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。

支持版本：2.0.7之后

(1) 服务提供者与消费者共享服务接口

IDemoService.java

interface IDemoService {
    public Person get(int id);
}

(2) 服务提供者实现

DemoServiceImpl.java

class NormalDemoService implements IDemoService {
    public Person get(int id) {
        return new Person(id, "charles`son", 4);
    }
}

(3) 服务提供者配置

provider.xml

<dubbo:application name="rpc-callback-demo" />
<dubbo:registry address="http://10.20.160.198/wiki/display/dubbo/10.20.153.186" />
<bean id="demoService" class="com.alibaba.dubbo.callback.implicit.NormalDemoService" />
<dubbo:service interface="com.alibaba.dubbo.callback.implicit.IDemoService" ref="demoService" version="1.0.0" group="cn"/>

(4) 服务消费者Callback接口及实现

Notify.java

interface Notify {
    public void onreturn(Person msg, Integer id);
    public void onthrow(Throwable ex, Integer id);
}

NotifyImpl.java

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);
    }
}

(5) 服务消费者Callback接口及实现

consumer.xml

<bean id ="demoCallback" class = "com.alibaba.dubbo.callback.implicit.NofifyImpl" />
<dubbo:reference id="demoService" interface="com.alibaba.dubbo.callback.implicit.IDemoService" version="1.0.0" group="cn" >
      <dubbo:method name="get" async="true" onreturn = "demoCallback.onreturn" onthrow="demoCallback.onthrow" />
</dubbo:reference>

callback与async功能正交分解：

• async=true，表示结果是否马上返回.
• onreturn 表示是否需要回调.

组合情况：(async=false 默认)

• 异步回调模式：async=true onreturn=“xxx”
• 同步回调模式：async=false onreturn=“xxx”
• 异步无回调 ：async=true
• 同步无回调 ：async=false

(6) TEST CASE

Test.java

IDemoService demoService = (IDemoService) context.getBean("demoService");
NofifyImpl notify = (NofifyImpl) 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());