Trivial way to inspect OpenJDK's Java Object Layout of your Scala classes.
Get the latest via:
// project/plugins.sbt
addSbtPlugin("pl.project13.sbt" % "sbt-jol" % pluginVersionHere)
Which allows you to (note auto-completion works nicely):
> jol:internals example.Entry
[info] # Running 64-bit HotSpot VM.
[info] # Using compressed oop with 3-bit shift.
[info] # Using compressed klass with 3-bit shift.
[info] # WARNING | Compressed references base/shifts are guessed by the experiment!
[info] # WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
[info] # WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
[info] # Objects are 8 bytes aligned.
[info] # Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
[info] # Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
[info]
[info] VM fails to invoke the default constructor, falling back to class-only introspection.
[info]
[info] example.Entry object internals:
[info] OFFSET SIZE TYPE DESCRIPTION VALUE
[info] 0 12 (object header) N/A
[info] 12 4 int Entry.value N/A
[info] 16 4 String Entry.key N/A
[info] 20 4 (loss due to the next object alignment)
[info] Instance size: 24 bytes
[info] Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
Currently uses jol 0.5.
Please note that this plugin is mostly developed on a "on demand" basis by and for myself, contributions are very (!) welcome since I most likely will not focus much on it unless I need more features (and for now estimates and internal are all I needed).
Apache v2