Question

Is it safe by default, like Java’s single-element-enum pattern or is it e. g. necessary to define readResolve or similar methods somewhere to prevent accidental or malicious breakage of the singleton contract?

Answer 1

Yes, it is safe by default:

object Singleton extends Serializable
// with Scala 2.8: @serializable object Singleton

import java.io._

val out = new ObjectOutputStream(new FileOutputStream("singleton"))
out.writeObject(Singleton)
out.close

val in = new ObjectInputStream(new FileInputStream("singleton"))
val obj = in.readObject
in.close

obj == Singleton // should print true

When you compile the object with scalac and decompile it (for example with JAD) you will get following Java-file:

public final class Singleton$ implements Serializable, ScalaObject
{
    public Object readResolve()
    {
        return MODULE$;
    }

    private Singleton$()
    {
    }

    public static final Singleton$ MODULE$ = new Singleton$1();
}