Support for exempting native libraries from JNI preload by grendello · Pull Request #10787 · dotnet/android

grendello · 2026-02-09T16:36:24Z

Fixes: #10617
Context: cba39dc

cba39dc introduced support for preloading of JNI native libraries at
application startup. However, it appears that in some scenarios this
behavior isn't desired.

This PR introduces a mechanism which allows exempting some or all (with
exception of the BCL libraries) libraries from the preload mechanism.

In order to not preload any JNI libraries it's now possible to set the
$(AndroidIgnoreAllJniPreload) MSBuild property to true.

It is also possible to exempt individual libraries from preload by
adding their name to the AndroidNativeLibraryNoJniPreload MSBuild
item group, for instance:

<ItemGroup>
  <AndroidNativeLibraryNoJniPreload Include="libMyLibrary.so" />
</ItemGroup>

jonathanpeppers · 2026-02-09T22:45:30Z

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

  <_AndroidStripNativeLibraries Condition=" '$(AndroidStripNativeLibraries)' != '' And '$(AndroidStripNativeLibraries)' == 'true' ">true</_AndroidStripNativeLibraries>
  <_AndroidStripNativeLibraries Condition=" '$(_AndroidStripNativeLibraries)' != 'true' ">false</_AndroidStripNativeLibraries>
+
+  <AndroidIgnoreAllJniPreload Condition=" '$(AndroidIgnoreAllJniPreload)' == '' ">false</AndroidIgnoreAllJniPreload>


How common do you think people would have to do this? Should it be a public, documented property?

Or is it an edge-case, and $(_AndroidIgnoreAllJniPreload) would be fine?

I don't know, to be honest. The original scenario it addresses appears to be quite unusual, but with the dependency hell that the AndroidX ecosystem is, who knows? I would make it a documented and supported property, just to be safe and not let people without easy choice.

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>

MonoVM tests are weirdly broken (appconfig reads incorrectly, need to investigate)

Copilot

Pull request overview

Adds an opt-out mechanism for JNI native library preloading, including per-library exemptions and a global “ignore all” switch, with supporting build/test infrastructure and documentation updates.

Changes:

Introduces $(AndroidIgnoreAllJniPreload) and @(AndroidNativeLibraryNoJniPreload) integration into native application config generation.
Adds a small “test JNI library” build path to enable preload-related tests.
Expands test utilities and adds new tests validating JNI preload inclusion/exclusion behavior.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/native/native.targets	Adds targets to configure/build a test JNI library as part of runtime build.
src/native/common/test-jni-library/stub.cc	Adds minimal `JNI_OnLoad` stub for preload tests.
src/native/common/test-jni-library/CMakeLists.txt	Builds the test JNI shared library and outputs to test directory.
src/native/CMakePresets.json.in	Adds `XA_TEST_OUTPUT_DIR` to presets cache variables.
src/native/CMakeLists.txt	Wires `BUILD_TEST_JNI_LIBRARY` flow and requires `XA_TEST_OUTPUT_DIR`.
src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets	Defines new property and item groups; passes new item lists into generator task.
src/Xamarin.Android.Build.Tasks/Utilities/MonoAndroidHelper.cs	Adds helper to normalize native library names.
src/Xamarin.Android.Build.Tasks/Utilities/ApplicationConfigNativeAssemblyGeneratorCLR.cs	Implements ignore/always-preload resolution for JNI preload list generation (CoreCLR path).
src/Xamarin.Android.Build.Tasks/Utilities/ApplicationConfigNativeAssemblyGenerator.cs	Applies same JNI preload ignore logic (MonoVM path).
src/Xamarin.Android.Build.Tasks/Tasks/GenerateNativeApplicationConfigSources.cs	Adds task parameters for preload ignore/always-preload lists.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/AndroidItem.cs	Adds test project item wrapper for `AndroidNativeLibraryNoJniPreload`.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/AndroidBuildActions.cs	Adds build action constant for `AndroidNativeLibraryNoJniPreload`.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Utilities/EnvironmentHelper.cs	Extends parsing helpers to read JNI preload index data from generated native sources.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest3.cs	Adds tests validating JNI preload inclusion/exemption and deduplication.
build-tools/xaprepare/xaprepare/xaprepare.targets	Adds placeholder replacement for `TestOutputDirectory`.
build-tools/xaprepare/xaprepare/Steps/Step_GenerateFiles.cs	Plumbs `XA_TEST_OUTPUT_DIR` into generated CMake presets.
build-tools/xaprepare/xaprepare/Application/Properties.Defaults.cs.in	Adds default for `TestOutputDirectory`.
build-tools/xaprepare/xaprepare/Application/KnownProperties.cs	Adds `TestOutputDirectory` known property.
Documentation/docs-mobile/building-apps/build-properties.md	Documents `AndroidIgnoreAllJniPreload`.
Documentation/docs-mobile/building-apps/build-items.md	Documents `AndroidNativeLibraryNoJniPreload`.

Copilot · 2026-02-18T15:55:32Z

src/native/native.targets

+  <Target Name="_ConfigureAndBuildTestJniLibrary"
+          Condition="'$(CMakeRuntimeFlavor)' != 'NativeAOT' "
+          DependsOnTargets="_PrepareCommonProperties;_ConfigureTestJniLibraryInputsAndOutputs"
+          Inputs="@_TestJniLibraryInput)"
+          Outputs="@(_TestJniLibraryOutput)">
+    <ItemGroup>
+      <_ConfigureTestJniLibraryCommands Include="@(_TestJniLibraryAbi)">
+        <Command>$(CmakePath)</Command>
+        <Arguments>-DOUTPUT_PATH="$(OutputPath.TrimEnd('\'))" -DRUNTIME_FLAVOR="$(CMakeRuntimeFlavor)" --preset default-release-%(_TestJniLibraryAbi.Identity) -DBUILD_TEST_JNI_LIBRARY=ON "$(MSBuildThisFileDirectory)"</Arguments>
+        <WorkingDirectory>$(FlavorIntermediateOutputPath)%(_TestJniLibraryAbi.AndroidRID)-test-jni-library</WorkingDirectory>
+      </_ConfigureTestJniLibraryCommands>
+    </ItemGroup>


Inputs="@_TestJniLibraryInput)" has an extra ) and is missing MSBuild item list syntax, which will break incremental build tracking for this target. Also, WorkingDirectory paths at lines 141 and 150 appear to be missing a directory separator after $(FlavorIntermediateOutputPath), producing an invalid path (note MakeDir uses $(FlavorIntermediateOutputPath)\... earlier). Fix the Inputs attribute to use the correct @(...) form, and make WorkingDirectory use the same path joining convention as the created directory.

Copilot · 2026-02-18T15:55:32Z

src/native/native.targets

+    <Exec
+        Command="$(NinjaPath) -v"
+        WorkingDirectory="$(FlavorIntermediateOutputPath)%(_TestJniLibraryAbi.AndroidRID)-test-jni-library" />


Inputs="@_TestJniLibraryInput)" has an extra ) and is missing MSBuild item list syntax, which will break incremental build tracking for this target. Also, WorkingDirectory paths at lines 141 and 150 appear to be missing a directory separator after $(FlavorIntermediateOutputPath), producing an invalid path (note MakeDir uses $(FlavorIntermediateOutputPath)\... earlier). Fix the Inputs attribute to use the correct @(...) form, and make WorkingDirectory use the same path joining convention as the created directory.

Copilot · 2026-02-18T15:55:33Z

src/Xamarin.Android.Build.Tasks/Utilities/MonoAndroidHelper.cs

+			}
+
+			if (!String.Equals (Path.GetExtension (ret), ".so", StringComparison.OrdinalIgnoreCase)) {
+				return "${ret}.so";


This returns the literal string "${ret}.so" instead of appending the .so extension to ret. Use C# string interpolation ($"{ret}.so") or concatenation (ret + ".so") so callers get the correct normalized filename.

Suggested change

return "${ret}.so";

return $"{ret}.so";

Copilot · 2026-02-18T15:55:33Z

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

-      NativeLibraries="@(AndroidNativeLibrary);@(EmbeddedNativeLibrary);@(FrameworkNativeLibrary)"
+      NativeLibraries="@(_AllNativeLibraries)"
+      NativeLibrariesNoJniPreload="@(_AndroidNativeLibraryNeverJniPreload);@(AndroidNativeLibraryNoJniPreload)"
+      NativeLibrarysAlwaysJniPreload="@(_AndroidNativeLibraryAlwaysJniPreload)"


The parameter name NativeLibrarysAlwaysJniPreload has a spelling/grammar error (“Librarys” vs “Libraries”). Since this name flows through MSBuild task parameters and generator state, consider renaming consistently everywhere (targets, task properties, state objects) to NativeLibrariesAlwaysJniPreload to avoid permanently cementing a typo into the public-ish build/task surface.

Suggested change

NativeLibrarysAlwaysJniPreload="@(_AndroidNativeLibraryAlwaysJniPreload)"

NativeLibrariesAlwaysJniPreload="@(_AndroidNativeLibraryAlwaysJniPreload)"

Copilot · 2026-02-18T15:55:33Z

Documentation/docs-mobile/building-apps/build-items.md

+Some framework libraries which must be loaded at application startup will not
+be affected if included in this item group.
+
+See also [`$(AndroidIgnoreAllJniPreload)`](build-items.md#androidignorealljnipreload)


This link points back to build-items.md, but $(AndroidIgnoreAllJniPreload) is documented in build-properties.md (added in this PR). Update the link target to the correct document/anchor so readers can actually navigate to the property description.

Suggested change

See also [`$(AndroidIgnoreAllJniPreload)`](build-items.md#androidignorealljnipreload)

See also [`$(AndroidIgnoreAllJniPreload)`](build-properties.md#androidignorealljnipreload)

Copilot · 2026-02-18T15:55:34Z

...Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Utilities/EnvironmentHelper.cs

+
+			NativeAssemblyParser.AssemblerSymbol dsoJniPreloadsIdxCount = GetNonEmptyRequiredSymbol (parser, envFile, DsoJniPreloadsIdxCountSymbolName);
+			ulong preloadsCount = GetSymbolValueAsUInt64 (dsoJniPreloadsIdxCount);
+			Assert.IsTrue (preloadsCount > 0, $"Symbol {dsoJniPreloadsIdxStride.Name} must have value larger than 0.");


The assertion message references dsoJniPreloadsIdxStride.Name, but the check is validating the count symbol. This will produce misleading failure output when the count is 0. Use dsoJniPreloadsIdxCount.Name (or the DsoJniPreloadsIdxCountSymbolName constant) in the message.

Suggested change

Assert.IsTrue (preloadsCount > 0, $"Symbol {dsoJniPreloadsIdxStride.Name} must have value larger than 0.");

Assert.IsTrue (preloadsCount > 0, $"Symbol {dsoJniPreloadsIdxCount.Name} must have value larger than 0.");

Copilot · 2026-02-18T15:55:34Z

...Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Utilities/EnvironmentHelper.cs

+		static string ReadStringBlob (EnvironmentFile envFile, NativeAssemblyParser.AssemblerSymbol contentsSymbol, NativeAssemblyParser parser)
+		{
+			NativeAssemblyParser.AssemblerSymbolItem contentsItem = contentsSymbol.Contents[0];
+			string[] field = GetField (envFile.Path, parser.SourceFilePath, contentsItem.Contents, contentsItem.LineNumber);;


There’s a double semicolon (;;) at the end of the line. Removing the extra ; will avoid noise and keep the file consistent with typical style.

Suggested change

string[] field = GetField (envFile.Path, parser.SourceFilePath, contentsItem.Contents, contentsItem.LineNumber);;

string[] field = GetField (envFile.Path, parser.SourceFilePath, contentsItem.Contents, contentsItem.LineNumber);

jonathanpeppers reviewed Feb 9, 2026

View reviewed changes

grendello force-pushed the dev/grendel/jni-preload-exemption branch 4 times, most recently from d399967 to afa6525 Compare February 13, 2026 13:00

grendello force-pushed the dev/grendel/jni-preload-exemption branch 2 times, most recently from f60c1c8 to fe5dc19 Compare February 18, 2026 15:17

grendello and others added 22 commits February 18, 2026 16:52

Basics

7f3a72a

Slightly broken, tbc tomorrow

6203596

Update src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

3da67bc

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>

Default config works (CLR only for now)

9951494

Beginnings of unit tests

dc8cc6c

Environment test helper progress

1bb29a7

[clr] first basic test works

1222f5c

A tiny refactoring in prep for more tests

3198516

Fix after rebase

8a17792

New test + small refactoring

4385500

Update defaults checks

da35357

Add a native library for JNI preload tests, tbc

0877364

Output the test JNI library to bin/Test${CONFIGURATION}/${RID}

9c59971

New test (WIP) + small refactoring

7d690c4

More tests + some cleanup

a2a34ee

New tests + fix test .so path on CI

7ebcdad

Yeah, that...

74a0366

Refactoring + MonoVM tests

5dd8c7a

MonoVM tests are weirdly broken (appconfig reads incorrectly, need to investigate)

Fix reading of the MonoVM application config

0447e16

MonoVM dso cache reader implemented

ebe6648

Mono tests work now

8ef26dc

Docs

2d3b823

grendello force-pushed the dev/grendel/jni-preload-exemption branch from 6df6659 to 2d3b823 Compare February 18, 2026 15:52

grendello marked this pull request as ready for review February 18, 2026 15:52

grendello requested a review from simonrozsival as a code owner February 18, 2026 15:52

Copilot AI review requested due to automatic review settings February 18, 2026 15:52

Copilot AI reviewed Feb 18, 2026

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Support for exempting native libraries from JNI preload#10787

Support for exempting native libraries from JNI preload#10787
grendello wants to merge 22 commits intomainfrom
dev/grendel/jni-preload-exemption

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	NativeLibrarysAlwaysJniPreload="@(_AndroidNativeLibraryAlwaysJniPreload)"
	NativeLibrariesAlwaysJniPreload="@(_AndroidNativeLibraryAlwaysJniPreload)"

	See also [`$(AndroidIgnoreAllJniPreload)`](build-items.md#androidignorealljnipreload)
	See also [`$(AndroidIgnoreAllJniPreload)`](build-properties.md#androidignorealljnipreload)

	Assert.IsTrue (preloadsCount > 0, $"Symbol {dsoJniPreloadsIdxStride.Name} must have value larger than 0.");
	Assert.IsTrue (preloadsCount > 0, $"Symbol {dsoJniPreloadsIdxCount.Name} must have value larger than 0.");

	string[] field = GetField (envFile.Path, parser.SourceFilePath, contentsItem.Contents, contentsItem.LineNumber);;
	string[] field = GetField (envFile.Path, parser.SourceFilePath, contentsItem.Contents, contentsItem.LineNumber);

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grendello commented Feb 9, 2026 •

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