远程调用——rmi协议
目标：介绍rmi协议的设计和实现，介绍dubbo-rpc-rmi的源码。
前言
dubbo支持rmi协议，主要基于spring封装的org.springframework.remoting.rmi包来实现，当然最原始还是依赖 JDK 标准的java.rmi.*包，采用阻塞式短连接和 JDK 标准序列化方式。关于rmi协议的介绍可以参考dubbo官方文档。
地址：http://dubbo.apache.org/zh-cn...
源码分析
（一）RmiRemoteInvocation
该类继承了RemoteInvocation，主要是在RemoteInvocation的基础上新增dubbo自身所需的附加值，避免这些附加值没有被传递，为了做一些验证处理。
public class RmiRemoteInvocation extends RemoteInvocation {private static final long serialVersionUID = 1L;private static final String dubboAttachmentsAttrName = "dubbo.attachments";/*** executed on consumer side*/public RmiRemoteInvocation(MethodInvocation methodInvocation) {super(methodInvocation);// 添加dubbo附加值的属性addAttribute(dubboAttachmentsAttrName, new HashMap<String, String>(RpcContext.getContext().getAttachments()));}/*** Need to restore context on provider side (Though context will be overridden by Invocation's attachment* when ContextFilter gets executed, we will restore the attachment when Invocation is constructed, check more* 需要在提供者端恢复上下文（尽管上下文将被Invocation的附件覆盖* 当ContextFilter执行时，我们将在构造Invocation时恢复附件，检查更多* from {@link com.alibaba.dubbo.rpc.proxy.InvokerInvocationHandler}*/@SuppressWarnings("unchecked")@Overridepublic Object invoke(Object targetObject) throws NoSuchMethodException, IllegalAccessException,InvocationTargetException {// 获得上下文RpcContext context = RpcContext.getContext();// 设置参数context.setAttachments((Map<String, String>) getAttribute(dubboAttachmentsAttrName));try {return super.invoke(targetObject);} finally {// 清空参数context.setAttachments(null);}}
}（二）RmiProtocol
该类继承了AbstractProxyProtocol类，是rmi协议实现的核心，跟其他协议一样，也实现了自己的服务暴露和服务引用方法。
1.doExport
@Override
protected <T> Runnable doExport(final T impl, Class<T> type, URL url) throws RpcException {// rmi暴露者final RmiServiceExporter rmiServiceExporter = new RmiServiceExporter();// 设置端口rmiServiceExporter.setRegistryPort(url.getPort());// 设置服务名称rmiServiceExporter.setServiceName(url.getPath());// 设置接口rmiServiceExporter.setServiceInterface(type);// 设置服务实现rmiServiceExporter.setService(impl);try {// 初始化bean的时候执行rmiServiceExporter.afterPropertiesSet();} catch (RemoteException e) {throw new RpcException(e.getMessage(), e);}return new Runnable() {@Overridepublic void run() {try {// 销毁rmiServiceExporter.destroy();} catch (Throwable e) {logger.warn(e.getMessage(), e);}}};
}该方法是服务暴露的逻辑实现。
2.doRefer
@Override
@SuppressWarnings("unchecked")
protected <T> T doRefer(final Class<T> serviceType, final URL url) throws RpcException {// FactoryBean对于RMI代理，支持传统的RMI服务和RMI调用者，创建RmiProxyFactoryBean对象final RmiProxyFactoryBean rmiProxyFactoryBean = new RmiProxyFactoryBean();// RMI needs extra parameter since it uses customized remote invocation object// 检测版本if (url.getParameter(Constants.DUBBO_VERSION_KEY, Version.getProtocolVersion()).equals(Version.getProtocolVersion())) {// Check dubbo version on provider, this feature only support// 设置RemoteInvocationFactory以用于此访问器rmiProxyFactoryBean.setRemoteInvocationFactory(new RemoteInvocationFactory() {@Overridepublic RemoteInvocation createRemoteInvocation(MethodInvocation methodInvocation) {// 自定义调用工厂可以向调用添加更多上下文信息return new RmiRemoteInvocation(methodInvocation);}});}// 设置此远程访问者的目标服务的URL。URL必须与特定远程处理提供程序的规则兼容。rmiProxyFactoryBean.setServiceUrl(url.toIdentityString());// 设置要访问的服务的接口。界面必须适合特定的服务和远程处理策略rmiProxyFactoryBean.setServiceInterface(serviceType);// 设置是否在找到RMI存根后缓存它rmiProxyFactoryBean.setCacheStub(true);// 设置是否在启动时查找RMI存根rmiProxyFactoryBean.setLookupStubOnStartup(true);// 设置是否在连接失败时刷新RMI存根rmiProxyFactoryBean.setRefreshStubOnConnectFailure(true);// // 初始化bean的时候执行rmiProxyFactoryBean.afterPropertiesSet();return (T) rmiProxyFactoryBean.getObject();
}该方法是服务引用的逻辑实现。
后记
该部分相关的源码解析地址：https://github.com/CrazyHZM/i...
该文章讲解了远程调用中关于rmi协议实现的部分，逻辑比较简单。接下来我将开始对rpc模块关于thrift协议部分进行讲解。
