[native-image] running play on GraalVM/SubstrateVM

[schmitch]

Hello,
I wanted to share my knowledge of running Play on GraalVM and wanted to point out some "issues" along the way.

Some error messages I couldn't really resolve are these:

• Error: Must not have a FileDescriptor in the image heap.
• Error: Must not have a started Thread in the image heap.

See: https://gist.github.com/schmitch/469eae4062373b6fe67def99ec220103
(I also created a repository to reproduce them: https://github.com/schmitch/play-scala-starter-example)

After that I tried to create a Play application in the "embedded" mode which is basically described here: https://www.playframework.com/documentation/2.6.x/ScalaEmbeddingPlayAkkaHttp
The good thing is, the issue's above aren't raised, however I had a problem because of the issues I described here: https://discuss.lightbend.com/t/akka-and-graal-s-native-image-tool/940/9

Currently I guess after fixing these "issues" there is still some work for play because in some cases we need proxy's or reflection where we might not need to. But a simple application could probably be made to run under substratevm.

[cstancu]

@schmitch thank you for your report. The Must not have a <unsupported-feature> in the image heap. errors have been updated and are more descriptive now:

Detected an <unsupported-feature> in the image heap. This is not supported. 
The object was reached from a static initializer. All static class initialization 
is done during native image construction, thus a static initializer cannot 
contain code that captures state dependent on the build machine. Write 
your own initialization methods and call them explicitly from your main entry point.

,where <unsupported-feature> currently can be: a started Thread, a FileDescriptor, or a ZipFile object.

In the near future we will also support loading resources via ClassLoader, probably similarly to how we support resource bundles currently, i.e., they need to be specified at image build time via a command line option.

We already support reflection through configuration files and are currently looking into supporting java.lang.reflect.Proxy.

That being said, the best way to make an application/framework compatible with SubstrateVM is to, if possible, avoid using unsupported features.

[schmitch]

Well we could probably look if we could rewrite some logic that supports SubstrateVM, however without support for ClassLoader#getResource would make it fail on runtime.
I all problems are Logging related, we could mostly lazy load the logger, however one file is problematic since it isn't our: org.slf4j.impl.StaticLoggerBinder.SINGLETON (logback-classic) which also uses a FileDescriptor. But that is not a problem on running Play in embedded mode since one can just leave out play-logback and run without logback.

I have created some work that now supports image creation, but fails on runtime:
Play branch https://github.com/schmitch/playframework/tree/substrate-vm-optimization
Play project https://github.com/schmitch/play-scala-starter-example/tree/substratevm

Part of Stacktrace:
Caused by: com.typesafe.config.ConfigException$BugOrBroken: Context class loader is not set for the current thread; if Thread.currentThread().getContextClassLoader() returns null, you must pass a ClassLoader explicitly to ConfigFactory.defaultApplication

(didn't tested rc2, yet)

[cstancu]

Related: #470 (comment)

[cstancu]

Is loading resources via Thread.getContextClassLoader().getResource() still a problem for you? We added support for ClassLoader.getSystemClassLoader() and Thread.getContextClassLoader(). See #470 (comment) for reference.

[schmitch]

Well unfortunatly it fails with:
Caused by: com.oracle.svm.core.jdk.UnsupportedFeatureError: Accessing an URL protocol that was not enabled. The URL protocol resource is not tested and might not work as expected. It can be enabled by adding the -H:EnableURLProtocols=resource option to the native-image command.
now.
But I tried to enable -H:EnableURLProtocols=resource

[cstancu]

Can you please provide the steps to reproduce this?

[schmitch]

Well just clone:

Play branch https://github.com/schmitch/playframework/tree/substrate-vm-optimization
and run cd framework && sbt publishLocal
after that clone:
Play project https://github.com/schmitch/play-scala-starter-example/tree/substratevm
and run sh build-2.12.sh (needs $GRAALVM_HOME to be set (to the latest version))

[cstancu]

As of 427e54b the resource URL protocol is enabled by default and it doesn't require any special config.

[SchrodingerZhu]

failed to read configurations for GraalVM 19+/Play 2.7