diff --git a/dubbo-rpc/dubbo-rpc-dubbo/src/main/java/org/apache/dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java b/dubbo-rpc/dubbo-rpc-dubbo/src/main/java/org/apache/dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java
index aa5c02b4a90..4d2f3c6756b 100644
--- a/dubbo-rpc/dubbo-rpc-dubbo/src/main/java/org/apache/dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java
+++ b/dubbo-rpc/dubbo-rpc-dubbo/src/main/java/org/apache/dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java
@@ -77,41 +77,41 @@ private static byte isCallBack(URL url, String methodName, int argIndex) {
      * @throws IOException
      */
     @SuppressWarnings({"unchecked", "rawtypes"})
-    private static String exportOrunexportCallbackService(Channel channel, URL url, Class clazz, Object inst, Boolean export) throws IOException {
+    private static String exportOrUnexportCallbackService(Channel channel, URL url, Class clazz, Object inst, Boolean export) throws IOException {
         int instid = System.identityHashCode(inst);
 
-        Map<String, String> params = new HashMap<String, String>(3);
+        Map<String, String> params = new HashMap<>(3);
         // no need to new client again
         params.put(Constants.IS_SERVER_KEY, Boolean.FALSE.toString());
         // mark it's a callback, for troubleshooting
         params.put(Constants.IS_CALLBACK_SERVICE, Boolean.TRUE.toString());
-        String group = url.getParameter(Constants.GROUP_KEY);
+        String group = (url == null ? null : url.getParameter(Constants.GROUP_KEY));
         if (group != null && group.length() > 0) {
             params.put(Constants.GROUP_KEY, group);
         }
         // add method, for verifying against method, automatic fallback (see dubbo protocol)
         params.put(Constants.METHODS_KEY, StringUtils.join(Wrapper.getWrapper(clazz).getDeclaredMethodNames(), ","));
 
-        Map<String, String> tmpmap = new HashMap<String, String>(url.getParameters());
-        tmpmap.putAll(params);
-        tmpmap.remove(Constants.VERSION_KEY);// doesn't need to distinguish version for callback
-        tmpmap.put(Constants.INTERFACE_KEY, clazz.getName());
-        URL exporturl = new URL(DubboProtocol.NAME, channel.getLocalAddress().getAddress().getHostAddress(), channel.getLocalAddress().getPort(), clazz.getName() + "." + instid, tmpmap);
+        Map<String, String> tmpMap = new HashMap<>(url.getParameters());
+        tmpMap.putAll(params);
+        tmpMap.remove(Constants.VERSION_KEY);// doesn't need to distinguish version for callback
+        tmpMap.put(Constants.INTERFACE_KEY, clazz.getName());
+        URL exportUrl = new URL(DubboProtocol.NAME, channel.getLocalAddress().getAddress().getHostAddress(), channel.getLocalAddress().getPort(), clazz.getName() + "." + instid, tmpMap);
 
         // no need to generate multiple exporters for different channel in the same JVM, cache key cannot collide.
         String cacheKey = getClientSideCallbackServiceCacheKey(instid);
-        String countkey = getClientSideCountKey(clazz.getName());
+        String countKey = getClientSideCountKey(clazz.getName());
         if (export) {
             // one channel can have multiple callback instances, no need to re-export for different instance.
             if (!channel.hasAttribute(cacheKey)) {
                 if (!isInstancesOverLimit(channel, url, clazz.getName(), instid, false)) {
-                    Invoker<?> invoker = proxyFactory.getInvoker(inst, clazz, exporturl);
+                    Invoker<?> invoker = proxyFactory.getInvoker(inst, clazz, exportUrl);
                     // should destroy resource?
                     Exporter<?> exporter = protocol.export(invoker);
                     // this is used for tracing if instid has published service or not.
                     channel.setAttribute(cacheKey, exporter);
-                    logger.info("export a callback service :" + exporturl + ", on " + channel + ", url is: " + url);
-                    increaseInstanceCount(channel, countkey);
+                    logger.info("Export a callback service :" + exportUrl + ", on " + channel + ", url is: " + url);
+                    increaseInstanceCount(channel, countKey);
                 }
             }
         } else {
@@ -119,7 +119,7 @@ private static String exportOrunexportCallbackService(Channel channel, URL url,
                 Exporter<?> exporter = (Exporter<?>) channel.getAttribute(cacheKey);
                 exporter.unexport();
                 channel.removeAttribute(cacheKey);
-                decreaseInstanceCount(channel, countkey);
+                decreaseInstanceCount(channel, countKey);
             }
         }
         return String.valueOf(instid);
@@ -245,17 +245,17 @@ private static void decreaseInstanceCount(Channel channel, String countkey) {
     public static Object encodeInvocationArgument(Channel channel, RpcInvocation inv, int paraIndex) throws IOException {
         // get URL directly
         URL url = inv.getInvoker() == null ? null : inv.getInvoker().getUrl();
-        byte callbackstatus = isCallBack(url, inv.getMethodName(), paraIndex);
+        byte callbackStatus = isCallBack(url, inv.getMethodName(), paraIndex);
         Object[] args = inv.getArguments();
         Class<?>[] pts = inv.getParameterTypes();
-        switch (callbackstatus) {
+        switch (callbackStatus) {
             case CallbackServiceCodec.CALLBACK_NONE:
                 return args[paraIndex];
             case CallbackServiceCodec.CALLBACK_CREATE:
-                inv.setAttachment(INV_ATT_CALLBACK_KEY + paraIndex, exportOrunexportCallbackService(channel, url, pts[paraIndex], args[paraIndex], true));
+                inv.setAttachment(INV_ATT_CALLBACK_KEY + paraIndex, exportOrUnexportCallbackService(channel, url, pts[paraIndex], args[paraIndex], true));
                 return null;
             case CallbackServiceCodec.CALLBACK_DESTROY:
-                inv.setAttachment(INV_ATT_CALLBACK_KEY + paraIndex, exportOrunexportCallbackService(channel, url, pts[paraIndex], args[paraIndex], false));
+                inv.setAttachment(INV_ATT_CALLBACK_KEY + paraIndex, exportOrUnexportCallbackService(channel, url, pts[paraIndex], args[paraIndex], false));
                 return null;
             default:
                 return args[paraIndex];