Project Crema: Open World for Native Image

[alina-yur]

TL;DR

Crema will lift Native Image's default closed-world assumption by allowing dynamic loading and execution of classes at run time. It builds upon Native Image Layers and the JDWP-based debugger by adding a Java interpreter to the application layer. This will significantly improve usability with libraries that produce JVM bytecode at run time. Additionally, it will enable dynamic loading of plugins, such as javac annotation processors.

Goals

• When enabled, be able to dynamically load classes, link them to AOT-compiled classes, and execute their bytecodes.
• Load classes from run time-created class loaders.
• Load classes from build time-created class loaders after re-linking them to their classpath.
• Configure which AOT classes can be linked to by dynamically loaded classes and which class loaders are enabled at run time.
• In future releases Crema can also potentially provide a JIT compiler for the dynamically loaded bytecodes.

Non-Goals

• It is not a goal to enable this capability by default, or to be able to add this capability to native images built prior to Crema.
• It is not a goal to be able to fallback to classloading for classes that are included in the AOT image (although some "AOT" methods could be executed via the bytecode interpreter).

[simonis]

Does this mean that the native image will have to include the byte codes (or AST) of all AOT compiled methods because they can potentially all be deoptimized after newly loaded classes invalidate assumptions that were made during AOT compilation?

[gilles-duboscq]

The open world mode (-H:-ClosedWorld, also used for non-final layers) doesn't make assumptions in its compiled code that can be invalidated by loading more classes. So we shouldn't need bytecode for methods that were AOT-compiled.

For methods that are not included in the image but that are declared in a type that is included in the image it might be necessary to keep bytecodes so that they can be executed if they become reachable from newly loaded code.