Large Scala Maps Fail Serialization

Jan 09, 2013

I spent some time dealing with a rather unusual error. Took me a little while to figure it out, but it turns out that large Maps in Scala are not serializable. The exception does not occur on serialization, though; it happens when you try to deserialize the map later.

java.lang.ArrayIndexOutOfBoundsException: -7119
at scala.collection.mutable.HashTable$class.addEntry(HashTable.scala:119)
at scala.collection.mutable.HashMap.addEntry(HashMap.scala:43)
at scala.collection.mutable.HashTable$class.init(HashTable.scala:81)
at scala.collection.mutable.HashMap.init(HashMap.scala:43)
at scala.collection.mutable.HashMap.readObject(HashMap.scala:130)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:969)

The number in the exception changes depending on the exact data, but it always looks roughly similar. I made some simple tests, and with a basic Map[String,Int] with short strings, the exceptions occur around 2.2 million elements. Everything works fine with Java maps, so I was able to work around the problem by stuffing everything into a java.util.HashMap instead.

Here’s a test case demonstrating the problem:

import collection.mutable
import java.io.{ObjectInputStream, ByteArrayInputStream, ByteArrayOutputStream, ObjectOutputStream}
import java.util
import org.scalatest.FunSuite
import org.scalatest.matchers.ShouldMatchers

class RandomTests extends FunSuite with ShouldMatchers {

  val m = new mutable.HashMap[String, Int]()
  val l = 2.5e6.toInt
  (0 to l).foreach {
    idx =>
      m(idx.toString) = idx

  test("mutable HashMap") {

  test("immutable HashMap") {
    testSerialization(collection.immutable.HashMap(m.toSeq: _*))

  test("java map") {
    val jmap = new util.HashMap[String, Int]()
    m.foreach{case(k,v) =>
    val deser = serializeDeserialize(jmap)
    deser.size() should be(m.size)
    (0 to l).foreach {
      idx =>
        if (deser.get(idx.toString) != idx) {
          fail("check failed for " + idx)

  def serializeDeserialize[A](a: A) = {
    val bytes = new ByteArrayOutputStream()
    val out = new ObjectOutputStream(bytes)

    val buf = bytes.toByteArray
    val in = new ObjectInputStream(new ByteArrayInputStream(buf))

  def testSerialization(m: collection.Map[String,Int]) {
    val deser = serializeDeserialize(m)
    deser.size should be(m.size)
    (0 to l).foreach {
      idx =>
        if (deser.get(idx.toString) != idx) {
          fail("check failed for " + idx)


The “java map” test passes, and the others fail.

I observed the problem on Scala 2.9.1