Clear all Firebase Cloud Firestore Collections

ClearCollections.java

import com.google.api.core.ApiFuture;
import com.google.auth.oauth2.GoogleCredentials;
import com.google.cloud.firestore.CollectionReference;
import com.google.cloud.firestore.Firestore;
import com.google.cloud.firestore.QueryDocumentSnapshot;
import com.google.cloud.firestore.QuerySnapshot;
import com.google.firebase.FirebaseApp;
import com.google.firebase.FirebaseOptions;
import com.google.firebase.cloud.FirestoreClient;

import java.io.IOException;
import java.util.List;

class ClearCollections {
  public static void main(String[] args) throws IOException {
    final int BATCH_SIZE = 5;

    Firestore firestore = initializeCloudFirestore();

    for (CollectionReference listCollection : firestore.listCollections()) {
      deleteCollection(listCollection, BATCH_SIZE);
    }
  }

  /**
   * One way of initializing Firestore,
   * see other options at https://firebase.google.com/docs/firestore/quickstart#initialize
   */
  private static Firestore initializeCloudFirestore() throws IOException {
    // Use the application default credentials
    GoogleCredentials credentials = GoogleCredentials.getApplicationDefault();
    FirebaseOptions options = new FirebaseOptions.Builder()
        .setCredentials(credentials)
        .setProjectId("projectId")
        .build();
    FirebaseApp.initializeApp(options);

    Firestore firestore = FirestoreClient.getFirestore();
    return firestore;
  }

  /**
   * Delete a collection in batches to avoid out-of-memory errors. Batch size may be tuned based on
   * document size (atmost 1MB) and application requirements.
   * See https://firebase.google.com/docs/firestore/manage-data/delete-data#java_5
   */
  static void deleteCollection(CollectionReference collection, int batchSize) {
    try {
      // retrieve a small batch of documents to avoid out-of-memory errors
      ApiFuture<QuerySnapshot> future = collection.limit(batchSize).get();
      int deleted = 0;
      // future.get() blocks on document retrieval
      List<QueryDocumentSnapshot> documents = future.get().getDocuments();
      for (QueryDocumentSnapshot document : documents) {
        document.getReference().delete();
        ++deleted;
      }
      if (deleted >= batchSize) {
        // retrieve and delete another batch
        deleteCollection(collection, batchSize);
      }
    } catch (Exception e) {
      System.err.println("Error deleting collection : " + e.getMessage());
    }
  }
}

In response to https://stackoverflow.com/questions/65355410/how-to-reset-firestore-emulator-in-integration-test