How do I use a @serializable scala object?

Question

I know that you can mark a scala object as @serializable, but I don't understand what to do with the object afterwards. Do I simply treat it as a Java Serializable object?

I want to serialize the object into a stream of bytes. Can someone show me the code to transform a serialize object into either a byte array or a string?

(the googles have not been helpful with this question)

FOLLOWUP: Thanks. I now understand that I can use it like a Java Serializable object. Sometimes the obvious answer escapes me.

Answer 1

To answer your first question: yes you can treat it as a Java Serializable object:

scala> @serializable object A
defined module A

scala> import java.io.ByteArrayOutputStream;
import java.io.ByteArrayOutputStream

scala> import java.io.ObjectOutputStream;
import java.io.ObjectOutputStream

scala> val baos = new ByteArrayOutputStream(1024)
baos: java.io.ByteArrayOutputStream = 

scala> val o = new ObjectOutputStream(baos)
o: java.io.ObjectOutputStream = java.io.ObjectOutputStream@3d689405

scala> o.writeObject(A)

scala> baos.toByteArray
res4: Array[Byte] = Array(-84, -19, 0, 5, 115, 114, 0, 24, 108, 105, 110, 101, 49, 51, 36, 111, 98, 106, 101, 99, 116, 36, 36, 105, 119, 36, 36, 105, 119, 36, 65, 36, 110, -104, -28, -53, -123, -97, -118, -36, 2, 0, 0, 120, 112)

scala> object B
defined module B

scala> o.writeObject(B)
java.io.NotSerializableException: B$
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
    at .(:13)
    at .()
    at RequestResult$.(:9)
    at RequestResult$.()
    at RequestResult$scala_repl_result()
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$18.apply(Interpreter.scala:981)
    at scala.tools.nsc.Interpreter$Request$...

If you want to serialize to some string format, perhaps this library may be useful, which serializes scala objects into JSON.

Answer 2

In general, I wouldn't recommend serializing Objects, however it could be a way to send your "global state" across the net.

As for how to send/receive data check out java.io.ObjectOutputStream and java.io.ObjectInputStream.

scala> trait Test { def x : Int }
defined trait Test

scala> @serializable object Foo { var x = 5 }
defined module Foo


scala> import java.io._                                                     
import java.io._

scala> def write() {
     | val output = new ObjectOutputStream(new FileOutputStream("test.obj"))
     | output.writeObject(Foo)
     | output.close()
     | }
write: ()Unit


scala> write()

scala> def read() = {
     | val input = new ObjectInputStream(new FileInputStream("test.obj"))
     | val obj = input.readObject()
     | input.close()
     | obj
     | }
read: ()java.lang.Object

scala> Foo.x = 7

scala> val r = read()
r: Test = Foo$@2855e552

scala> r.x
res39: Int = 7

You see... top level objects don't really work well with serialization. However a nested object could be serialized. For example:

scala> @serializable
     | class SomeClass(var y : Int) {
     |   @serializable object X extends Test {  def x  = y }
     | }
defined class SomeClass


scala> def write(x : AnyRef) {
     | val output = new ObjectOutputStream(new FileOutputStream("test.obj"))
     | output.writeObject(x)
     | output.close()
     | }
write: (x : AnyRef)Unit

scala> def read[A] = {
     | val input = new ObjectInputStream(new FileInputStream("test.obj"))
     | val obj = input.readObject()
     | input.close()
     | obj.asInstanceOf[A]
     | }
read: [A]A

scala> write(x.X)

scala> val y = read[Test]
y: Test = SomeClass$X$@58e39f23

scala> y.x
res51: Int = 10

scala> x.y = 20  

scala> x.X.x
res52: Int = 20

scala> y.x
res53: Int = 10

Hope that helps!