效果：Consumer調用Provider方法，在Provider方法中回調Consumer提供的方法。

參數回調方式與調用本地 callback 或 listener 相同，只需要在 Spring 的配置文件中聲明哪個參數是 callback 類型即可。Dubbo 將基於長連接生成反向代理，這樣就可以從服務器端調用客戶端邏輯。

服務接口示例

CallbackService.java

public interface CallbackService {
    void addListener(String key, CallbackListener listener);
}

CallbackListener.java

public interface CallbackListener {
    void changed(String msg);
}

服務提供者接口實現示例

public class CallbackServiceImpl implements CallbackService{
    private final Map<String, CallbackListener> listeners = new ConcurrentHashMap<String, CallbackListener>();
    
    public CallbackServiceImpl() {
        Thread t 
 
= new Thread(new Runnable() {
            public void run() {
                while(true) {
                    try {
                        for(Map.Entry<String, CallbackListener> entry : listeners.entrySet()){
                           try {
                               entry.getValue().changed(getChanged(entry.getKey()));
                           } 
 
catch (Throwable t) {
                               listeners.remove(entry.getKey());
                           }
                        }
                        Thread.sleep(5000); // 定時觸發變更通知
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        t.setDaemon(true);
        t.start();
    }
    
    public void addListener(String key, CallbackListener listener) {
        listeners.put(key, listener);
        listener.changed(getChanged(key)); // 發送變更通知
    }

    private String getChanged(String key) {
        return "Changed: " + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date());
    }
}

服務提供者配置示例

<bean id="callbackService" class="com.ssmp.serviceImpl.CallbackServiceImpl" />
<dubbo:service interface="com.ssmp.service.CallbackService" ref="callbackService" connections="1" callbacks="1000">
    <dubbo:method name="addListener">
        <dubbo:argument index="1" callback="true" />
        <!--也可以通過指定類型的方式-->
        <!--<dubbo:argument type="com.demo.CallbackListener" callback="true" />-->
    </dubbo:method>
</dubbo:service>

服務消費者配置示例

<dubbo:reference id="callbackService" interface="com.ssmp.service.CallbackService" />

服務消費者調用示例

ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:META-INF/spring/user-consumer.xml");
context.start();

CallbackService callbackService = (CallbackService) context.getBean("callbackService");
callbackService.addListener("http://10.20.160.198/wiki/display/dubbo/foo.bar", new CallbackListener(){
    public void changed(String msg) {
        System.out.println("callback1:" + msg);
    }
});
System.in.read();

參數回調 callbacks