GraalVM Native Image Requires Config for lazy vals

Compiler version

3.1.0

Background

GraalVM Native Image Ahead-of-Time compiles JVM apps into native executables. To do the AOT compilation, reflection points must be specified.

Issue 1

Scala’s scala.runtime.LazyVals uses reflection so it must be declared. This could be done in the Scala stdlib so that others don’t need to do it themselves. But this approach is a bit brittle. Any chance of removing the use of sun.misc.Unsafe there? If not, then I can send a PR to include the GraalVM reflection config:

  {
    "name":"sun.misc.Unsafe",
    "allDeclaredFields":true
  }

Issue 2

Code generated by the compiler for lazy vals also uses reflection and must be declared, like:

  {
    "name":"App$package$",
    "fields":[{"name":"0bitmap$1"}]
  }

Also, ideally reflection is not required. But I haven’t looked into what the reflection is doing. If someone uses a Scala library that uses lazy vals, and does NOT provide their reflection config for GraalVM, then the user must specify the config for the library on their own. Ideally the library authors nor the users need to specify this info.

Steps to Reproduce

1. Clone https://github.com/jamesward/scala3-graalvm-tester.git
2. Switch to the lazy-reflect-broke branch
3. Run ./sbt nativeImage nativeImageRun

You will see a runtime error showing Issue 1:

Exception in thread "main" scala.runtime.LazyVals$$anon$1
	at scala.runtime.LazyVals$.$init$$$anonfun$3(LazyVals.scala:18)
	at scala.Option.getOrElse(Option.scala:201)
	at scala.runtime.LazyVals$.<clinit>(LazyVals.scala:19)
	at App$package$.<clinit>(App.scala:2)
	at run.main(App.scala:4)

(Note: It should be possible to fail the build in this case instead of producing a runtime error but there seems to be a bug: https://github.com/oracle/graal/issues/4010)

If you switch to the lazy-reflect-works branch and re-run ./sbt nativeImage nativeImageRun you will see that it works given the requisite config for the app in that repo that has one lazy val.

One Approach (If Reflection Must Stay)

The Spring team has developed a set of annotations which provide the reflection info, like: https://docs.spring.io/spring-native/docs/0.9.1-SNAPSHOT/api/org/springframework/nativex/hint/TypeHint.html

There has been talk that this work will merge in some way into GraalVM itself. In that case, the Scala compiler could generate the necessary TypeHint annotations for places it generates code that uses reflection.

[jamesward]

I bumped the https://github.com/jamesward/scala3-graalvm-tester.git branch lazy-reflect-broke to 3.2.0-RC3 and still have the issue.

[carlosedp]

An FYI, scala-cli does one of the options mentioned by @szymon-rd. It manipulates the bytecode and patches the jars to “fix” lazyvals on current libraries.

https://github.com/VirtusLab/scala-cli/tree/main/modules/scala3-graal/src/main/scala/scala/cli/graal