A Issue with Using db4o in Scala Programs

In the other article I showed that we can use db4o in Scala programs. I also mentioned that there are some limitations: db4o cannot correctly store Scala objects with fields of the type scala.collection.mutable.Map and scala.collection.mutable.Set. This article discuss this issue in detail.

The Scala class Programmer (Listing-1) has a field named skillMap whose type is scalca.collection.mutable.Map. The map is used to record the programmer's skills and respective skill levels.

Listing-1 

package tutorial.db4o

class Programmer (val name: String, var phone: String) {
    private val skillMap = scala.collection.mutable.Map[String, Int]()
    
    def skills = skillMap.keySet
    
    def hasSkill(skill : String) = skills.contains(skill)
    
    def skillLevel(skill: String) : Option[Int] = {
        if (hasSkill(skill)) Some(skillMap(skill))
        else None
    }
    
    def addSkill(skill: String, level: Int) {skillMap(skill) = level}
}

In the program in Listing-2, we create an instance of the Programmer class and store it in the db4o database. In the program in Listin-3, we retrieve the object stored in db4o, and print out the programer's name, phone number, skills and corresponding levels. However, the execution runs into an exception.

Listing-2 - A Program to store an object into db4o

package tutorial.db4o

import com.db4o.Db4oEmbedded
import Adapter._

object AddProgrammers extends App {
    // a connection to the tutorial (db4o) database
    val connection = Db4oEmbedded.openFile("tutorial.db4o")
    
    // create a Programmer instance and store it in the database
    
    val steve = new Programmer("Steve", "513-206-3276")
    steve.addSkill("Java", 6)
    steve.addSkill("Perl", 3)
    connection.store(steve)
    
    connection.commit()
    
    connection.close()
}

Listing-3 - A Program to retrieve an object from db4o

package tutorial.db4o

import com.db4o.Db4oEmbedded
import Adapter._
import com.db4o.query.Predicate

object ReadProgrammers extends App {
    // a connection to the tutorial (db4o) database
    val connection = Db4oEmbedded.openFile("tutorial.db4o")
    
    // Retrieve programmers with the name Steve from the database
    val rs = connection.query{programmer: Programmer => programmer.name == "Steve"}
        
    // Print out each programmer's name, phone number, and skills
    rs.foreach{programmer =>
        println("Programmer name: %s, phone number: %s, skills:".format(programmer.name, programmer.phone))
        programmer.skills.foreach{skill => println("    %s at level %d".format(skill, programmer.skillLevel(skill).get))}
    }
    
    connection.close()
}

Output of execution of the program in Listing-3:

Programmer name: Steve, phone number: 513-206-3276, skills: Exception in thread "main" java.lang.NullPointerException at scala.collection.mutable.HashTable$$anon$1.(HashTable.scala:159) at scala.collection.mutable.HashTable$class.entriesIterator(HashTable.scala:157) at scala.collection.mutable.HashMap.entriesIterator(HashMap.scala:45) at scala.collection.mutable.HashTable$class.foreachEntry(HashTable.scala:190) at scala.collection.mutable.HashMap.foreachEntry(HashMap.scala:45) at scala.collection.mutable.HashMap$$anon$1.foreach(HashMap.scala:99) at tutorial.db4o.ReadProgrammers$$anonfun$2.apply(ReadProgrammers.scala:17) at tutorial.db4o.ReadProgrammers$$anonfun$2.apply(ReadProgrammers.scala:15) at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59) at scala.collection.immutable.List.foreach(List.scala:76) at tutorial.db4o.ReadProgrammers$delayedInit$body.apply(ReadProgrammers.scala:15) at scala.Function0$class.apply$mcV$sp(Function0.scala:34) at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12) at scala.App$$anonfun$main$1.apply(App.scala:60) at scala.App$$anonfun$main$1.apply(App.scala:60) at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59) at scala.collection.immutable.List.foreach(List.scala:76) at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:30) at scala.App$class.main(App.scala:60) at tutorial.db4o.ReadProgrammers$.main(ReadProgrammers.scala:7) at tutorial.db4o.ReadProgrammers.main(ReadProgrammers.scala)

Clearly, the Programmer object was not stored correctly in db4o. If we replace the Scala Map by a Java Map (e.g. java.util.HashMap), the programs will work correctly. It is however a awkward work-around to use Java map instead of Scala map in a Scala program.