How to get client username in PublishInboundInterceptor in HiveMQ

IN PublishInboundInterceptor, I can get client id, then I guess I can get username use ClientService, but I failed.

Is there a way to get client username and password?

there is no out of the box method to fetch username/password in the PublishInboundInterceptor. You would have to fetch the credentials via SimpleAuthenticator, EnhancedAuthenticator or ClientLifecycleEventListener#onMqttConnectionStart and then store them in the ConnectionAttributeStore (this is a store each client has for himself and stores attributes that are deleted when the client disconnects).

I put together an example to showcase this:

// use a SimpleAuthenticator to store username/password (if they are set by the client) in the connection attribute store of the client
// other options to fetch username/password: ClientLifecycleEventListener#onMqttConnectionStart or EnhancedAuthenticator
Services.securityRegistry().setAuthenticatorProvider(providerInput ->
        (SimpleAuthenticator) (simpleAuthInput, simpleAuthOutput) -> {
            final ConnectionAttributeStore connectionAttributeStore = simpleAuthInput.getConnectionInformation().getConnectionAttributeStore();
            final ConnectPacket connectPacket = simpleAuthInput.getConnectPacket();

            connectPacket.getUserName().ifPresent(username -> connectionAttributeStore.put("username", ByteBuffer.wrap(username.getBytes(StandardCharsets.UTF_8))));
            connectPacket.getPassword().ifPresent(password -> connectionAttributeStore.put("password", password));
        });

// in the publish inbound interceptor fetch username/password from the connection attribute store (check optional if they are set)
Services.initializerRegistry().setClientInitializer((initializerInput, clientContext) ->
        clientContext.addPublishInboundInterceptor(
                (publishInboundInput, publishInboundOutput) -> {
                    final Optional<String> usernameOptional = publishInboundInput.getConnectionInformation().getConnectionAttributeStore().getAsString("username");
                    final Optional<@Immutable ByteBuffer> passwordOptional = publishInboundInput.getConnectionInformation().getConnectionAttributeStore().get("password");

                    //happy coding
                }
        )
);

Hope this helps!