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ModuleSystemSupportRuntime.md

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Runtime module system support

The core of the runtime module system support consists of a set of substitutions and a feature (com.oracle.svm.hosted.ModuleLayerFeature) that provides runtime module-graphs.

The purpose of the runtime module support:

  • Correct information when querying module instances
  • Correct runtime module-aware access checks
  • Correct runtime module graphs
  • Dynamic module system – defining Module and ModuleLayer instances at runtime

Substitutions

  • com.oracle.svm.core.jdk.Target_java_lang_Module:
    • addReads0(), addExports0(), etc - module graph modifications
    • defineModule0() - crucial for dynamic module support
  • com.oracle.svm.core.jdk.Target_java_lang_ModuleLayer:
    • boot() - replaces the hosted boot module layer with our own

Some data structures also need to be substituted/resetted as to not pull in hosted modules (see runtime module synthesizing):

  • com.oracle.svm.core.jdk.Target_java_lang_Module_ReflectionData

ModuleLayerFeature

Synthesizes the runtime boot module layer and all reachable module layers initialized at image-build time. It also replicates build-time module relations at runtime (i.e., reads, opens and exports).

Why don't we reuse build-time modules/layers?

  • Module graphs are not the same – even if they were, we would like to optimize and not include modules that we do not need (as long as it does not break compatibility)
  • Hosted module instances capture state (e.g., class loader), and patching them and all relevant data structures is harder than synthesizing new instances

Synthesizing runtime module layers and modules (com.oracle.svm.hosted.ModuleLayerFeature#afterAnalysis):

  • Calculate runtime root module set:
  • Find all reachable named modules
  • Collect all extra modules (addmods, resources)
  • Find all synthetic modules
  • Find all reachable runtime module layers and synthesize them in parent-to-child order using the root module set for the boot module layer
  • Replicate module graph modifications
  • Replicate native access information

Runtime boot module layer

Calculating root modules – done in the same way as in jdk.internal.module.ModuleBootstrap#boot2, for compatibility.

Substitution for ModuleLayer#boot2 returns the synthesized boot module layer from the com.oracle.svm.core.jdk.RuntimeModuleSupport

An object replacer makes sure that hosted module instances are replaced with their runtime counterparts.

Tests

  • mx hellomodule - smoke test
  • vm-enterprise/tests/native-image/module-graph - test compatibility and correctness of the runtime module graph