How to call a function from a java library

Question
1

Can someone help me figure out what I need to do to call the function getCollectionCount() from this java library?

What do I need to import, what do I need to add to my build.sbt etc?

2

This is what I’ve tried, but it doesn’t work.

import java.lang.management._
val beans: Array[GarbageCollectorMXBean] = ManagementFactory.getGarbageCollectorMXBeans().toArray

val gcCount0 = beans(0).getCollectionCount()
val gcTime0 = beans(0).getCollectionTime()
println(s"gcCount0=$gcCount0   gcTime0=$gcTime0")

Here is the message I get from IntelliJ
Screenshot 2020-03-05 at 09.06.57

And I get the following compiler error. Which I don’t understand. Do I really need to use a wildcard such as _ >: java.lang.management.GarbageCollectorMXBean from SLS 3.2.10

My feeling is that this 3.2.10 error is a red herring, that I’m simply not accessing the getCollectionCount and getCollectionTime methods correctly.

Error:(198, 99) type mismatch;
 found   : Array[Object]
 required: Array[java.lang.management.GarbageCollectorMXBean]
Note: Object >: java.lang.management.GarbageCollectorMXBean, but class Array is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: java.lang.management.GarbageCollectorMXBean`. (SLS 3.2.10)
        val beans: Array[GarbageCollectorMXBean] = ManagementFactory.getGarbageCollectorMXBeans().toArray
3

Your problem is actually with the java.util.List API. Its toArray method returns a Array[AnyRef].

You can use this other toArray method, which I find quite ridiculous to be honest:

ManagementFactory
  .getGarbageCollectorMXBeans()
  .toArray(Array.empty[GarbageCollectorMXBean])

Or you can move to the Scala universe where toArray does what one would expect:

import scala.jdk.CollectionConverters._

ManagementFactory
  .getGarbageCollectorMXBeans()
  .asScala
  .toArray
4

thanks. Is scala.jdk correct or a typo? My system doesn’t find it. Maybe I need to update my build.sbt file?

5

In Scala 2.13 that should work.
In older versions it’s scala.collection.JavaConverters.

6

So, here is the code I’m running. It seems to work, although I’m still analyzing the results.
As I’m not sure how many beans I will have (perhaps just 1), I’m folding over the results to add up all of them. That’s probably fine for the gc-count, but perhaps dubious for the time. Perhaps time should be maximized rather than summed???

import java.lang.management._

// Thanks Jasper M for the following recipe.
// https://users.scala-lang.org/u/jasper-m
// https://users.scala-lang.org/t/how-to-call-a-function-from-a-java-library/5722/2
val beans: Array[GarbageCollectorMXBean] = ManagementFactory
       .getGarbageCollectorMXBeans()
       .toArray(Array.empty[GarbageCollectorMXBean])

def gcCount(): Long = beans.foldLeft(0L) { (acc, b) => b.getCollectionCount() + acc }

def gcTime(): Long = beans.foldLeft(0L) { (acc, b) => b.getCollectionTime() + acc }

val gcCount0 = gcCount().toDouble
val gcTime0 = gcTime().toDouble
val (colorization, bdd) = graphToBdd(List(start), uniGraph, biGraph, numNodes,
                                     (n: Double, size: () => Double) => {
                                       newGcCount(n, gcCount() - gcCount0)
                                       newGcTime(n, gcTime() - gcTime0)
                                       newSize(n, size())
                                       val (hashSize, numAllocations) = Bdd.getBddSizeCount()
                                       newHashSize(n, hashSize.toDouble)
                                       newNumAllocations(n, numAllocations.toDouble)
                                     },
                                     differentColor,
                                     fold = fold)
7

I have 2. Probably one is for young generation and another one is for old generation. For information about objects’ generations look at Tracing garbage collection - Wikipedia

Time should be summed if you want total time. In most GCs young and old GC cycles don’t overlap.

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