Callback parameter

Callback parameter in dubbo

The parameter callback is the same as calling a local callback or listener, just declare which parameter is a callback type in Spring’s configuration file, and Dubbo will generate a reverse proxy based on the long connection so that client logic can be called from the server.Can ref to Sample code in the dubbo project.

Example of service interface

package com.callback;

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

package com.callback;

public interface CallbackListener {
    void changed(String msg);

Example of service provider interface implementation

package com.callback.impl;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import com.callback.CallbackListener;
import com.callback.CallbackService;

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 {
                           } catch (Throwable t) {
                        Thread.sleep(5000); // Timed trigger change notification
                    } catch (Throwable t) { // Defense fault tolerance

    public void addListener(String key, CallbackListener listener) {
        listeners.put(key, listener);
        listener.changed(getChanged(key)); // send change notification

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

Example of service provider configuration

<bean id="callbackService" class="com.callback.impl.CallbackServiceImpl" />
<dubbo:service interface="com.callback.CallbackService" ref="callbackService" connections="1" callbacks="1000">
    <dubbo:method name="addListener">
        <dubbo:argument index="1" callback="true" />
        <!--also can via specified argument type-->
        <!--<dubbo:argument type="com.demo.CallbackListener" callback="true" />-->

Example of service consumer configuration

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

Example of service consumer call

ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:consumer.xml");

CallbackService callbackService = (CallbackService) context.getBean("callbackService");

callbackService.addListener("", new CallbackListener(){
    public void changed(String msg) {
        System.out.println("callback1:" + msg);

Last modified December 16, 2022: Fix check (#1745) (c82daa41ea)