scala serialization always throw exception, why?

Question

I've this small scala code snipet:

@SerialVersionUID(43L) class p(val a:Int=14,val b:Double=3.0) extends Serializable{
}
import java.io._

val out = new ObjectOutputStream(new FileOutputStream("my.obj"))
out.writeObject(new p(5,7))
out.close()

val in = new ObjectInputStream(new FileInputStream("my.obj"))
val savedPerson=in.readObject.asInstanceOf[p]
println(savedPerson.a)

When I run it on windows or mac, it prints out a huge exception information:

java.io.NotSerializableException: Main$$anon$1
    at java.base/java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.base/java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
    at java.base/java.io.ObjectOutputStream.writeSerialData(Unknown Source)
    at java.base/java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
    at java.base/java.io.ObjectOutputStream.writeObject0(Unknown Source)
    at java.base/java.io.ObjectOutputStream.writeObject(Unknown Source)
    at Main$$anon$1.errorFunction(ch09.scala:52)
    at Main$$anon$1.<init>(ch09.scala:110)
    at Main$.main(ch09.scala:1)
    at Main.main(ch09.scala)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.base/java.lang.reflect.Method.invoke(Unknown Source)
    at scala.reflect.internal.util.ScalaClassLoader.$anonfun$run$2(ScalaClassLoader.scala:98)
    at scala.reflect.internal.util.ScalaClassLoader.asContext(ScalaClassLoader.scala:32)
    at scala.reflect.internal.util.ScalaClassLoader.asContext$(ScalaClassLoader.scala:30)
    at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:129)
    at scala.reflect.internal.util.ScalaClassLoader.run(ScalaClassLoader.scala:98)
    at scala.reflect.internal.util.ScalaClassLoader.run$(ScalaClassLoader.scala:90)
    at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:129)
    at scala.tools.nsc.CommonRunner.run(ObjectRunner.scala:22)
    at scala.tools.nsc.CommonRunner.run$(ObjectRunner.scala:21)
    at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:39)
    at scala.tools.nsc.CommonRunner.runAndCatch(ObjectRunner.scala:29)
    at scala.tools.nsc.CommonRunner.runAndCatch$(ObjectRunner.scala:28)
    at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:39)
    at scala.tools.nsc.ScriptRunner.runCompiled(ScriptRunner.scala:170)
    at scala.tools.nsc.ScriptRunner.$anonfun$runScript$1(ScriptRunner.scala:187)
    at scala.tools.nsc.ScriptRunner.$anonfun$runScript$1$adapted(ScriptRunner.scala:187)
    at scala.tools.nsc.ScriptRunner.$anonfun$withCompiledScript$2(ScriptRunner.scala:156)
    at scala.tools.nsc.ScriptRunner.runScript(ScriptRunner.scala:124)
    at scala.tools.nsc.ScriptRunner.runScriptAndCatch(ScriptRunner.scala:200)
    at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:70)
    at scala.tools.nsc.MainGenericRunner.run$1(MainGenericRunner.scala:85)
    at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:96)
    at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:101)
    at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

What error is this? How can I fix it? I don't quite get this. Thanks!

It works for me. I've checked it for Scala 2.11.7 and Scala 2.12.6. Try to recompile classes. If you use sbt, do clean and compile. — Duelist
works for me too. make sure you are not reading an unexpected file — pedrorijo91

som-snytt som-snytt · Accepted Answer · 2018-06-10T21:30:10

I see from your stack trace that you're using the script runner:

scala -nc noser.scala

where I've named the script as in Yessir, no sir, because there is no serialization happening.

-nc avoids starting the compile server daemon.

If the script runner doesn't see an object with a main method, it packages your script code in a class that is local to a main method it constructs for you.

def main(args: Array[String]) = { class anon_class { code } ; new anon_class() }

Your code is run in the constructor, which isn't a good idea either.

You can see this with -Xprint:typer,flatten. Or experiment with -Xprint:all.

Your serializable class has an outer pointer to the anonymous local class, and the anonymous class is not serializable.

Maybe the outer pointer is just a bug. Or not, see the reference below. It needs a hint to eliminate the outer pointer, such as making the class final.

I see there have been similar bugs such as this one where the enclosing module is captured by a lambda.

Your workaround is to put your code in the main method of an object, or wrap everything in App.

object Main extends App {
  // code
}

Another workaround is to make your serializable class final. There is discussion on this ticket and the related ticket and fix.

scala serialization always throw exception, why?

1 Answers