[Xamarin.Android.Build.Tasks] Add opt-in R8 runtime remapping

Documentation/docs-mobile/TOC.yml

@@ -370,6 +370,12 @@
         href: messages/xa4325.md
       - name: XA4326
         href: messages/xa4326.md
+      - name: XA4327
+        href: messages/xa4327.md
+      - name: XA4328
+        href: messages/xa4328.md
+      - name: XA4329
+        href: messages/xa4329.md
     - name: "XA5xxx: GCC and toolchain"
       items:
       - name: "XA5xxx: GCC and toolchain"

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

@@ -469,6 +469,33 @@ removing the existing one(s) and adding your own AOT profiles.
 This property is `False` by default.
 
 
+## AndroidEnableR8Obfuscation
+
+A boolean property that opts an Android application into R8 name obfuscation.
+The default is `false`; setting
+[`$(AndroidR8ObfuscationMode)`](#androidr8obfuscationmode) alone does not enable it.
+This feature is experimental.
+
+The current implementation requires `AndroidLinkTool=r8`,
+`AndroidTypeMapImplementation=trimmable`, `PublishTrimmed=true`, and either the
+CoreCLR or NativeAOT runtime. Explicit incompatible settings produce
+[XA4329](../messages/xa4329.md) rather than being silently changed.
+This property has no effect on library projects.
+
+For example:
+
+```xml
+<PropertyGroup Condition="'$(Configuration)' == 'Release'">
+  <AndroidLinkTool>r8</AndroidLinkTool>
+  <AndroidTypeMapImplementation>trimmable</AndroidTypeMapImplementation>
+  <PublishTrimmed>true</PublishTrimmed>
+  <AndroidEnableR8Obfuscation>true</AndroidEnableR8Obfuscation>
+  <AndroidR8ObfuscationMode>runtime-remapping</AndroidR8ObfuscationMode>
+</PropertyGroup>
+```
+
+Added in .NET 11.
+
 ## AndroidEnableRestrictToAttributes
 
 An enum-style property with valid values of `obsolete` and `disable`.
@@ -1095,6 +1122,31 @@ r8 dex-compiler and shrinker. The default value is a path into the
 .NET for Android workload installation. For further information see our
 documentation on [D8 and R8][d8-r8].
 
+## AndroidR8ObfuscationMode
+
+Selects how managed JNI references are reconciled with R8's obfuscated Java
+names. It is only used when
+[`$(AndroidEnableR8Obfuscation)`](#androidenabler8obfuscation) is `true`.
+The default is `runtime-remapping`.
+
+| Value | Behavior |
+|---|---|
+| `runtime-remapping` | Keeps managed assemblies unchanged and translates JNI type/member lookups using generated native remapping tables. Available for trimmed CoreCLR and NativeAOT applications. |
+| `experimental-rewriting` | Reserved for the separate managed-assembly rewriting implementation. This SDK does not yet include its build pipeline; selecting it reports [XA4329](../messages/xa4329.md). |
+
+The runtime-remapping mode leaves managed assemblies unchanged. It runs R8 once,
+after managed trimming or ILC, then uses the resulting R8 mapping to
+generate native runtime remapping tables. CoreCLR selects remaps from linked
+assemblies. NativeAOT selects remaps from retained JNI literals in ILC's native
+object and statically links the table afterward.
+
+Runtime-generated JNI names may require explicit remapping or keep rules.
+Conservative keep rules still protect native callbacks, bootstrap code, and
+resource-referenced names. Neither mode is selected as a fallback for another
+mode; unrecognized values report XA4329 when obfuscation is enabled.
+
+Added in .NET 11.
+
 ## AndroidResgenExtraArgs
 
 Specifies

Documentation/docs-mobile/messages/index.md

@@ -257,6 +257,9 @@ Either change the value in the AndroidManifest.xml to match the $(SupportedOSPla
 + [XA4324](xa4324.md): [{arch}] Unable to delete source file '{file}'.
 + [XA4325](xa4325.md): Failed to rewrite managed JNI names for R8. {message}
 + [XA4326](xa4326.md): Unable to safely rewrite a JNI member lookup because its class handle does not have one structurally unambiguous `JNIEnv.FindClass` source.
++ [XA4327](xa4327.md): Failed to generate the R8 JNI remapping data. {message}
++ [XA4328](xa4328.md): The R8 JNI remapping data is incomplete. {message}
++ [XA4329](xa4329.md): Invalid or unsupported R8 obfuscation configuration.
 
 ## XA5xxx: GCC and toolchain
 

Documentation/docs-mobile/messages/xa4327.md

@@ -0,0 +1,53 @@
+---
+title: .NET for Android error XA4327
+description: XA4327 error code
+ms.date: 09/04/2026
+f1_keywords:
+  - "XA4327"
+---
+
+# .NET for Android error XA4327
+
+## Example messages
+
+```
+error XA4327: Failed to generate the R8 JNI remapping data. The R8 mapping file 'obj/Release/net11.0/android-arm64/r8-jni-final-mapping.txt' was not found.
+```
+
+## Issue
+
+The build could not produce the data that lets the runtime translate the
+original JNI names in the managed assemblies into the names R8 chose.
+
+This only happens when R8 obfuscation is enabled with
+`$(AndroidEnableR8Obfuscation)=true` and
+`$(AndroidR8ObfuscationMode)=runtime-remapping`. The remapping is built from the
+mapping file produced by the final R8 pass after managed trimming or ILC. On
+NativeAOT, this also reports a missing or invalid ILC native object: remapping
+data is selected from the surviving JNI literals in that object before the final
+native link.
+
+NativeAOT filtering supports normal generated JNI bindings whose class names,
+member names, and descriptors are literal strings. It inspects the initialized
+data of the 32-bit or 64-bit ILC ELF object, including UTF-16 literals and UTF-8
+metadata. Shared strings can retain extra mappings; they do not make arbitrary
+runtime-constructed JNI names safe. JNI names or descriptors constructed at
+runtime require explicit remapping XML or R8 keep rules that preserve the
+affected Java types and members.
+
+## Solution
+
+The message names the specific file that is missing or unreadable.
+
+* Build with `-v:diag` (or check the binary log) for the output of the final R8
+  pass that should have produced the mapping file, and address any failure it reports.
+* Delete the `obj` directory and rebuild if the intermediate output is in an
+  inconsistent state.
+* For NativeAOT, ensure ILC completed and its `NativeObject` output exists before
+  remapping runs. Pre-ILC assemblies and dependency graphs cannot substitute for
+  that object. Missing or invalid retention data fails the build instead of
+  falling back to an unfiltered mapping.
+* If the failure persists, [report an issue][report-issue] and include the full
+  error, a binary log, and, if possible, a project that reproduces it.
+
+[report-issue]: https://github.com/dotnet/android/issues/new/choose

Documentation/docs-mobile/messages/xa4328.md

@@ -0,0 +1,44 @@
+---
+title: .NET for Android warning XA4328
+description: XA4328 warning code
+ms.date: 09/04/2026
+f1_keywords:
+  - "XA4328"
+---
+
+# .NET for Android warning XA4328
+
+## Example message
+
+```
+warning XA4328: The R8 JNI remapping data is incomplete. The 'replace-type' entry for 'T com/contoso/MainActivity' was not emitted: another JNI remapping input already maps it to 'com/contoso/Renamed', which conflicts with 'a/b'.
+```
+
+## Issue
+
+The R8 JNI runtime remapping is generated from the final R8 mapping file and is
+merged with every other JNI remapping input in the build, such as the Intune
+(MAM) mapping.
+
+An entry produced from the R8 mapping described the same type or member as an
+entry that another input already contributed, but mapped it somewhere else. The
+pre-existing input wins and the conflicting entry is not emitted.
+
+When the conflict is on a type, the type's reverse mapping and all of its
+members are left to the other input as well, so the type named in the message is
+not remapped for R8 at all.
+
+The warning is also emitted when a Java signature in the R8 mapping file cannot
+be converted to a JNI descriptor. That entry is skipped as well.
+
+## Solution
+
+Only one remapping input can own a given type or member.
+
+* If the app uses the Intune (MAM) mapping, exclude the affected types from the
+  R8 renaming with a `-keep` rule in a `@(ProguardConfiguration)` file so the
+  final R8 pass does not rename them.
+* If the conflict is unexpected, [report an issue][report-issue] and include the
+  full warning, the final R8 mapping file, and the other remapping input.
+
+[report-issue]: https://github.com/dotnet/android/issues/new/choose

Documentation/docs-mobile/messages/xa4329.md

@@ -0,0 +1,40 @@
+---
+title: .NET for Android error XA4329
+description: XA4329 error code
+ms.date: 09/05/2026
+f1_keywords:
+  - "XA4329"
+---
+
+# .NET for Android error XA4329
+
+## Example messages
+
+```
+Invalid value for AndroidEnableR8Obfuscation: 'yes'. Valid values are: true, false.
+```
+
+```
+AndroidR8ObfuscationMode='experimental-rewriting' is not available in this SDK. Use 'runtime-remapping' or set AndroidEnableR8Obfuscation=false.
+```
+
+## Issue
+
+An R8 obfuscation property has an invalid value, the selected mode is unavailable,
+or the application's build configuration is incompatible with obfuscation.
+
+## Solution
+
+Set `AndroidEnableR8Obfuscation` to `true` or `false`. When enabled, use
+`AndroidR8ObfuscationMode=runtime-remapping` (the default), `AndroidLinkTool=r8`,
+`AndroidTypeMapImplementation=trimmable`, and `PublishTrimmed=true` with CoreCLR
+or NativeAOT.
+
+The `experimental-rewriting` value is reserved for a separate implementation
+whose build pipeline is not included in this SDK. It does not fall back to
+runtime remapping. Setting a mode alone does not enable obfuscation.
+Runtime remapping does not rewrite managed assemblies; it uses the final R8
+mapping to generate runtime lookup tables after trimming or ILC.
+
+See [AndroidEnableR8Obfuscation](../building-apps/build-properties.md#androidenabler8obfuscation)
+and [AndroidR8ObfuscationMode](../building-apps/build-properties.md#androidr8obfuscationmode).

external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceFields.cs

@@ -27,8 +27,24 @@ public JniFieldInfo GetFieldInfo (string encodedMember)
 			return InstanceFields.GetOrAdd (encodedMember, static (member, fields) => {
 				string field, signature;
 				JniPeerMembers.GetNameAndSignature (member, out field, out signature);
-				return fields.Members.JniPeerType.GetInstanceField (field, signature);
+				return fields.GetFieldInfo (field, signature);
 			}, this);
 		}
+
+		JniFieldInfo GetFieldInfo (string field, string signature)
+		{
+			var newField = JniPeerMembers.GetReplacementFieldInfo (Members.JniPeerTypeName, Members.ManagedPeerType, field, signature);
+			if (newField.HasValue) {
+				var typeName     = newField.Value.TargetJniType ?? Members.JniPeerTypeName;
+				var fieldName    = newField.Value.TargetJniFieldName ?? field;
+				var fieldSig     = newField.Value.TargetJniFieldSignature ?? signature;
+
+				using var t = new JniType (typeName);
+				if (t.TryGetInstanceField (fieldName, fieldSig, out var f)) {
+					return f;
+				}
+			}
+			return Members.JniPeerType.GetInstanceField (field, signature);
+		}
 	}}
 }

external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs

@@ -24,12 +24,16 @@ internal JniInstanceMethods (JniPeerMembers members)
 							declaringType.FullName));
 
 			DeclaringType   = declaringType;
-			jniPeerType     = new JniType (info.Name);
+			targetJniTypeName   = info.Name;
+			jniPeerType     = new JniType (targetJniTypeName);
 			jniPeerType.RegisterWithRuntime ();
 		}
 
 		JniPeerMembers?                                     members;
 		JniType?                                            jniPeerType;
+		readonly string?                                    targetJniTypeName;
+
+		string TargetJniTypeName => targetJniTypeName ?? Members.JniPeerTypeName;
 
 		internal    JniPeerMembers                          Members => members ?? throw new InvalidOperationException ();
 
@@ -59,7 +63,23 @@ public JniMethodInfo GetConstructor (string signature)
 			if (signature == null)
 				throw new ArgumentNullException (nameof (signature));
 			return InstanceMethods.GetOrAdd (signature, static (member, methods) =>
-					methods.JniPeerType.GetConstructor (member), this);
+					methods.GetConstructorCore (member), this);
+		}
+
+		JniMethodInfo GetConstructorCore (string signature)
+		{
+			// Constructors are never renamed, but their parameter types can be, so the descriptor
+			// still has to be translated.
+			var newMethod = JniPeerMembers.GetReplacementMethodInfo (TargetJniTypeName, DeclaringType, "<init>", signature, searchBaseTypes: false);
+			var targetSignature = newMethod?.TargetJniMethodSignature;
+			if (targetSignature != null && !string.Equals (targetSignature, signature, StringComparison.Ordinal)) {
+				var typeName = newMethod?.TargetJniType ?? TargetJniTypeName;
+				using var t = new JniType (typeName);
+				if (t.TryGetInstanceMethod ("<init>", targetSignature, out var m)) {
+					return m;
+				}
+			}
+			return JniPeerType.GetConstructor (signature);
 		}
 
 		internal JniInstanceMethods GetConstructorsForType (Type declaringType)
@@ -104,9 +124,9 @@ public JniMethodInfo GetMethodInfo (string encodedMember)
 		JniMethodInfo GetMethodInfo (string method, string signature)
 		{
 			var m              = (JniMethodInfo?) null;
-			var newMethod      = JniEnvironment.Runtime.TypeManager.GetReplacementMethodInfo (Members.JniPeerTypeName, method, signature);
+			var newMethod      = JniPeerMembers.GetReplacementMethodInfo (TargetJniTypeName, DeclaringType, method, signature);
 			if (newMethod.HasValue) {
-				var typeName   = newMethod.Value.TargetJniType ?? Members.JniPeerTypeName;
+				var typeName   = newMethod.Value.TargetJniType ?? TargetJniTypeName;
 				var methodName = newMethod.Value.TargetJniMethodName ?? method;
 				var methodSig  = newMethod.Value.TargetJniMethodSignature ?? signature;
 
@@ -120,7 +140,7 @@ JniMethodInfo GetMethodInfo (string method, string signature)
 				if (t.TryGetInstanceMethod (methodName, methodSig, out m)) {
 					return m;
 				}
-				Console.Error.WriteLine ($"warning: For declared method `{Members.JniPeerTypeName}.{method}.{signature}`, could not find requested method `{typeName}.{methodName}.{methodSig}`!");
+				Console.Error.WriteLine ($"warning: For declared method `{TargetJniTypeName}.{method}.{signature}`, could not find requested method `{typeName}.{methodName}.{methodSig}`!");
 			}
 			return JniPeerType.GetInstanceMethod (method, signature);
 		}

external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniStaticFields.cs

@@ -22,10 +22,26 @@ public JniFieldInfo GetFieldInfo (string encodedMember)
 			return StaticFields.GetOrAdd (encodedMember, static (member, fields) => {
 				string field, signature;
 				JniPeerMembers.GetNameAndSignature (member, out field, out signature);
-				return fields.Members.JniPeerType.GetStaticField (field, signature);
+				return fields.GetFieldInfo (field, signature);
 			}, this);
 		}
 
+		JniFieldInfo GetFieldInfo (string field, string signature)
+		{
+			var newField = JniPeerMembers.GetReplacementFieldInfo (Members.JniPeerTypeName, Members.ManagedPeerType, field, signature);
+			if (newField.HasValue) {
+				var typeName     = newField.Value.TargetJniType ?? Members.JniPeerTypeName;
+				var fieldName    = newField.Value.TargetJniFieldName ?? field;
+				var fieldSig     = newField.Value.TargetJniFieldSignature ?? signature;
+
+				using var t = new JniType (typeName);
+				if (t.TryGetStaticField (fieldName, fieldSig, out var f)) {
+					return f;
+				}
+			}
+			return Members.JniPeerType.GetStaticField (field, signature);
+		}
+
 		internal void Dispose ()
 		{
 			StaticFields.Clear ();

external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniStaticMethods.cs

@@ -34,7 +34,7 @@ public JniMethodInfo GetMethodInfo (string encodedMember)
 		JniMethodInfo GetMethodInfo (string method, string signature)
 		{
 			var m              = (JniMethodInfo?) null;
-			var newMethod      = JniEnvironment.Runtime.TypeManager.GetReplacementMethodInfo (Members.JniPeerTypeName, method, signature);
+			var newMethod      = JniPeerMembers.GetReplacementMethodInfo (Members.JniPeerTypeName, Members.ManagedPeerType, method, signature);
 			if (newMethod.HasValue) {
 				using var t = new JniType (newMethod.Value.TargetJniType ?? Members.JniPeerTypeName);
 				if (t.TryGetStaticMethod (