How does serialization of lazy fields work? - scala

Question

I know the benefits of lazy fields when a postponed evaluation of values is needed for some reasons. I was wondering what was the behavior of lazy fields in terms of serialization.
Consider the following class.
class MyClass {
lazy val myLazyVal = {...}
...
}
Questions:
If an instance of MyClass is serialized, does the lazy field get serialized too ?
Does the behavior of serialization change if the field has been accessed or not before the serialization ? I mean, if I don't cause the evaluation of the field, is it considered as null ?
Does the serialization mechanism provoke an implicit evaluation of the lazy field ?
Is there a simple way to avoid the serialization of the variable and getting the value recomputed one more time lazily after the deserialization ? This should happen independently from the evaluation of the field.

Answer 1

Answers
Yes if field was already initialized, if not you can tread it as a method. Value is not computed -> not serialized, but available after de serialization.
If you didn't touch field it's serialized almost as it's a simple 'def' method, you don't need it's type to be serializable itself, it will be recalculated after de-serialization
No
You can add #transient before lazy val definition in my code example, as I understand it will do exactly what you want
Code to prove
object LazySerializationTest extends App {
def serialize(obj: Any): Array[Byte] = {
val bytes = new ByteArrayOutputStream()
val out = new ObjectOutputStream(bytes)
out.writeObject(obj)
out.close()
bytes.toByteArray
}
def deSerialise(bytes: Array[Byte]): MyClass = {
new ObjectInputStream(new ByteArrayInputStream(bytes)).
readObject().asInstanceOf[MyClass]
}
def test(obj: MyClass): Unit = {
val bytes = serialize(obj)
val fromBytes = deSerialise(bytes)
println(s"Original cnt = ${obj.x.cnt}")
println(s"De Serialized cnt = ${fromBytes.x.cnt}")
}
object X {
val cnt = new AtomicInteger()
}
class X {
// Not Serializable
val cnt = X.cnt.incrementAndGet
println(s"Create instance of X #$cnt")
}
class MyClass extends Serializable {
lazy val x = new X
}
// Not initialized
val mc1 = new MyClass
test(mc1)
// Force lazy evaluation
val mc2 = new MyClass
mc2.x
test(mc2) // Failed with NotSerializableException
}