Dubbo服务消费端远程调用过程剖析

1 Dubbo服务消费端远程调用过程概述

（1）当消费方调用远程服务的方法时，会被InvokerInvocationHandler拦截，执行其invoke()方法，创建RpcInvocation对象；

（2）接着会选择远程调用的负载均衡策略；

（3）最后执行DubboInvoker的doInvoke()方法发起远程调用。

2 消费端远程调用执行入口

服务消费端启动时，会为需要调用的远程服务类创建代理类（通过ReferenceConfig的get()方法），其中代理类的方法拦截器为InvokerInvocationHandler。代码如下所示。

    public <T> T getProxy(Invoker<T> invoker, Class<?>[] interfaces) {try {return (T) Proxy.getProxy(interfaces).newInstance(new InvokerInvocationHandler(invoker));} catch (Throwable fromJavassist) {// try fall back to JDK proxy factorytry {T proxy = jdkProxyFactory.getProxy(invoker, interfaces);logger.error(PROXY_FAILED, "", "", "Failed to generate proxy by Javassist failed. Fallback to use JDK proxy success. " +"Interfaces: " + Arrays.toString(interfaces), fromJavassist);return proxy;} catch (Throwable fromJdk) {logger.error(PROXY_FAILED, "", "", "Failed to generate proxy by Javassist failed. Fallback to use JDK proxy is also failed. " +"Interfaces: " + Arrays.toString(interfaces) + " Javassist Error.", fromJavassist);logger.error(PROXY_FAILED, "", "", "Failed to generate proxy by Javassist failed. Fallback to use JDK proxy is also failed. " +"Interfaces: " + Arrays.toString(interfaces) + " JDK Error.", fromJdk);throw fromJavassist;}}}

所以，当消费方调用远程服务的方法时，会被InvokerInvocationHandler拦截，执行其invoke()方法。其中method为调用的方法，args为参数。此处创建的RpcInvocation对象会一直传递下去，直到发起远程调用。invoke()方法代码如下所示。

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {if (method.getDeclaringClass() == Object.class) {return method.invoke(invoker, args);}String methodName = method.getName();Class<?>[] parameterTypes = method.getParameterTypes();if (parameterTypes.length == 0) {if ("toString".equals(methodName)) {return invoker.toString();} else if ("$destroy".equals(methodName)) {invoker.destroy();return null;} else if ("hashCode".equals(methodName)) {return invoker.hashCode();}} else if (parameterTypes.length == 1 && "equals".equals(methodName)) {return invoker.equals(args[0]);}RpcInvocation rpcInvocation = new RpcInvocation(serviceModel, method.getName(), invoker.getInterface().getName(), protocolServiceKey, method.getParameterTypes(), args);if (serviceModel instanceof ConsumerModel) {rpcInvocation.put(Constants.CONSUMER_MODEL, serviceModel);rpcInvocation.put(Constants.METHOD_MODEL, ((ConsumerModel) serviceModel).getMethodModel(method));}return InvocationUtil.invoke(invoker, rpcInvocation);}