0. dubbo同步调用、异步调用和是否返回结果配置
（1）dubbo默认为同步调用，并且有返回结果。
（2）dubbo异步调用配置，设置 async="true"，异步调用可以提高效率。
（3）dubbo默认是有返回结果，不需要返回，可以设置return="false"，不需要返回值，可以减少等待结果时间。
1. 源码分析（dubbo版本：2.6.0）
dubbo自身底层调用是使用netty异步实现的，默认同步调用返回结果，是通过获取ResponseFuture，然后使用ReentrantLock的await使当前线程等待结果，设置返回的。下面是部分核心代码部分：
com.alibaba.dubbo.rpc.protocol.dubbo.DubboInvoker.doInvoke(Invocation)
@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;
if (clients.length == 1) {
currentClient = clients[0];
} else {
currentClient = clients[index.getAndIncrement() % clients.length];
}
try {
// 获取是否异步配置async
boolean isAsync = RpcUtils.isAsync(getUrl(), invocation);
// 获取是否需要返回结果配置return
boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);
// 获取超时配置timeout
int timeout = getUrl().getMethodParameter(methodName, Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);
if (isOneway) {
// 不管是否异步，只要不需要返回结果，直接异步调用，设置结果为null
boolean isSent = getUrl().getMethodParameter(methodName, Constants.SENT_KEY, false);
currentClient.send(inv, isSent);
RpcContext.getContext().setFuture(null);
return new RpcResult();
} else if (isAsync) {
// 如果异步，并且需要返回结果，调用后设置结果future
ResponseFuture future = currentClient.request(inv, timeout);
RpcContext.getContext().setFuture(new FutureAdapter<Object>(future));
return new RpcResult();
} else {
// 如果同步，并且需要返回结果，调用后在此等待，直到有结果设置结果，或者超时抛出异常。
RpcContext.getContext().setFuture(null);
return (Result) currentClient.request(inv, timeout).get();
}
} catch (TimeoutException e) {
throw new RpcException(RpcException.TIMEOUT_EXCEPTION, "Invoke remote method timeout. method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
} catch (RemotingException e) {
throw new RpcException(RpcException.NETWORK_EXCEPTION, "Failed to invoke remote method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
}
}
2. 实例
这里使用dubbo源码自带的例子
（1）在dubbo-demo-api中定义异步服务
package com.alibaba.dubbo.demo;
public interface AsyncDemoService {
String sayHello(String name);
}
（2）在dubbo-demo-provider中实现
package com.alibaba.dubbo.demo.provider; import java.text.SimpleDateFormat;
import java.util.Date; import com.alibaba.dubbo.demo.AsyncDemoService;
import com.alibaba.dubbo.rpc.RpcContext; public class AsyncDemoServiceImpl implements AsyncDemoService{ @Override
public String sayHello(String name) {
System.out.println("[" + new SimpleDateFormat("HH:mm:ss").format(new Date()) + "] Hello " + name + ", request from consumer: " + RpcContext.getContext().getRemoteAddress());
return "Hello " + name + ", response form provider: " + RpcContext.getContext().getLocalAddress();
} }
（3）在dubbo-demo-provider.xml中配置服务信息
<bean id="asyncDemoService" class="com.alibaba.dubbo.demo.provider.AsyncDemoServiceImpl"/>
<dubbo:service interface="com.alibaba.dubbo.demo.AsyncDemoService" ref="asyncDemoService"/>
（4）在dubbo-demo-consumer.xml中配置调用服务信息，设置为异步调用async="true"
<dubbo:reference id="asyncDemoService" check="false" interface="com.alibaba.dubbo.demo.AsyncDemoService">
<dubbo:method name="sayHello" async="true"/>
</dubbo:reference>
（5）在dubbo-demo-consumer的Consumer类中增加调用
AsyncDemoService asyncDemoService = (AsyncDemoService) context.getBean("asyncDemoService");
asyncDemoService.sayHello("world");
// 通过 1.源码分析 中可以知道异步返回结果放到了RpcContext.getContext()中
Future<String> future = RpcContext.getContext().getFuture();
try {
String hello = future.get(1, TimeUnit.SECONDS);
System.out.println(hello);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
} catch (TimeoutException e) {
e.printStackTrace();
}
（6）不需要返回结果，配置dubbo-demo-consumer.xml 中return="false"，同步异步的调用方式一样的，很简单，只需要调用一下就可以继续其他操作，下面是异步的例子。
AsyncDemoService asyncDemoService = (AsyncDemoService) context.getBean("asyncDemoService");
asyncDemoService.sayHello("world");