jol

readme.md

• https://github.com/openjdk/jol

build.sbt

scalaVersion := "3.3.0"

libraryDependencies += "org.openjdk.jol" % "jol-core" % "0.17"

repl

Welcome to Scala 3.3.0 (11.0.19, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                                                                                                                                                   
scala> org.openjdk.jol.info.ClassLayout.parseClass(classOf[::[?]]).toPrintable
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
# WARNING: Unable to attach Serviceability Agent. Unable to attach even with module exceptions: [org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed.]
val res0: String = "scala.collection.immutable.$colon$colon object internals:
OFF  SZ                              TYPE DESCRIPTION               VALUE
  0   8                                   (object header: mark)     N/A
  8   4                                   (object header: class)    N/A
 12   4                  java.lang.Object $colon$colon.head         N/A
 16   4   scala.collection.immutable.List $colon$colon.next         N/A
 20   4                                   (object alignment gap)    
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
"
                                                                                                                                                                                                                                                                   
scala> org.openjdk.jol.info.ClassLayout.parseClass(classOf[List[?]]).toPrintable
val res1: String = "scala.collection.immutable.List object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     N/A
  8   4        (object header: class)    N/A
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
"
                                                                                                                                                                                                                                                                   
scala> org.openjdk.jol.info.ClassLayout.parseClass(classOf[LazyList[?]]).toPrintable
val res2: String = "scala.collection.immutable.LazyList object internals:
OFF  SZ                                        TYPE DESCRIPTION                                                    VALUE
  0   8                                             (object header: mark)                                          N/A
  8   4                                             (object header: class)                                         N/A
 12   1                                     boolean LazyList.scala$collection$immutable$LazyList$$stateEvaluated   N/A
 13   1                                     boolean LazyList.midEvaluation                                         N/A
 14   1                                     boolean LazyList.bitmap$0                                              N/A
 15   1                                             (alignment/padding gap)                                        
 16   4   scala.collection.immutable.LazyList.State LazyList.scala$collection$immutable$LazyList$$state            N/A
 20   4                             scala.Function0 LazyList.lazyState                                             N/A
Instance size: 24 bytes
Space losses: 1 bytes internal + 0 bytes external = 1 bytes total
"