在调用之前、调用之后、出现异常时，会触发 oninvoke、onreturn、onthrow 三个事件，可以配置当事件发生时，通知哪个类的哪个方法。
调用服务方法前我们可以记录开始时间，调用结束后统计整个调用耗费，发生异常时我们可以告警或打印错误日志或者调用服务前后记录请求日志、响应日志等。
参考用例 dubbo-samples-notify
interface IDemoService {
public Person get(int id);
}
class NormalDemoService implements IDemoService {
public Person get(int id) {
return new Person(id, "charles`son", 4);
}
}
<dubbo:application name="rpc-callback-demo" />
<dubbo:registry address="zookeeper://127.0.0.1:2181"/>
<bean id="demoService" class="org.apache.dubbo.callback.implicit.NormalDemoService" />
<dubbo:service interface="org.apache.dubbo.callback.implicit.IDemoService" ref="demoService" version="1.0.0" group="cn"/>
interface Notify {
public void onreturn(Person msg, Integer id);
public void onthrow(Throwable ex, Integer id);
}
class NotifyImpl implements Notify {
public Map<Integer, Person> ret = new HashMap<Integer, Person>();
public Map<Integer, Throwable> errors = new HashMap<Integer, Throwable>();
public void onreturn(Person msg, Integer id) {
System.out.println("onreturn:" + msg);
ret.put(id, msg);
}
public void onthrow(Throwable ex, Integer id) {
errors.put(id, ex);
}
}
两者叠加存在以下几种组合情况：
async=true onreturn="xxx"async=false onreturn="xxx"async=trueasync=falsecallback 与 async 功能正交分解，async=true 表示结果是否马上返回，async=false 默认，onreturn 表示是否需要回调。
<bean id ="demoCallback" class = "org.apache.dubbo.callback.implicit.NotifyImpl" />
<dubbo:reference id="demoService" interface="org.apache.dubbo.callback.implicit.IDemoService" version="1.0.0" group="cn" >
<dubbo:method name="get" async="true" onreturn = "demoCallback.onreturn" onthrow="demoCallback.onthrow" />
</dubbo:reference>
IDemoService demoService = (IDemoService) context.getBean("demoService");
NotifyImpl notify = (NotifyImpl) context.getBean("demoCallback");
int requestId = 2;
Person ret = demoService.get(requestId);
Assert.assertEquals(null, ret);
//for Test：只是用来说明callback正常被调用，业务具体实现自行决定.
for (int i = 0; i < 10; i++) {
if (!notify.ret.containsKey(requestId)) {
Thread.sleep(200);
} else {
break;
}
}
Assert.assertEquals(requestId, notify.ret.get(requestId).getId());