[Xamarin.Android.Build.Tasks] Add opt-in R8 runtime remapping by simonrozsival · Pull Request #12692 · dotnet/android

simonrozsival · 2026-09-05T19:02:09Z

Context: #12535

The SDK disables R8 obfuscation because managed JNI callers retain the
original Java names. This change adds an experimental alternative to
assembly rewriting: use R8's mapping to translate JNI lookups through the
existing runtime remapping machinery, keeping managed assemblies unchanged.

This is an alternative to the rewriting pipelines in #12632 and #12634,
not an integration of those pipelines.

Build and runtime changes

Run a naming-only seed R8 pass before ILLink/ILC, then apply that mapping
during final R8.
Select CoreCLR mappings from linked assemblies. For NativeAOT, filter
against surviving JNI literals in allocated sections of ILC's ELF object,
generate a native table object, and link it after ILC. Do not treat
NativeAOT's original publish assemblies as trimmed output.
Extend runtime lookups for forward/reverse types, method descriptors,
fields, inherited members, and managed-peer activation. Share the native
lookup implementation between CoreCLR and NativeAOT; weak empty tables
preserve the disabled NativeAOT path.
Preserve JNI bootstrap names and manifest/resource keep rules. Track
AAPT rules in their incremental parent target so no-op builds cannot
delete rules needed by resource-inflated classes.
Track generated tables, native link inputs, and public configuration
changes, including missing-output recovery and switching obfuscation off.

Public opt-in

For a trimmed CoreCLR or NativeAOT application:

<PropertyGroup Condition="'$(Configuration)' == 'Release'">
<AndroidLinkTool>r8</AndroidLinkTool>
<AndroidTypeMapImplementation>trimmable</AndroidTypeMapImplementation>
<PublishTrimmed>true</PublishTrimmed>
<AndroidEnableR8Obfuscation>true</AndroidEnableR8Obfuscation>
<AndroidR8ObfuscationMode>runtime-remapping</AndroidR8ObfuscationMode>
</PropertyGroup>

$(AndroidEnableR8Obfuscation) defaults to false.
$(AndroidR8ObfuscationMode) defaults to runtime-remapping; selecting a
mode alone does not enable obfuscation. Library projects are unaffected.

experimental-rewriting is reserved for the separate rewriting workstream
and currently reports XA4329 rather than silently falling back. Invalid
values and incompatible enabled configurations also produce a localized
configuration error. MonoVM is not supported by this opt-in.

Validation

88 focused R8/remapping host cases passed.
15 public-option configuration cases passed.
CoreCLR and NativeAOT emulator cases passed with public options enabled,
then disabled on the same project. Coverage includes actually renamed
classes, constructors, overloads, instance/static fields, peer activation,
no-op builds, missing-table recovery without ILC, and AAPT rule recovery.
NativeAOT MAUI sample-content baseline and remapped APKs exercised project
and task navigation, keyboard editing, save/reopen, and Manage Meta on an
arm64 API-35 emulator. Both used identical sample sources with local
InputMethodManager constructor preservation and XamlC for NativeAOT.

The earlier MAUI NativeAOT snapshot produced a 314,376-byte remapping object
instead of retaining the full seed map. Those APKs used the prototype's
former private switch; the public API was subsequently covered by the
configuration/device cases above. No new startup-performance claim is
made by this PR.

Experimental limitations

NativeAOT literal matching is conservative and can retain extra entries.
Arbitrarily computed JNI names need explicit remaps or keep rules.
Conservative class/interface/native-callback keeps still limit obfuscation,
and existing Intune/R8 conflict handling is not full remapping-chain
composition. This is not a production-readiness claim.

Useful description of why the change is necessary.
Links to related issues.
Unit tests.

Fixes: #12535

Context: #12535 R8 obfuscation changes JNI names that managed bindings still use. Add an experimental alternative to assembly rewriting by translating those names through the existing runtime remapping machinery. Run a naming-only R8 seed before ILLink or ILC and apply its mapping during final R8. Select CoreCLR remaps from linked assemblies and NativeAOT remaps from retained ELF literals, then link the NativeAOT table after ILC. Extend lookups for reverse types, descriptors, fields, and peers. Expose AndroidEnableR8Obfuscation, defaulting to false, and AndroidR8ObfuscationMode, defaulting to runtime-remapping. Reserve experimental-rewriting with a clear error until its separate pipeline is available. Diagnose incompatible configurations with XA4329. Preserve JNI bootstrap and resource keep rules, track incremental table and native-link inputs, and support switching obfuscation back off. Include task, configuration, device regression tests, and documentation. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot

Copilot review overview

🔵 Needs a closer look

It makes cross-cutting changes across MSBuild targets, build tasks, native runtime lookup code, and public API surface that require careful human validation beyond automated review.

Review tier: Lite
Findings: 3

New issues introduced by this change (3)

Severity	Finding
	`src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/GenerateJniRemappingNativeCodeTests.cs` — ❌ error: Avoid the null-forgiving operator (`!`) in tests as well; it hides real nullability issues…
	`src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/R8Tests.cs` — ⚠️ warning: Two [TestCase] attributes are on the same line, which makes the test metadata harder to…
	`src/Xamarin.Android.Build.Tasks/Utilities/JniRemapping/R8Mapping.cs` — ⚠️ warning: Two method declarations are on the same line, which breaks the repo’s typical…

What changed in this PR

Adds an experimental opt-in for R8 obfuscation in .NET for Android by generating and consuming runtime JNI remapping tables (rather than rewriting managed assemblies), enabling obfuscated DEX outputs while preserving managed JNI name expectations.

Changes:

Introduces a two-pass R8 pipeline (seed mapping pre-trim + final R8 -applymapping) and generates JNI remapping XML/native tables for CoreCLR and NativeAOT.
Extends runtime remapping to cover reverse type lookups, rewritten method descriptors, and field remapping; shares native lookup code between CoreCLR and NativeAOT.
Adds/updates tests and documentation for new public properties and XA4327/8/9 diagnostics.

File	Description
tests/MSBuildDeviceIntegration/Tests/R8RuntimeRemappingTests.cs	Device test validating obfuscated members/types and incremental/missing-output recovery.
src/Xamarin.Android.Build.Tasks/Xamarin.Android.D8.targets	Wires new R8 inputs/outputs and enables mapping input/output + obfuscation flag.
src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets	Adds opt-in properties, validation (XA4329), incremental inputs, and AAPT rules tracking changes.
src/Xamarin.Android.Build.Tasks/Utilities/JniRemapping/R8Mapping.cs	Extends mapping parsing/projection for class/method/field data used by remapping generation.
src/Xamarin.Android.Build.Tasks/Utilities/JniRemapping/NativeAotJniRetention.cs	NativeAOT ELF-based literal retention to conservatively select required remap entries.
src/Xamarin.Android.Build.Tasks/Utilities/JniRemapping/JniDescriptorText.cs	Converts Java source-form types to JNI tokens + builds method descriptors.
src/Xamarin.Android.Build.Tasks/Utilities/JniRemapping/JniAssemblyRewriter.cs	Adds scan-only entrypoint for linked-assembly analysis (no rewriting).
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/R8Tests.cs	Adds unit coverage for keep-option and config generation behavior.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/GenerateTrimmableTypeMapTests.cs	Verifies NativeAOT proguard generation respects allowobfuscation.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/GenerateJniRemappingNativeCodeTests.cs	New tests for native remap table emission, ordering, and legacy compatibility.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/InvalidConfigTests.cs	Tests defaults + invalid configuration errors for new MSBuild properties.
src/Xamarin.Android.Build.Tasks/Tasks/R8.cs	Adds seed mapping mode, applymapping support, and conditional dontobfuscate removal.
src/Xamarin.Android.Build.Tasks/Tasks/GenerateR8JniRemapping.cs	Generates JNI remapping XML from R8 mapping + existing remaps; supports NativeAOT retention path.
src/Xamarin.Android.Build.Tasks/Tasks/GenerateR8JniManifestProguardConfiguration.cs	Generates manifest keep rules to stabilize seed mapping applicability.
src/Xamarin.Android.Build.Tasks/Tasks/GenerateProguardConfiguration.cs	Emits allowobfuscation on keep rules when runtime remapping is enabled.
src/Xamarin.Android.Build.Tasks/Tasks/GenerateNativeAotProguardConfiguration.cs	Emits allowobfuscation for NativeAOT-generated keep rules when enabled.
src/Xamarin.Android.Build.Tasks/Tasks/GenerateJniRemappingNativeCode.cs	Extends generated tables to include reverse types + fields; exposes info for tests.
src/Xamarin.Android.Build.Tasks/Resources/proguard_xamarin.cfg	Adds/adjusts keep rules needed for stable seed/final graphs and bootstrap types.
src/Xamarin.Android.Build.Tasks/Resources/proguard_trimmable_nativeaot.cfg	Aligns NativeAOT baseline keep rules with remapping needs and seed/final stability.
src/Xamarin.Android.Build.Tasks/Properties/Resources.resx	Adds XA4327/8/9 localized strings for errors/warnings/validation.
src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs	Updates generated resource accessors for XA4327/8/9.
src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Aapt2.targets	Moves AAPT proguard rule tracking to incremental parent target.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.Trimmable.targets	Imports new R8 JNI remapping targets last to override pre-trim outputs as needed.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.Trimmable.NativeAOT.targets	Includes new properties in incremental stamps; passes obfuscation state into proguard generation.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.Trimmable.CoreCLR.targets	Reworks linked-assembly proguard inputs; adds remapping-assembly prep + incremental inputs.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.LlvmIr.targets	Adds remapping enable flag into proguard generation and incremental inputs.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.R8JniRemapping.targets	New MSBuild pipeline for seed mapping, remap XML, and NativeAOT late-linked table build.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.NativeAOT.targets	Links remapping object into NativeAOT shared library and updates incremental inputs.
src/native/nativeaot/include/runtime-base/internal-pinvokes.hh	Adds internal pinvokes for reverse-type and field lookup.
src/native/nativeaot/host/jni-remapping-tables-stub.cc	Provides weak empty table symbols for apps without remapping objects.
src/native/nativeaot/host/internal-pinvoke-stubs.cc	Removes now-shared remapping pinvoke stubs from NativeAOT host stubs.
src/native/nativeaot/host/host.cc	Plumbs `jniRemappingInUse` via shared `JniRemapping::is_in_use()`.
src/native/nativeaot/host/CMakeLists.txt	Adds shared remapping sources and stub table compilation to NativeAOT host build.
src/native/native.targets	Includes shared remapping sources/headers in NativeAOT flavor build inputs.
src/native/mono/xamarin-app-stub/xamarin-app.hh	Updates stub ABI structs to include target_signature + field remapping structures.
src/native/mono/xamarin-app-stub/application_dso_stub.cc	Updates stub table initializers for new method signature field.
src/native/mono/runtime-base/internal-pinvokes.hh	Adds internal pinvoke declarations for reverse-type and field lookup.
src/native/mono/pinvoke-override/pinvoke-tables.include	Extends pinvoke table entries/count for the new remapping exports.
src/native/mono/pinvoke-override/generate-pinvoke-tables.cc	Adds new internal pinvoke names to generator input list.
src/native/mono/monodroid/internal-pinvokes.cc	Adds MonoVM-safe placeholder exports for new remapping entrypoints.
src/native/clr/xamarin-app-stub/application_dso_stub.cc	Extends CLR stub tables to include reverse types + fields + signature pinning.
src/native/clr/runtime-base/jni-remapping.cc	Implements binary-search remapping lookups (types, reverse types, methods, fields) + `is_in_use()`.
src/native/clr/pinvoke-override/precompiled.cc	Maps new internal pinvoke entrypoints to implementations.
src/native/clr/include/xamarin-app.hh	Declares remapping table symbols and adds field + reverse type structures.
src/native/clr/include/runtime-base/jni-remapping.hh	Declares shared lookup surface including reverse type and field lookup.
src/native/clr/include/runtime-base/internal-pinvokes.hh	Declares new remapping pinvokes for CoreCLR runtime.
src/native/clr/host/internal-pinvokes-shared.cc	Centralizes shared remapping pinvoke implementations for CoreCLR/NativeAOT.
src/native/clr/host/internal-pinvokes-clr.cc	Removes remapping implementations now provided by shared file.
src/native/clr/host/host.cc	Uses `JniRemapping::is_in_use()` for init flag and includes remapping header.
src/Mono.Android/Microsoft.Android.Runtime/TrimmableTypeMapValueManager.cs	Ensures FindClass uses replacement type name when remapping is enabled.
src/Mono.Android/Microsoft.Android.Runtime/TrimmableTypeMapTypeManager.cs	Adds reverse-type handling for Java-to-managed lookups; uses replacement type for signatures.
src/Mono.Android/Microsoft.Android.Runtime/TrimmableTypeMap.cs	Uses reverse type for proxy lookup and replacement type for FindClass checks.
src/Mono.Android/Microsoft.Android.Runtime/JniRemappingLookup.cs	Adds reverse type + field lookup plumbing and supports target-method-signature.
src/Mono.Android/Android.Runtime/RuntimeNativeMethods.cs	Adds LibraryImport declarations for reverse type + field lookup pinvokes.
src/Mono.Android/Android.Runtime/AndroidRuntime.cs	Exposes GetOriginalTypeCore via reverse-type lookup.
external/Java.Interop/tests/Java.Interop-Tests/Java.Interop/JniPeerMembersTests.cs	Adds tests validating remapped field names and pinned target signatures.
external/Java.Interop/tests/Java.Interop-Tests/Java.Interop/JavaVMFixture.cs	Extends test runtime type manager with field replacement support.
external/Java.Interop/src/Java.Interop/PublicAPI.Unshipped.txt	Records new public API surface additions for replacement fields + original type.
external/Java.Interop/src/Java.Interop/Java.Interop/JniType.cs	Adds TryGet{Static,Instance}Field helpers to support remapped field probing.
external/Java.Interop/src/Java.Interop/Java.Interop/JniRuntime.ReflectionJniTypeManager.cs	Adds null default implementation for field replacement in reflection manager.
external/Java.Interop/src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs	Adds ReplacementFieldInfo + original type + replacement field APIs.
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniStaticMethods.cs	Uses replacement lookup keyed by original type name (compat + remapping).
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniStaticFields.cs	Adds remapped static field probing and fallback to original lookup.
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs	Tracks original vs effective JNI type names; remaps ctor/method lookup accordingly.
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceFields.cs	Adds remapped instance field probing and fallback behavior.
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.cs	Tracks original type name and performs replacement lookups across base types/types.
Documentation/docs-mobile/TOC.yml	Adds XA4327/8/9 docs to TOC.
Documentation/docs-mobile/messages/xa4329.md	Documents invalid/unsupported configuration errors and resolutions.
Documentation/docs-mobile/messages/xa4328.md	Documents remapping incompleteness warnings (conflicts/signature conversion).
Documentation/docs-mobile/messages/xa4327.md	Documents remapping generation failures and troubleshooting steps.
Documentation/docs-mobile/messages/index.md	Adds XA4327/8/9 to messages index.
Documentation/docs-mobile/building-apps/build-properties.md	Documents AndroidEnableR8Obfuscation + AndroidR8ObfuscationMode properties.

Files not reviewed (1)

src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs: Generated file

Suppressed comments (1)

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/GenerateJniRemappingNativeCodeTests.cs:112

❌ error: This uses the null-forgiving operator (info!) after Assert.IsNotNull, which the compiler can’t reason about. Prefer ?? throw so nullability is enforced without suppressions.

+ string TestDirectory {
+ get {
+ Assert.IsNotNull (directory);
+ return directory!;
+ }
+ }
+
+ List<BuildErrorEventArgs> Errors {
+ get {
+ Assert.IsNotNull (errors);
+ return errors!;
+ }
+ }



- [TestCase ("package com.example.app;\npublic class Foo {}", "com.example.app")]
- [TestCase ("package com.example.app ;\npublic class Foo {}", "com.example.app")] // space before ';'
+ [TestCase ("package com.example.app;\npublic class Foo {}", "com.example.app")] [TestCase ("package com.example.app ;\npublic class Foo {}", "com.example.app")] // space before ';'


+ return javaMethodName.Length != 0;
+ }
+
+ internal static string BuildClassEntry (string className) => $"C\t{className}"; internal static string BuildFieldEntry (string className, string fieldName) => $"F\t{className}\t{fieldName}";


Context: #12692 Follow the existing Intune contract: member lookup keys contain the replaced owner type and the original managed member name and descriptor. Generate R8 member entries and collision keys in that same form rather than teaching every consumer to try both original and replaced owners. Remove JniPeerOriginalTypeName, the duplicate constructor identity, and the now-unused Java.Interop reverse-type hook. Keep reverse lookup for peer activation and derive desugared companion names in the Android remapping helper where the original name is actually needed. Cover residual-owner keys, descriptors, MAM conflicts and duplicates, and real renamed-class construction and member access on both runtimes. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

CopilotAI lite review requested due to automatic review settings September 5, 2026 19:02

simonrozsival requested a review from jonathanpeppers as a code owner September 5, 2026 19:02

Copilotstarted reviewing on behalf of simonrozsival September 5, 2026 19:02 View session

simonrozsival marked this pull request as draft September 5, 2026 19:02

CopilotAI reviewed Sep 5, 2026
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[Xamarin.Android.Build.Tasks] Add opt-in R8 runtime remapping - #12692

[Xamarin.Android.Build.Tasks] Add opt-in R8 runtime remapping#12692
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simonrozsival commented Sep 5, 2026

Build and runtime changes

Public opt-in

Validation

Experimental limitations

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