Alpakka MongoDB：自定义实现MongoSource遇ObservableToPublisher私有类问题求助

Hey there, let's work through this issue you're facing—totally get why hitting a private internal class is frustrating when you're trying to roll your own MongoSource implementation. Here are a few solid ways to get around this:

1. Use Alpakka & RxJava's Public Conversion Helpers #

The key here is that you don't need to touch that private ObservableToPublisher at all. Alpakka and RxJava already have public tools to bridge MongoDB's Observable to Akka Streams' Source:

If you're using Scala, you can leverage Akka Streams' Source.fromPublisher alongside RxJava's built-in conversion from MongoDB's Observable to a Reactive Streams Publisher. Here's a quick example:

import akka.stream.scaladsl.Source
import com.mongodb.rx.client.Observable
import io.reactivex.rxjava3.core.Observable as RxObservable

// Your existing MongoDB query Observable
val mongoDataObservable: Observable[YourCustomDocument] = yourMongoCollection.find()

// Convert MongoDB's Observable to a Publisher using RxJava's public API
val reactivePublisher = RxObservable.from(mongoDataObservable).toPublisher()

// Wrap the Publisher into an Akka Streams Source
val finalSource = Source.fromPublisher(reactivePublisher)

Make sure you have the RxJava 3 dependency in your build (it's often included indirectly with Alpakka MongoDB, but double-check if needed).

2. Roll Your Own Minimal Observable-to-Publisher Adapter #

If you'd rather avoid adding extra RxJava dependencies, you can easily write a tiny adapter class that implements the Reactive Streams Publisher interface, wrapping MongoDB's Observable. This is basically what the private ObservableToPublisher does, but as your own public code:

import org.reactivestreams.{Publisher, Subscriber, Subscription}
import com.mongodb.rx.client.{Observable, Observer, Subscription => MongoSubscription}

class ObservableToPublisherAdapter[T](private val observable: Observable[T]) extends Publisher[T] {
  override def subscribe(subscriber: Subscriber[_ >: T]): Unit = {
    observable.subscribe(new Observer[T] {
      private var streamSubscription: Subscription = _

      override def onSubscribe(mongoSub: MongoSubscription): Unit = {
        streamSubscription = new Subscription {
          override def request(n: Long): Unit = mongoSub.request(n)
          override def cancel(): Unit = mongoSub.unsubscribe()
        }
        subscriber.onSubscribe(streamSubscription)
      }

      override def onNext(item: T): Unit = subscriber.onNext(item)
      override def onError(error: Throwable): Unit = subscriber.onError(error)
      override def onComplete(): Unit = subscriber.onComplete()
    })
  }
}

// Usage in your code
val mongoObservable: Observable[YourCustomDocument] = ...
val source = Source.fromPublisher(new ObservableToPublisherAdapter(mongoObservable))

This is lightweight, doesn't rely on internal Alpakka code, and works exactly like the private implementation you were trying to use.

3. Stick to Alpakka's Public MongoSource API (Avoid Rolling Your Own) #

Before going full custom, double-check if you can use Alpakka's built-in MongoSource directly. It's designed to handle typed documents as long as your MongoDB client has the right codecs registered. For example:

import akka.stream.alpakka.mongodb.scaladsl.MongoSource
import com.mongodb.rx.client.MongoCollection
import org.bson.codecs.configuration.CodecRegistries.{fromProviders, fromRegistries}
import org.bson.codecs.pojo.PojoCodecProvider

// Register codec for your custom type
val codecRegistry = fromRegistries(
  com.mongodb.MongoClient.getDefaultCodecRegistry,
  fromProviders(PojoCodecProvider.builder().automatic(true).build())
)

// Get a typed collection
val typedCollection: MongoCollection[YourCustomDocument] = 
  mongoDatabase.getCollection("your-collection", classOf[YourCustomDocument])
    .withCodecRegistry(codecRegistry)

// Use Alpakka's built-in MongoSource directly
val source = MongoSource(typedCollection.find())

This is the most maintainable approach because you're using the official, supported API—no need to reinvent the wheel, and you avoid future breakage if Alpakka changes its internal implementation.

Final Tip #

Avoid relying on private internal classes whenever possible—they're not part of the public contract, so they can change or disappear in any Alpakka update. Using the public APIs or your own adapter ensures your code stays stable.

内容的提问来源于stack exchange，提问作者Funny