We are working on a dubbo gateway and need to support mutual access between different network clusters. This should be a common scenario. This requires dubbo to do two things for us:

1. Dynamically modify the ip and change the access of the local cluster to remote. This extension can be done, although there are some bugs, https://dubbo.apache.org/zh/docs3-v2/java-sdk/advanced-features-and-usage/service/specify-ip/

2. Forward the dubbo request. When the gateway receives the access request from cluster A, it should forward it to cluster B (now it returns an exception).

  From the code execution process.
  current:
  gateway-dubbo-provider receives the request -> parses -> judges whether the request interface has a corresponding cache URL -> no -> throws IOException -> netty returns

  ```java
  void handleRequest(final ExchangeChannel channel, Request req) throws RemotingException {
          Response res = new Response(req.getId(), req.getVersion());
          if (req.isBroken()) {
              Object data = req.getData();
              String msg;
              if (data == null) {
                  msg = null;
              } else if (data instanceof Throwable) {
                  msg = StringUtils.toString((Throwable) data);
              } else {
                  msg = data.toString();
              }
              res.setErrorMessage("Fail to decode request due to: " + msg);
              res.setStatus(Response.BAD_REQUEST);

              channel.send(res);
              return;
          }
  }
  ```

  ​

  expect:
  gateway-dubbo-provider receives the request -> parses -> judges whether the request interface has a corresponding cache URL -> no -> obtains the exception handler through spi (the request can be forwarded here) -> netty returns

```java
  void handleRequest(final ExchangeChannel channel, Request req) throws RemotingException {
          Response res = new Response(req.getId(), req.getVersion());
          if (req.isBroken()) {
              Object data = req.getData();
           //expert begin
          List<ExceptionHandler> list = spiExtensions.listByType(ExceptionHandler); 
          list.forEach(handler.handle(path,version))
           //expert end
              String msg;
              if (data == null) {
                  msg = null;
              } else if (data instanceof Throwable) {
                  msg = StringUtils.toString((Throwable) data);
              } else {
                  msg = data.toString();
              }
              res.setErrorMessage("Fail to decode request due to: " + msg);
              res.setStatus(Response.BAD_REQUEST);

              channel.send(res);
              return;
          }
  }
```


Since dubbo.extension provides dynamic ip replacement, then we should consider whether the target ip can be accepted and processed. I think this exception handler is an extension function of the dubbo.extension package.
We can provide ExceptionHandler similar to springboot, which can be more flexible

ServiceDescriptor serviceDescriptor = null;
                if (CollectionUtils.isNotEmpty(providerModels)) {
                    for (ProviderModel providerModel : providerModels) {
                        serviceDescriptor = providerModel.getServiceModel();
                        if (serviceDescriptor != null) {
                            break;
                        }
                    }
                }
                if (serviceDescriptor == null) {
                    
                    for (ApplicationModel applicationModel : frameworkModel.getApplicationModels()) {
                        for (ModuleModel moduleModel : applicationModel.getModuleModels()) {
                            serviceDescriptor = moduleModel.getServiceRepository().lookupService(path);
                            if (serviceDescriptor != null) {
                                break;
                            }
                        }
                    }
                }
                if (serviceDescriptor != null) {
                    MethodDescriptor methodDescriptor = serviceDescriptor.getMethod(getMethodName(), desc);
                    if (methodDescriptor != null) {
                        pts = methodDescriptor.getParameterClasses();
                        this.setReturnTypes(methodDescriptor.getReturnTypes());
                        
                        if (CollectionUtils.isNotEmpty(providerModels)) {
                            if (providerModels.size() == 1) {
                                Thread.currentThread().setContextClassLoader(providerModels.get(0).getClassLoader());
                            } else {