[GR-49770] Safe Composition of Metadata

[vjovanov]

TL;DR

To make usage of third-party reachability metadata safe we must guarantee that metadata addition can't break existing programs.
This is currently not the case:

1. For reflective methods such as java.lang.Class#getDeclaredMethods that return values based on reachability of other reflective elements.
   Because of this adding new metadata makes more elements reachable and can change program functionality in undesirable ways.

   We must require that all reflective methods on java.lang.Class require a metadata entry and that they: either return all elements when the metadata entry is present, or throw a missing-metadata exception when there is no metadata entry.

2. For resource metadata that can currently contain exclude patterns. An added exclude pattern can accidentally remove resources from the image and break the functionality of a working program.

   We must require that exclude patterns are regarded only within the scope of a single resource-config.json file.

Safe Composition of Reflection Metadata

In Native Image, adding new metadata can break user programs because methods such as java.lang.Class#getDeclaredMethods return the set of currently reachable elements.
In the following snippet

if (AClass.class.getMethods().size > 5) {
    System.exit(1);
}

adding a metadata entry for a method from AClass can cause the System.exit(1) to be reached.
Furthermore, this can happen by simply using methods from AClass anywhere in the code as this makes new methods reachable.

To allow safe composition of metadata we need to make sure that every reflective call on java.lang.Class requires a metadata entry.
The following tables show metadata entries for all of those methods:

Method	Existing Metadata Entry
java.lang.Class#getDeclaredMethods	"queryAllDeclaredMethods": true
java.lang.Class#getDeclaredConstructors	"queryAllDeclaredConstructors": true
java.lang.Class#getMethods	"queryAllPublicMethods": true
java.lang.Class#getConstructors	"queryAllPublicConstructors": true
java.lang.Class#getFields	"allPublicFields": true
java.lang.Class#getDeclaredFields	"allDeclaredFields": true

Newly Tracked Method	New Metadata Entry
java.lang.Class#getClasses	"queryAllPublicClasses": true
java.lang.Class#getDeclaredClasses	"queryAllDeclaredClasses": true
java.lang.Class#getPermittedSubclasses	"queryAllPermittedSubclasses": true
java.lang.Class#getNestMembers	"queryAllNestMembers": true

All methods above would return all elements when metadata entry is present, or throw a missing-metadata exception if the metadata entry is not present.
All methods above and corresponding metadata entries would be tracked by the Native Image agent.

Safe Composition of Resource Metadata

Resource metadata allows for include patterns such as

{
  "resources": {
    "includes":[{
      "pattern": "assets/.*"
    }]
  }
}

and exclude patterns

{
  "resources": {
    "excludes":[{
      "pattern": "assets/.*.png"
    }]
  }
}

This allows metadata from third-party jars to remove a resource by accident. The second example above removes all png files from assets of another library.

To allow safe composition, exclude patterns must apply only to resources that were matched by the include patterns in the same file. With this change the above
example would still include all files from assets as the exclude pattern doesn't have a corresponding include pattern.
To remove png files from assets the file would have to be specified as:

{
  "resources": {
    "includes":[{
      "pattern": "assets/.*"
    }],
    "excludes":[{
      "pattern": "assets/.*.png"
    }]
  }
}

Note that composing all examples above would still include all resources from assets as the first file doesn't have an exclude pattern.

[JnRouvignac]

Hello! Release notes seem to say this issue is fixed, and yet, the issue is still open here. Who is right? :D

[vjovanov]

The reflection part is fixed but marked as experimental. When you use -H:ThrowMissingRegistrationErrors=<your_package> the reflection will be safely composable.
The reason for -H:ThrowMissingRegistrationErrors=<your_package> being experimental is that we are waiting for the frameworks (e.g., Spring Boot) to adopt the new behavior.

This feature will become available as non-experimental in the next release (GraalVM for Java 22), and will be promoted to the default behavior as the community adopts it.

The composability of the resource patterns is tracked at #7487

[vjovanov]

This item is now mostly complete when the -H:ThrowMissingRegistrationErrors= is set. To be complete it still depends on #7487

[vjovanov]

This item is now complete as we have finished: #7487