Make the applicationCodeRegister only for single code.

apm-collector/apm-collector-agent/agent-grpc/agent-grpc-provider/src/main/java/org/apache/skywalking/apm/collector/agent/grpc/provider/handler/ApplicationRegisterServiceHandler.java

@@ -24,7 +24,7 @@ import org.apache.skywalking.apm.collector.analysis.register.define.service.IApp
 import org.apache.skywalking.apm.collector.core.module.ModuleManager;
 import org.apache.skywalking.apm.collector.server.grpc.GRPCHandler;
 import org.apache.skywalking.apm.network.proto.Application;
-import org.apache.skywalking.apm.network.proto.ApplicationMappings;
+import org.apache.skywalking.apm.network.proto.ApplicationMapping;
 import org.apache.skywalking.apm.network.proto.ApplicationRegisterServiceGrpc;
 import org.apache.skywalking.apm.network.proto.KeyWithIntegerValue;
 import org.slf4j.Logger;
@@ -44,16 +44,16 @@ public class ApplicationRegisterServiceHandler extends ApplicationRegisterServic
     }
 
     @Override
-    public void applicationCodeRegister(Application request, StreamObserver<ApplicationMappings> responseObserver) {
+    public void applicationCodeRegister(Application request, StreamObserver<ApplicationMapping> responseObserver) {
         logger.debug("register application");
 
-        ApplicationMappings.Builder builder = ApplicationMappings.newBuilder();
+        ApplicationMapping.Builder builder = ApplicationMapping.newBuilder();
         String applicationCode = request.getApplicationCode();
         int applicationId = applicationIDService.getOrCreateForApplicationCode(applicationCode);
 
         if (applicationId != 0) {
             KeyWithIntegerValue value = KeyWithIntegerValue.newBuilder().setKey(applicationCode).setValue(applicationId).build();
-            builder.addApplications(value);
+            builder.setApplication(value);
         }
         responseObserver.onNext(builder.build());
         responseObserver.onCompleted();

apm-protocol/apm-network/src/main/proto/ApplicationRegisterService.proto

@@ -7,7 +7,7 @@ import "KeyWithIntegerValue.proto";
 
 //register service for ApplicationCode, this service is called when service starts.
 service ApplicationRegisterService {
-    rpc applicationCodeRegister (Application) returns (ApplicationMappings) {
+    rpc applicationCodeRegister (Application) returns (ApplicationMapping) {
     }
 }
 
@@ -15,6 +15,6 @@ message Application {
     string applicationCode = 1;
 }
 
-message ApplicationMappings {
-    repeated KeyWithIntegerValue applications = 1;
+message ApplicationMapping {
+    KeyWithIntegerValue application = 1;
 }

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/remote/AppAndServiceRegisterClient.java

@@ -41,7 +41,7 @@ import org.apache.skywalking.apm.network.proto.Application;
 import org.apache.skywalking.apm.network.proto.ApplicationInstance;
 import org.apache.skywalking.apm.network.proto.ApplicationInstanceHeartbeat;
 import org.apache.skywalking.apm.network.proto.ApplicationInstanceMapping;
-import org.apache.skywalking.apm.network.proto.ApplicationMappings;
+import org.apache.skywalking.apm.network.proto.ApplicationMapping;
 import org.apache.skywalking.apm.network.proto.ApplicationRegisterServiceGrpc;
 import org.apache.skywalking.apm.network.proto.InstanceDiscoveryServiceGrpc;
 import org.apache.skywalking.apm.network.proto.NetworkAddressRegisterServiceGrpc;
@@ -109,10 +109,10 @@ public class AppAndServiceRegisterClient implements BootService, GRPCChannelList
             try {
                 if (RemoteDownstreamConfig.Agent.APPLICATION_ID == DictionaryUtil.nullValue()) {
                     if (applicationRegisterServiceBlockingStub != null) {
-                        ApplicationMappings applicationMapping = applicationRegisterServiceBlockingStub.applicationCodeRegister(
+                        ApplicationMapping applicationMapping = applicationRegisterServiceBlockingStub.applicationCodeRegister(
                             Application.newBuilder().setApplicationCode(Config.Agent.APPLICATION_CODE).build());
-                        if (applicationMapping.getApplicationsCount() > 0) {
-                            RemoteDownstreamConfig.Agent.APPLICATION_ID = applicationMapping.getApplications(0).getValue();
+                        if (applicationMapping != null) {
+                            RemoteDownstreamConfig.Agent.APPLICATION_ID = applicationMapping.getApplication().getValue();
                             shouldTry = true;
                         }
                     }