How to recognize boxing/unboxing in a decompiled Scala code?

Question

In the accepted best response to this question, there is a clear explanation why boxing happens.

However, if I decompile the code (using java decompiler) I cannot see use of scala.runtime.BoxesRunTime. Furthermore, if I profile the code (using JProfiler) I cannot see any instances of BoxesRunTime.

So, how do I really see a proof of boxing/unboxing taking place?

Answer 1

In this code:

class Foo[T] {
  def bar(i: T) = i
}


object Main {
  def main(args: Array[String]) {
    val f = new Foo[Int]
    f.bar(5)
  }
}

The invocation of bar should first box the integer. Compiling with Scala 2.8.1 and using:

javap -c -l -private -verbose -classpath <dir> Main$

to see the bytecode produced for the main method of the Main class yields:

public void main(java.lang.String[]);                                                      
...                                                                                   
   9:   iconst_5                                                                                          
   10:  invokestatic    #24; //Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;      
   13:  invokevirtual   #28; //Method Foo.bar:(Ljava/lang/Object;)Ljava/lang/Object;        
   16:  pop                                                                                            
   17:  return                                                                                     
...

You can see the call to BoxesRunTime before the call to bar.

BoxesRunTime is an object which contains boxing methods for primitive types, so there should be exactly one instance in total. The trick here is that this particular file in the library was written in Java, and the conversions are static methods. For this reason there aren't any instances of it at runtime, although using it in Scala code feels as if it were an object.

You should probably look for boxed primitives (e.g. java.lang.Integer) with JProfile, though I am uncertain how the JVM works and whether it may actually rewrite the code at runtime and optimize it to avoid boxing. To my knowledge, it shouldn't apply specialization (but I believe CLR does). A few microbenchmarks with and without the boxing situation are another way to figure out what happens at runtime.

EDIT:

The above is assuming that a type parameter wasn't annotated with the @specialized annotation. In this case, the boxing/unboxing can be avoided. Certain classes in the standard library are specialized. See this sid.

Answer 2

Given the following Test.scala program:

object Test {
  def main(args:Array[String]) {
    val list = List(1,5,15)
    val res = list.map(e => e*2).filter(e => e>10)
  }
}

If I compile with scalac -Xprint:jvm Test.scala, I get this snippet suggesting that specialization occurs (sorry for wide paste):

package <empty> {
  final class Test extends java.lang.Object with ScalaObject {
    def main(args: Array[java.lang.String]): Unit = {
      val list: List = immutable.this.List.apply(scala.this.Predef.wrapIntArray(Array[Int]{1, 5, 15}));
      val res: List = list.map({
        (new Test$$anonfun$1(): Function1)
      }, immutable.this.List.canBuildFrom()).$asInstanceOf[scala.collection.TraversableLike]().filter({
        (new Test$$anonfun$2(): Function1)
      }).$asInstanceOf[List]();
      ()
    };
    def this(): object Test = {
      Test.super.this();
      ()
    }
  };
  @SerialVersionUID(0) @serializable final <synthetic> class Test$$anonfun$1 extends scala.runtime.AbstractFunction1$mcII$sp {
    final def apply(e: Int): Int = Test$$anonfun$1.this.apply$mcII$sp(e);
    <specialized> def apply$mcII$sp(v1: Int): Int = v1.*(2);
    final <bridge> def apply(v1: java.lang.Object): java.lang.Object = scala.Int.box(Test$$anonfun$1.this.apply(scala.Int.unbox(v1)));
    def this(): Test$$anonfun$1 = {
      Test$$anonfun$1.super.this();
      ()
    }
  };
  @SerialVersionUID(0) @serializable final <synthetic> class Test$$anonfun$2 extends scala.runtime.AbstractFunction1$mcZI$sp {
    final def apply(e: Int): Boolean = Test$$anonfun$2.this.apply$mcZI$sp(e);
    <specialized> def apply$mcZI$sp(v1: Int): Boolean = v1.>(10);
    final <bridge> def apply(v1: java.lang.Object): java.lang.Object = scala.Boolean.box(Test$$anonfun$2.this.apply(scala.Int.unbox(v1)));
    def this(): Test$$anonfun$2 = {
      Test$$anonfun$2.super.this();
      ()
    }
  }
}

Could be why you don't see any evidence of boxing in bytecode...