) {
            service_share_connect = true;
            connections = 1;
        }
 
        ExchangeClient[] clients = new ExchangeClient[connections];
        for (int i = 0; i < clients.length; i++) {
            if (service_share_connect) {
                clients[i] = getSharedClient(url);
            } else {
                clients[i] = initClient(url);
            }
        }
        return clients;
    }

默认向指定的服务器创建一个连接，可以通过指定connections设置建立多个连接，在并发比较大的情况下可以设置多个；

private ExchangeClient initClient(URL url) {
 
        // client type setting.
        String str = url.getParameter(Constants.CLIENT_KEY, url.getParameter(Constants.SERVER_KEY, Constants.DEFAULT_REMOTING_CLIENT));
 
        url = url.addParameter(Constants.CODEC_KEY, DubboCodec.NAME);
        // enable heartbeat by default
        url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));
 
        // BIO is not allowed since it has severe performance issue.
        if (str != null && str.length() > 0 && !ExtensionLoader.getExtensionLoader(Transporter.class).hasExtension(str)) {
            throw new RpcException("Unsupported client type: " + str + "," +
                    " supported client type is " + StringUtils.join(ExtensionLoader.getExtensionLoader(Transporter.class).getSupportedExtensions(), " "));
        }
 
        ExchangeClient client;
        try {
            // connection should be lazy
            if (url.getParameter(Constants.LAZY_CONNECT_KEY, false)) {
                client = new LazyConnectExchangeClient(url, requestHandler);
            } else {
                client = Exchangers.connect(url, requestHandler);
            }
        } catch (RemotingException e) {
            throw new RpcException("Fail to create remoting client for service(" + url + "): " + e.getMessage(), e);
        }
        return client;
    }

此方法主要通过Exchange层接口来和服务端建立连接，同时提供了懒连接的方式，要等到真正发送请求的时候才建立连接，返回ExchangeClient；DubboInvoker内部通过ExchangeClient来发送请求给服务端；再来看一下export方法：

public  Exporter export(Invoker invoker) throws RpcException {
       URL url = invoker.getUrl();
 
       // export service.
       String key = serviceKey(url);
       DubboExporter exporter = new DubboExporter(invoker, key, exporterMap);
       exporterMap.put(key, exporter);
 
       //export an stub service for dispatching event
       Boolean isStubSupportEvent = url.getParameter(Constants.STUB_EVENT_KEY, Constants.DEFAULT_STUB_EVENT);
       Boolean isCallbackservice = url.getParameter(Constants.IS_CALLBACK_SERVICE, false);
       if (isStubSupportEvent && !isCallbackservice) {
           String stubServiceMethods = url.getParameter(Constants.STUB_EVENT_METHODS_KEY);
           if (stubServiceMethods == null || stubServiceMethods.length() == 0) {
               if (logger.isWarnEnabled()) {
                   logger.warn(new IllegalStateException("consumer [" + url.getParameter(Constants.INTERFACE_KEY) +
                           "], has set stubproxy support event ,but no stub methods founded."));
               }
           } else {
               stubServiceMethodsMap.put(url.getServiceKey(), stubServiceMethods);
           }
       }
 
       openServer(url);
       optimizeSerialization(url);
       return exporter;
   }

每个dubbo:service都会绑定一个Exporter，首先通过url获取一个key(包括：port，serviceName，serviceVersion，serviceGroup)，然后将实例化的DubboExporter通过key值保存在一个Map中，后续在接收到消息的时候从新定位到具体的Exporter；接下来就是创建服务器：

private void openServer(URL url) {
        // find server.
        String key = url.getAddress();
        //client can export a service which's only for server to invoke
        boolean isServer = url.getParameter(Constants.IS_SERVER_KEY, true);
        if (isServer) {
            ExchangeServer server = serverMap.get(key);
            if (server == null) {
                serverMap.put(key, createServer(url));
            } else {
                // server supports reset, use together with override
                server.reset(url);
            }
        }
    }
 
    private ExchangeServer createServer(URL url) {
        // send readonly event when server closes, it's enabled by default
        url = url.addParameterIfAbsent(Constants.CHANNEL_READONLYEVENT_SENT_KEY, Boolean.TRUE.toString());
        // enable heartbeat by default
        url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));
        String str = url.getParameter(Constants.SERVER_KEY, Constants.DEFAULT_REMOTING_SERVER);
 
        if (str != null && str.length() > 0 && !ExtensionLoader.getExtensionLoader(Transporter.class).hasExtension(str))
            throw new RpcException("Unsupported server type: " + str + ", url: " + url);
 
        url = url.addParameter(Constants.CODEC_KEY, DubboCodec.NAME);
        ExchangeServer server;
        try {
            server = Exchangers.bind(url, requestHandler);
        } catch (RemotingException e) {
            throw new RpcException("Fail to start server(url: " + url + ") " + e.getMessage(), e);
        }
        str = url.getParameter(Constants.CLIENT_KEY);
        if (str != null && str.length() > 0) {
            Set supportedTypes = ExtensionLoader.getExtensionLoader(Transporter.class).getSupportedExtensions();
            if (!supportedTypes.contains(str)) {
                throw new RpcException("Unsupported client type: " + str);
            }
        }
        return server;
    }

以上主要就是通过Exchangers的bind方法来启动服务器，并返回对应的ExchangeServer，同样也保存在本地的Map中；最后同样做了序列化优化处理；

Invoker类分析

refer()返回的Invoker由协议实现，协议通常需要在此Invoker中发送远程请求，export()传入的Invoker由框架实现并传入，协议不需要关心；接口类如下：

public interface Invoker extends Node {
 
    Class getInterface();
 
    Result invoke(Invocation invocation) throws RpcException;
}

本节介绍的是refer方法返回的Invoker，默认的dubbo协议下，实现了DubboInvoker，实现了其中的invoke方法，此方法在客户端调用远程方法的时候会被调用；

public Result invoke(Invocation inv) throws RpcException {
    if (destroyed.get()) {
        throw new RpcException("Rpc invoker for service " + this + " on consumer " + NetUtils.getLocalHost()
                + " use dubbo version " + Version.getVersion()
                + " is DESTROYED, can not be invoked any more!");
    }
    RpcInvocation invocation = (RpcInvocation) inv;
    invocation.setInvoker(this);
    if (attachment != null && attachment.size() > 0) {
        invocation.addAttachmentsIfAbsent(attachment);
    }
    Map contextAttachments = RpcContext.getContext().getAttachments();
    if (contextAttachments != null) {
        /**
         * invocation.addAttachmentsIfAbsent(context){@link RpcInvocation#addAttachmentsIfAbsent(Map)}should not be used here,
         * because the {@link RpcContext#setAttachment(String, String)} is passed in the Filter when the call is triggered
         * by the built-in retry mechanism of the Dubbo. The attachment to update RpcContext will no longer work, which is
         * a mistake in most cases (for example, through Filter to RpcContext output traceId and spanId and other information).
         */
        invocation.addAttachments(contextAttachments);
    }
    if (getUrl().getMethodParameter(invocation.getMethodName(), Constants.ASYNC_KEY, false)) {
        invocation.setAttachment(Constants.ASYNC_KEY, Boolean.TRUE.toString());
    }
    RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation);
 
 
    try {
        return doInvoke(invocation);
    } catch (InvocationTargetException e) { // biz exception
        Throwable te = e.getTargetException();
        if (te == null) {
            return new RpcResult(e);
        } else {
            if (te instanceof RpcException) {
                ((RpcException) te).setCode(RpcException.BIZ_EXCEPTION);
            }
            return new RpcResult(te);
        }
    } catch (RpcException e) {
        if (e.isBiz()) {
            return new RpcResult(e);
        } else {
            throw e;
        }
    } catch (Throwable e) {
        return new RpcResult(e);
    }
}
 
protected abstract Result doInvoke(Invocation invocation) throws Throwable;

在DubboInvoker的抽象类中提供了invoke方法，做统一的附件(Attachment)处理，方法传入的参数是一个RpcInvocation对象，包含了方法调用的相关参数：

public class RpcInvocation implements Invocation, Serializable {
 
    private static final long serialVersionUID = -4355285085441097045L;
 
    private String methodName;
 
    private Class[] parameterTypes;
 
    private Object[] arguments;
 
    private Map attachments;
 
    private transient Invoker invoker;
     
    ....省略...
}

包含了方法名称，方法参数，参数值，附件信息；可能你会发现没有接口，版本等信息，这些信息其实包含在附件中；在invoke方法中首先处理的就是把attachment信息保存到RpcInvocation中；接下来就是调用DubboInvoker中的doInvoke方法：

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;
        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