RpcContext

RpcContext内部有一个ThreadLocal变量，它是作为ThreadLocalMap的key，表明每个线程有一个RpcContext。

public class RpcContext {
    private static final ThreadLocal<RpcContext> LOCAL = new ThreadLocal<RpcContext>() {
        @Override
        protected RpcContext initialValue() {
            return new RpcContext();
        }
    };

    //如果get在set之前，则get会返回initialValue()创建的对象
    public static RpcContext getContext() {
        return LOCAL.get();
    }
}

1. RpcContext的一种用法是：存放Future。Future封装了consumer的请求和响应，发送请求时会创建Future对象，此时响应是null，而DubboClientHandler线程会设置响应值。

设置调用方式为异步：

<dubbo:service interface="com.zhang.HelloService" ref="helloServiceImpl" protocol="dubbo">
    <dubbo:method name="sayHello" retries="0" async="true"></dubbo:method>
</dubbo:service>

RpcContext设置Future：

//DubboInvoker
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) {
            //异步调用，设置Future
            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();
        }
    } catch (TimeoutException e) {
    } catch (RemotingException e) {
    }
}

调用时直接返回：

HelloService helloService = (HelloService) appCtx.getBean("hello");
//返回null
String str = helloService.sayHello();

Future<String> future = RpcContext.getContext().getFuture();
//阻塞获取结果
String str2 = future.get();