What's the relationship with ServerServiceDefinition and a concrete class?

Question

I am learning to use grpc recently and feel confused about one thing after I instantiated a Server class named server:

server = ServerBuilder.forPort(port)
            .addService(new MetadataStoreImpl(this.config, port))
            .executor(Executors.newFixedThreadPool(numThreads))
            .build()
            .start();

I read the java doc and found the ServerBuilder.addService:

public abstract T addService(ServerServiceDefinition service);

In this case, I use my own class "MetadataStoreImpl" to replace "ServerServiceDefinition" but still don't know why. Moreover,how can I get reference the member variable of this MetadataStoreImpl class from class server? Could anyone help me?

Answer 1

If your MetadataStoreImpl extends a generated service stub (e.g., MetadataStoreImplBase) then it will implement gRPC's BindableService interface. ServerBuilder has another addService method that accepts a BindableService instead of a ServerServiceDefinition. Here is an example of our generated code's implementation of the bindService method, which just builds and returns a ServerServiceDefinition.

For your second question, you can get a list of ServerServiceDefinitions via Server#getServices, but this won't directly give you back your original MetadataStoreImpl, but rather the ServerServiceDefinition returned by the generated code's bindService method. Typically, you wouldn't use getServices, as you'd just retain a reference to your service after you added it to the builder.

You can find more information gRPC Java's generated code at https://grpc.io/docs/reference/java/generated-code.html.