Question

In Java, when using an object across multiple threads (and in general), it is good practice to make fields final. For example,

public class ShareMe {
    private final MyObject obj;
    public ShareMe(MyObject obj) {
        this.obj = obj;
    }
}

In this case, the visibility of obj will be consistent across multiple threads (let’s assume obj has all final fields as well) since it is safely constructed using the final keyword.

In scala, it doesn’t appear val compiles down to a final reference, but rather val is semantics in scala that prevents you from reassigning a variable (Scala final variables in constructor). If scala constructor variables are not defined as final, will they suffer from the same problem (when using these objects in actors)?

Answer 1

The answer to the other question is misleading. There are two meanings of the term final: a) for Scala fields/methods and Java methods it means “cannot be overridden in a subclass” and b) for Java fields and in JVM bytecode it means “the field must be initialized in the constructor and cannot be reassigned”.

Class parameters marked with val (or, equivalently, case class parameters without a modifier) are indeed final in second sense, and hence thread safe.

Here’s proof:

scala>  class A(val a: Any); class B(final val b: Any); class C(var c: Any)
defined class A
defined class B
defined class C

scala> import java.lang.reflect._
import java.lang.reflect._

scala> def isFinal(cls: Class[_], fieldName: String) = {
     |   val f = cls.getDeclaredFields.find(_.getName == fieldName).get
     |   val mods = f.getModifiers
     |   Modifier.isFinal(mods)
     | }
isFinal: (cls: Class[_], fieldName: String)Boolean

scala> isFinal(classOf[A], "a")
res32: Boolean = true

scala> isFinal(classOf[B], "b")
res33: Boolean = true

scala> isFinal(classOf[C], "c")
res34: Boolean = false

Or with javap, which can be conveniently run from the REPL:

scala> class A(val a: Any)
defined class A

scala> :javap -private A
Compiled from "<console>"
public class A extends java.lang.Object implements scala.ScalaObject{
    private final java.lang.Object a;
    public java.lang.Object a();
    public A(java.lang.Object);
}