[generator] Add nullable reference types (NRT) support. (#563)

Fixes: #468
Context: dotnet/android#4227
Add [C#8 nullable reference type][0] (NRT) support to `generator` when
given `generator -lang-features=nullable-reference-types`. This uses a
variety of Java annotations to infer nullable information (18c29b7)
via the `//method/@return-not-null` and `//parameter/@not-null`
attribute values within `api.xml` to "forward" nullability information
to the generated C# binding.
~~ Goals ~~
`generator` should be able to interpret the nullable annotations
provided by an input `.jar` file (via `class-parse`). It should use this information to generate bindings that expose similar nullable
annotations on the produced public C# API.
For example, this Java:
// Java
public class Foo {
public void bar (@NotNull Object baz, String value) { … }
}
Should generate this C# API:
// C# Binding
public class Foo : Java.Lang.Object {
public void Bar (Java.Lang.Object baz, string? value) { … }
}
Additionally, the generated binding code should not produce any
additional warnings *on its own*. That is, the internal plumbing code
itself should not create warnings.
~~ Non-Goals ~~
There exists cases in our generated plumbing code that do not play
nicely with the provability of C#8 nullable reference types.
For example, we may generate code like this:
int Java.Lang.IComparable.CompareTo (Java.Lang.Object o)
{
return CompareTo (global::Java.Interop.JavaObjectExtensions.JavaCast<Android.Util.Half>(o));
}
Technically `.JavaCast<>()` can return `null`, which cannot be passed
to `.CompareTo (object o)` because it does not accept `null`. In these
cases we liberally use the [null forgiving operator (`!`)][1] to
suppress warnings. It may be desirable to change how this code is
structured to be better provably `null`-safe, however this PR does not
attempt to make those modification. It is assumed that the code is
currently working, so `null` is prevented here via other mechanisms.
No functional changes are made to generated code.
Additionally, there are cases where Java nullable annotations can
create scenarios that will produce warnings in C#, particularly around
inheritance. For example:
// Java
public class Base {
public void m (@NotNull Object baz) { … }
}
public class Derived extends Base {
@OverRide public void m (Object baz) { … }
}
This would produce a C# warning such as:
CS8610: Nullability of reference types in type of parameter 'M' doesn't match overridden member. `generator` will not attempt to resolve this error, it is an exercise
for the user. This can be accomplished by fixing the Java code or
using `metadata` to override the `//@not-null` attribute such as:
<attr path="/api/package[@name='blah']/class[@name='Foo2']/method[@name='Bar' and count(parameter)=1 and parameter[1][@type='object']]/parameter" name="not-null">true</attr>
~~ Unit Test Changes ~~
Several of the unit test "expected output" files changed their property
type from `java.lang.String` to `string`. This occurred due to a
related refactoring of parameter & return type generation code. This
change shouldn't be "user visible" because the unit tests don't go
through a "complete" pipeline which would involve ensuring that get-
and set-method pairs have consistent parameter & return types.
[0]: https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references
[1]: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-forgiving

Author: jpobst

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers.csproj

@@ -2,6 +2,8 @@
 
  <PropertyGroup>
  <TargetFramework>netstandard2.0</TargetFramework>
+ <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
+ <LangVersion>8.0</LangVersion>
  <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>
 
@@ -23,6 +25,9 @@
  <CompileInclude="..\Java.Interop.Tools.TypeNameMappings\Java.Interop.Tools.TypeNameMappings\JavaNativeTypeManager.cs">
  <Link>JavaNativeTypeManager.cs</Link>
  </Compile>
+ <CompileInclude="..\Java.Interop\NullableAttributes.cs">
+ <Link>NullableAttributes.cs</Link>
+ </Compile>
  </ItemGroup>
 
  <ItemGroup>

src/Java.Interop.Tools.TypeNameMappings/Java.Interop.Tools.TypeNameMappings/JavaNativeTypeManager.cs

@@ -19,6 +19,7 @@
  <DefineConstants>DEBUG;$(DefineConstants)</DefineConstants>
  </PropertyGroup>
  <ItemGroup>
+ <CompileCondition=" '$(TargetFramework)' != 'netstandard2.0' "Remove="NullableAttributes.cs" />
  <CompileRemove="Java.Interop\JniLocationException.cs" />
  </ItemGroup>
  <PropertyGroup>

src/Java.Interop/Java.Interop.csproj

@@ -158,7 +158,7 @@ public override IList<T> CreateGenericValue (ref JniObjectReference reference, J
 });
 }
 
-publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState(IList<T>value,ParameterAttributessynchronize)
+publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState([MaybeNull]IList<T>value,ParameterAttributessynchronize)
 {
 returnJavaArray<T>.CreateArgumentState(value,synchronize,(list,copy)=>{
 vara=copy
@@ -169,7 +169,7 @@ public override JniValueMarshalerState CreateGenericObjectReferenceArgumentState
 });
 }
 
-publicoverridevoidDestroyGenericArgumentState(IList<T>value,refJniValueMarshalerStatestate,ParameterAttributessynchronize)
+publicoverridevoidDestroyGenericArgumentState([AllowNull]IList<T>value,refJniValueMarshalerStatestate,ParameterAttributessynchronize)
 {
 JavaArray<T>.DestroyArgumentState<JavaObjectArray<T>>(value,refstate,synchronize);
 }

src/Java.Interop/Java.Interop/JavaObjectArray.cs

@@ -2,6 +2,7 @@
 
 usingSystem;
 usingSystem.Collections.Generic;
+usingSystem.Diagnostics.CodeAnalysis;
 usingSystem.Linq;
 usingSystem.Linq.Expressions;
 usingSystem.Reflection;
@@ -203,7 +204,7 @@ public override JniValueMarshalerState CreateArgumentState (object? value, Param
 thrownewNotSupportedException();
 }
 
-publicoverrideJniValueMarshalerStateCreateGenericArgumentState(IntPtrvalue,ParameterAttributessynchronize)
+publicoverrideJniValueMarshalerStateCreateGenericArgumentState([MaybeNull]IntPtrvalue,ParameterAttributessynchronize)
 {
 thrownewNotSupportedException();
 }
@@ -213,7 +214,7 @@ public override JniValueMarshalerState CreateObjectReferenceArgumentState (objec
 thrownewNotSupportedException();
 }
 
-publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState(IntPtrvalue,ParameterAttributessynchronize)
+publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState([MaybeNull]IntPtrvalue,ParameterAttributessynchronize)
 {
 thrownewNotSupportedException();
 }

src/Java.Interop/Java.Interop/JniRuntime.JniMarshalMemberBuilder.cs

@@ -155,7 +155,7 @@ static Type GetUnderlyingType (Type type, out int rank)
 }
 
 // `type` will NOT be an array type.
-protectedvirtualstringGetSimpleReference(Typetype)
+protectedvirtualstring?GetSimpleReference(Typetype)
 {
 returnGetSimpleReferences(type).FirstOrDefault();
 }

src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

@@ -199,7 +199,7 @@ public virtual void DisposePeerUnlessReferenced (IJavaPeerable value)
 DisposePeer(h,value);
 }
 
-publicabstractIJavaPeerablePeekPeer(JniObjectReferencereference);
+publicabstractIJavaPeerable?PeekPeer(JniObjectReferencereference);
 
 publicobject?PeekValue(JniObjectReferencereference)
 {
@@ -261,7 +261,7 @@ static Type GetPeerType (Type type)
 returntype;
 }
 
-publicvirtualIJavaPeerableCreatePeer(refJniObjectReferencereference,JniObjectReferenceOptionstransfer,Type?targetType)
+publicvirtualIJavaPeerable?CreatePeer(refJniObjectReferencereference,JniObjectReferenceOptionstransfer,Type?targetType)
 {
 if(disposed)
 thrownewObjectDisposedException(GetType().Name);
@@ -396,7 +396,9 @@ public T CreateValue<T> (ref JniObjectReference reference, JniObjectReferenceOpt
 targetType=targetType??typeof(T);
 
 if(typeof(IJavaPeerable).IsAssignableFrom(targetType)){
+#pragma warning disable CS8601// Possible null reference assignment.
 return(T)JavaPeerableValueMarshaler.Instance.CreateGenericValue(refreference,options,targetType);
+#pragma warning restore CS8601// Possible null reference assignment.
 }
 
 varmarshaler=GetValueMarshaler<T>();
@@ -473,7 +475,9 @@ public T GetValue<T> (ref JniObjectReference reference, JniObjectReferenceOption
 }
 
 if(typeof(IJavaPeerable).IsAssignableFrom(targetType)){
+#pragma warning disable CS8601// Possible null reference assignment.
 return(T)JavaPeerableValueMarshaler.Instance.CreateGenericValue(refreference,options,targetType);
+#pragma warning restore CS8601// Possible null reference assignment.
 }
 
 varmarshaler=GetValueMarshaler<T>();
@@ -607,12 +611,12 @@ public override void DestroyArgumentState (object? value, ref JniValueMarshalerS
 }
 }
 
-sealedclassJavaPeerableValueMarshaler:JniValueMarshaler<IJavaPeerable>{
+sealedclassJavaPeerableValueMarshaler:JniValueMarshaler<IJavaPeerable?>{
 
 internalstaticJavaPeerableValueMarshalerInstance=newJavaPeerableValueMarshaler();
 
 [return:MaybeNull]
-publicoverrideIJavaPeerableCreateGenericValue(refJniObjectReferencereference,JniObjectReferenceOptionsoptions,Type?targetType)
+publicoverrideIJavaPeerable?CreateGenericValue(refJniObjectReferencereference,JniObjectReferenceOptionsoptions,Type?targetType)
 {
 varjvm=JniEnvironment.Runtime;
 varmarshaler=jvm.ValueManager.GetValueMarshaler(targetType??typeof(IJavaPeerable));
@@ -621,15 +625,15 @@ public override IJavaPeerable CreateGenericValue (ref JniObjectReference referen
 returnjvm.ValueManager.CreatePeer(refreference,options,targetType);
 }
 
-publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState(IJavaPeerablevalue,ParameterAttributessynchronize)
+publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState([MaybeNull]IJavaPeerable?value,ParameterAttributessynchronize)
 {
 if(value==null||!value.PeerReference.IsValid)
 returnnewJniValueMarshalerState();
 varr=value.PeerReference.NewLocalRef();
 returnnewJniValueMarshalerState(r);
 }
 
-publicoverridevoidDestroyGenericArgumentState(IJavaPeerablevalue,refJniValueMarshalerStatestate,ParameterAttributessynchronize)
+publicoverridevoidDestroyGenericArgumentState([MaybeNull]IJavaPeerable?value,refJniValueMarshalerStatestate,ParameterAttributessynchronize)
 {
 varr=state.ReferenceValue;
 JniObjectReference.Dispose(refr);
@@ -694,12 +698,12 @@ public override T CreateGenericValue (ref JniObjectReference reference, JniObjec
 return(T)ValueMarshaler.CreateValue(refreference,options,targetType??typeof(T))!;
 }
 
-publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState(Tvalue,ParameterAttributessynchronize)
+publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState([MaybeNull]Tvalue,ParameterAttributessynchronize)
 {
 returnValueMarshaler.CreateObjectReferenceArgumentState(value,synchronize);
 }
 
-publicoverridevoidDestroyGenericArgumentState(Tvalue,refJniValueMarshalerStatestate,ParameterAttributessynchronize)
+publicoverridevoidDestroyGenericArgumentState([AllowNull]Tvalue,refJniValueMarshalerStatestate,ParameterAttributessynchronize)
 {
 ValueMarshaler.DestroyArgumentState(value,refstate,synchronize);
 }
@@ -720,12 +724,12 @@ public override Expression CreateReturnValueFromManagedExpression (JniValueMarsh
 }
 }
 
-sealedclassProxyValueMarshaler:JniValueMarshaler<object>{
+sealedclassProxyValueMarshaler:JniValueMarshaler<object?>{
 
 internalstaticProxyValueMarshalerInstance=newProxyValueMarshaler();
 
 [return:MaybeNull]
-publicoverrideobjectCreateGenericValue(refJniObjectReferencereference,JniObjectReferenceOptionsoptions,Type?targetType)
+publicoverrideobject?CreateGenericValue(refJniObjectReferencereference,JniObjectReferenceOptionsoptions,Type?targetType)
 {
 varjvm=JniEnvironment.Runtime;
 
@@ -748,7 +752,7 @@ public override object CreateGenericValue (ref JniObjectReference reference, Jni
 returnjvm.ValueManager.CreatePeer(refreference,options,targetType);
 }
 
-publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState(objectvalue,ParameterAttributessynchronize)
+publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState([MaybeNull]object?value,ParameterAttributessynchronize)
 {
 if(value==null)
 returnnewJniValueMarshalerState();
@@ -765,7 +769,7 @@ public override JniValueMarshalerState CreateGenericObjectReferenceArgumentState
 returnnewJniValueMarshalerState(p!.PeerReference.NewLocalRef());
 }
 
-publicoverridevoidDestroyGenericArgumentState(objectvalue,refJniValueMarshalerStatestate,ParameterAttributessynchronize)
+publicoverridevoidDestroyGenericArgumentState(object?value,refJniValueMarshalerStatestate,ParameterAttributessynchronize)
 {
 varvm=state.ExtraasJniValueMarshaler;
 if(vm!=null){

src/Java.Interop/Java.Interop/JniRuntime.JniValueManager.cs

@@ -1,6 +1,7 @@
 #nullable enable
 
 usingSystem;
+usingSystem.Diagnostics.CodeAnalysis;
 usingSystem.Linq.Expressions;
 usingSystem.Reflection;
 usingSystem.Runtime.CompilerServices;
@@ -18,7 +19,7 @@ sealed class JniStringValueMarshaler : JniValueMarshaler<string?> {
 returnJniEnvironment.Strings.ToString(refreference,options,targetType??typeof(string));
 }
 
-publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState(string?value,ParameterAttributessynchronize)
+publicoverrideJniValueMarshalerStateCreateGenericObjectReferenceArgumentState([MaybeNull]string?value,ParameterAttributessynchronize)
 {
 varr=JniEnvironment.Strings.NewString(value);
 returnnewJniValueMarshalerState(r);

src/Java.Interop/Java.Interop/JniStringValueMarshaler.cs

@@ -5,7 +5,7 @@ int Count {
 	[Register ("set_Count", "(I)V", "Getset_Count_IHandler:java.code.IMyInterfaceInvoker, ")] set;
 }
 
-java.lang.String Key {
+string Key {
 	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='get_Key' and count(parameter)=0]"
 	[Register ("get_Key", "()Ljava/lang/String;", "Getget_KeyHandler:java.code.IMyInterfaceInvoker, ")] get;
 	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='set_Key' and count(parameter)=1 and parameter[1][@type='java.lang.String']]"

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/Common/WriteInterfaceProperties.txt

@@ -39,7 +39,7 @@ public partial interface IMyInterface : IJavaObject, IJavaPeerable {
  [Register ("set_Count", "(I)V", "Getset_Count_IHandler:java.code.IMyInterfaceInvoker, ")] set;
 	}
 
-java.lang.String Key {
+string Key {
  // Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='get_Key' and count(parameter)=0]"
  [Register ("get_Key", "()Ljava/lang/String;", "Getget_KeyHandler:java.code.IMyInterfaceInvoker, ")] get;
  // Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='set_Key' and count(parameter)=1 and parameter[1][@type='java.lang.String']]"