@Override
    public void registerUserProcessor(UserProcessor<?> processor) {
        UserProcessorRegisterHelper.registerUserProcessor(processor, this.userProcessors);
        // startup the processor if it registered after component startup
        if (isStarted() && !processor.isStarted()) {
            processor.startup();
        }
    }
Use UserProcessorRegisterHelper{@link UserProcessorRegisterHelper} to help register user processor for server side.

@see AbstractRemotingServer#registerUserProcessor(com.alipay.remoting.rpc.protocol.UserProcessor)
src/main/java/com/alipay/remoting/rpc/RpcServer.java