add spi

workspace-gateway/src/main/java/com/aliware/tianchi/CallbackListenerImpl.java

+package com.aliware.tianchi;
+
+import org.apache.dubbo.rpc.listener.CallbackListener;
+
+/**
+ * @author daofeng.xjf
+ */
+public class CallbackListenerImpl implements CallbackListener {
+
+    @Override
+    public void receiveServerMsg(String msg) {
+        System.out.println("receive msg from server :" + msg);
+    }
+
+}

workspace-gateway/src/main/resources/META-INF/services/org.apache.dubbo.rpc.listener.CallbackListener

+com.aliware.tianchi.CallbackListenerImpl
\ No newline at end of file

workspace-provider/src/main/java/com/aliware/tianchi/CallbackServiceImpl.java

+package com.aliware.tianchi;
+
+import org.apache.dubbo.common.extension.SPI;
+import org.apache.dubbo.rpc.listener.CallbackListener;
+import org.apache.dubbo.rpc.service.CallbackService;
+
+import java.util.Date;
+import java.util.Map;
+import java.util.Timer;
+import java.util.TimerTask;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * @author daofeng.xjf
+ */
+public class CallbackServiceImpl implements CallbackService {
+
+    public CallbackServiceImpl() {
+        timer.schedule(new TimerTask() {
+            @Override
+            public void run() {
+                if (!listeners.isEmpty()) {
+                    for (Map.Entry<String, CallbackListener> entry : listeners.entrySet()) {
+                        try {
+                            entry.getValue().receiveServerMsg(new Date().toString());
+                        } catch (Throwable t1) {
+                            listeners.remove(entry.getKey());
+                        }
+                    }
+                }
+            }
+        }, 5000, 5000);
+    }
+
+    private Timer timer = new Timer();
+
+    /**
+     * key: listener type
+     * value: callback listener
+     */
+    private final Map<String, CallbackListener> listeners = new ConcurrentHashMap<>();
+
+    @Override
+    public void addListener(String key, CallbackListener listener) {
+        listeners.put(key, listener);
+        listener.receiveServerMsg(new Date().toString()); // send notification for change
+    }
+}

workspace-provider/src/main/resources/META-INF/services/org.apache.dubbo.rpc.service.CallbackService

+com.aliware.tianchi.CallbackServiceImpl
\ No newline at end of file