Creating a DB factory using kotlin

Question

So I'm trying to create a MongoDB factory with kotlin... but I think I don't really understand the concept of companion object very well because I can't even get this to compile:

package org.jgmanzano.storage

import com.mongodb.MongoClient
import com.mongodb.MongoClientURI
import com.mongodb.client.MongoDatabase

class MongoConnectionFactory(private val connectionURI: String) {
    private var database: MongoDatabase

    init {
        val connectionString = MongoClientURI(connectionURI)
        val mongoClient = MongoClient(connectionString)
        database = mongoClient.getDatabase("paybotDB")
    }

    companion object {
        fun getDatabase() : MongoDatabase {
            return database
        }
    }
}

How would you guys achieve this? My idea is to create what in Java would be a kind of factory method. I can't seem to get the syntax right tho.

Furthermore, would this be a correct approach to DB connection factories?

Answer 1

Move everything to the companion object, pass the connection URI to the getDatabase method. Companion objects get compiled as a static field inside the containing (outer class). Since the field is static, it cannot access outer class's fields because the outer class is an instance.

I assume you want to cache database objects.

class MongoConnectionFactory() {

    companion object {
        private var database: MongoDatabae? = null

        fun getDatabase(connectionURI: String) : MongoDatabase {
            if (database != null) {
                return database
            {
            val connectionString = MongoClientURI(connectionURI)
            val mongoClient = MongoClient(connectionString)
            database = mongoClient.getDatabase("paybotDB")
            return database
        }
    }
}

But then you don't need a companion object nested inside containing class. You can create an object instead.

object MongoConnectionFactory {
    private var database: MongoDatabae? = null

    fun getDatabase(connectionURI: String) : MongoDatabase {
        if (database != null) {
            return database
        {
        val connectionString = MongoClientURI(connectionURI)
        val mongoClient = MongoClient(connectionString)
        database = mongoClient.getDatabase("paybotDB")
        return database
    }
}

If you need multiple databases with different connection URIs then store them inside the hash table.