[Java.Interop] Remove IJavaObject constraint on JavaObjectArray.

(Work in progress!)

Allow e.g. JavaObjectArray<object> (i.e. a non-IJavaObject type). This
requires the introduction of JavaProxyObject, which allows the JVM to
"hold onto" a non-IJavaObject type.

This in turn requires the generation of the new "java-interop.jar"
file, which MUST be included in the $CLASSPATH of all
Java.Interop-using JVMs.

TODO:

  * Support builtin types (JavaObjectArray<int> causes things to blow
    up, as JNIEnv::FindClass("I") throws),
  * "Full" collection marshaling (JavaObjectArray<int[]> should
    deep-marshal the array instead of using JavaProxyObject)

Note: Xamarin.Android's JavaProxyObject equivalent --
Android.Runtime.JavaObject -- overrides Java.Lang.Object.Clone().
JavaProxyObject does not, because ICloneable isn't part of the PCL
profile that Java.Interop is targeting, and thus there's no way to
reference the ICloneable.Clone() method w/o resorting to Reflection.

src/Java.Interop/Java.Interop.csproj

@@ -67,11 +67,17 @@
     <Compile Include="Java.Interop\JavaObjectArray.cs" />
     <Compile Include="Java.Interop\JniTypeInfoAttribute.cs" />
     <Compile Include="Java.Interop\JniTypeInfo.cs" />
+    <Compile Include="Java.Interop\JavaProxyObject.cs" />
+    <Compile Include="Java.Interop\JniMarshal.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <CompileJavaInteropJar Include="java\com\xamarin\android\internal\JavaProxyObject.java" />
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" />
   <PropertyGroup>
     <BuildDependsOn>
       BuildJniEnvironment_g_cs;
+      BuildInteropJar;
       $(BuildDependsOn)
     </BuildDependsOn>
   </PropertyGroup>
@@ -81,4 +87,12 @@
   <Target Name="BuildJniEnvironment_g_cs" Inputs="$(OutputPath)\jnienv-gen.exe" Outputs="Java.Interop\JniEnvironment.g.cs">
     <Exec Command="$(Runtime) &quot;$(OutputPath)\jnienv-gen.exe&quot; Java.Interop\JniEnvironment.g.cs" />
   </Target>
+  <ItemGroup>
+    <JavaInteropJar Include="$(OutputPath)java-interop.jar" />
+  </ItemGroup>
+  <Target Name="BuildInteropJar" Inputs="@(CompileJavaInteropJar)" Outputs="@(JavaInteropJar)">
+    <MakeDir Directories="$(OutputPath)ji-classes" />
+    <Exec Command="javac -d &quot;$(OutputPath)ji-classes&quot; @(CompileJavaInteropJar -&gt; '%(Identity)', ' ')" />
+    <Exec Command="jar cf &quot;$(OutputPath)java-interop.jar&quot; -C &quot;$(OutputPath)ji-classes&quot; ." />
+  </Target>
 </Project>

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

@@ -3,7 +3,6 @@
 namespace Java.Interop
 {
 	public class JavaObjectArray<T> : JavaArray<T>
-		where T : class, IJavaObject
 	{
 		public JavaObjectArray (JniReferenceSafeHandle handle, JniHandleOwnership transfer)
 			: base (handle, transfer)
@@ -25,23 +24,18 @@ public JavaObjectArray (int length)
 		{
 		}
 
-		// TODO: remove `IJavaObject` constraint
 		public override T this [int index] {
 			get {
 				if (index < 0 || index >= Length)
 					throw new ArgumentOutOfRangeException ("index", "index < 0 || index >= Length");
 				var lref = JniEnvironment.Arrays.GetObjectArrayElement (SafeHandle, index);
-				return (T) JniEnvironment.Current.JavaVM.GetObject (lref, JniHandleOwnership.Transfer, typeof (T));
+				return JniMarshal.GetValue<T> (lref, JniHandleOwnership.Transfer);
 			}
 			set {
 				if (index < 0 || index >= Length)
 					throw new ArgumentOutOfRangeException ("index", "index < 0 || index >= Length");
-				if (value != null && !value.SafeHandle.IsInvalid)
-					value.RegisterWithVM ();
-				JniEnvironment.Arrays.SetObjectArrayElement (SafeHandle, index,
-						value == null || value.SafeHandle.IsInvalid
-						? JniReferenceSafeHandle.Null
-						: value.SafeHandle);
+				using (var h = JniMarshal.CreateLocalRef (value))
+					JniEnvironment.Arrays.SetObjectArrayElement (SafeHandle, index, h);
 			}
 		}
 	}

src/Java.Interop/Java.Interop/JavaProxyObject.cs

@@ -0,0 +1,93 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace Java.Interop {
+
+	[JniTypeInfo (JavaProxyObject.JniTypeName)]
+	sealed class JavaProxyObject : JavaObject
+	{
+		internal const string JniTypeName = "com/xamarin/android/internal/JavaProxyObject";
+
+		static  readonly    JniType                                         TypeRef;
+		static  readonly    ConditionalWeakTable<object, JavaProxyObject>   CachedValues;
+
+		static JavaProxyObject ()
+		{
+			TypeRef = new JniType (JniTypeName);
+			TypeRef.RegisterWithVM ();
+			TypeRef.RegisterNativeMethods (
+					new JniNativeMethodRegistration ("equals",      "(Ljava/lang/Object;)Z",    (Func<IntPtr, IntPtr, IntPtr, bool>)    _Equals),
+					new JniNativeMethodRegistration ("hashCode",    "()I",                      (Func<IntPtr, IntPtr, int>)             _GetHashCode),
+					new JniNativeMethodRegistration ("toString",    "()Ljava/lang/String;",     (Func<IntPtr, IntPtr, IntPtr>)          _ToString));
+			CachedValues = new ConditionalWeakTable<object, JavaProxyObject> ();
+		}
+
+		JavaProxyObject (object value)
+		{
+			if (value == null)
+				throw new ArgumentNullException ("value");
+			Value = value;
+		}
+
+		public object Value {get; private set;}
+
+		public override int GetHashCode ()
+		{
+			return Value.GetHashCode ();
+		}
+
+		public override bool Equals (object obj)
+		{
+			var other = obj as JavaProxyObject;
+			if (other != null)
+				return object.Equals (Value, other.Value);
+			return object.Equals (Value, obj);
+		}
+
+		public override string ToString ()
+		{
+			return Value.ToString ();
+		}
+
+		public static JavaProxyObject GetProxy (object value)
+		{
+			if (value == null)
+				return null;
+
+			lock (CachedValues) {
+				JavaProxyObject proxy;
+				if (CachedValues.TryGetValue (value, out proxy))
+					return proxy;
+				proxy = new JavaProxyObject (value);
+				proxy.RegisterWithVM ();
+				CachedValues.Add (value, proxy);
+				return proxy;
+			}
+		}
+
+		static bool _Equals (IntPtr jnienv, IntPtr n_self, IntPtr n_value)
+		{
+			JniEnvironment.CheckCurrent (jnienv);
+			var self    = JniEnvironment.Current.JavaVM.GetObject<JavaProxyObject> (n_self);
+			var value   = JniEnvironment.Current.JavaVM.GetObject (n_value);
+			return self.Equals (value);
+		}
+
+		static int _GetHashCode (IntPtr jnienv, IntPtr n_self)
+		{
+			JniEnvironment.CheckCurrent (jnienv);
+			var self = JniEnvironment.Current.JavaVM.GetObject<JavaProxyObject> (n_self);
+			return self.GetHashCode ();
+		}
+
+		static IntPtr _ToString (IntPtr jnienv, IntPtr n_self)
+		{
+			JniEnvironment.CheckCurrent (jnienv);
+			var self    = JniEnvironment.Current.JavaVM.GetObject<JavaProxyObject> (n_self);
+			var s       = self.ToString ();
+			using (var r = JniEnvironment.Strings.NewString (s))
+				return JniEnvironment.Handles.NewReturnToJniRef (r);
+		}
+	}
+}
+

src/Java.Interop/Java.Interop/JniMarshal.cs

@@ -0,0 +1,26 @@
+using System;
+
+namespace Java.Interop {
+
+	static class JniMarshal {
+
+		public static T GetValue<T> (JniReferenceSafeHandle handle, JniHandleOwnership transfer)
+		{
+			var target  = JniEnvironment.Current.JavaVM.GetObject (handle, transfer, typeof (T));
+			var proxy   = target as JavaProxyObject;
+			if (proxy != null)
+				return (T) proxy.Value;
+			return (T) target;
+		}
+
+		public static JniLocalReference CreateLocalRef<T> (T value)
+		{
+			var o = (value as IJavaObject) ??
+				JavaProxyObject.GetProxy (value);
+			if (o == null || o.SafeHandle.IsInvalid)
+				return new JniLocalReference ();
+			return o.SafeHandle.NewLocalRef ();
+		}
+	}
+}
+

src/Java.Interop/Tests/Java.Interop/JavaObjectArrayTest.cs

@@ -7,8 +7,7 @@
 
 namespace Java.InteropTests
 {
-	public class JavaObjectArrayContractTest<T> : JavaArrayContract<T>
-		where T : class, IJavaObject, new()
+	public abstract class JavaObjectArrayContractTest<T> : JavaArrayContract<T>
 	{
 		protected override System.Collections.Generic.ICollection<T> CreateCollection (System.Collections.Generic.IEnumerable<T> values)
 		{
@@ -18,25 +17,27 @@ protected override System.Collections.Generic.ICollection<T> CreateCollection (S
 				array [i] = items [i];
 			return array;
 		}
+	}
 
-		protected override T CreateValueA ()
-		{
-			return new T ();
-		}
-
-		protected override T CreateValueB ()
-		{
-			return new T ();
-		}
+	[TestFixture]
+	public class JavaObjectArrayContractTest : JavaObjectArrayContractTest<JavaObject> {
+		protected override JavaObject CreateValueA () {return new JavaObject ();}
+		protected override JavaObject CreateValueB () {return new JavaObject ();}
+		protected override JavaObject CreateValueC () {return new JavaObject ();}
+	}
 
-		protected override T CreateValueC ()
-		{
-			return new T ();
-		}
+	[TestFixture]
+	public class JavaObjectArray_string_ContractTest : JavaObjectArrayContractTest<string> {
+		protected override string CreateValueA () {return "a";}
+		protected override string CreateValueB () {return "b";}
+		protected override string CreateValueC () {return "c";}
 	}
 
 	[TestFixture]
-	public class JavaObjectArrayContractTest : JavaObjectArrayContractTest<JavaObject> {
+	public class JavaObjectArray_object_ContractTest : JavaObjectArrayContractTest<object> {
+		protected override object CreateValueA () {return new object ();}
+		protected override object CreateValueB () {return new object ();}
+		protected override object CreateValueC () {return new object ();}
 	}
 }
 

src/Java.Interop/java/com/xamarin/android/internal/JavaProxyObject.java

@@ -0,0 +1,13 @@
+package com.xamarin.android.internal;
+
+/* package */ class JavaProxyObject extends java.lang.Object {
+
+	@Override
+	public native boolean equals(Object obj);
+
+	@Override
+	public native int hashCode();
+
+	@Override
+	public native String toString();
+}

tests/TestJVM/TestJVM.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Diagnostics;
 using System.Linq;
@@ -16,7 +17,11 @@ public class TestJVM : JreVM {
 		static JreVMBuilder CreateBuilder (string[] jars)
 		{
 			var builder = new JreVMBuilder ();
-			builder.AddSystemProperty ("java.class.path", string.Join (":", jars));
+			var _jars   = new List<string> (jars) {
+				"java-interop.jar",
+			};
+			_jars.AddRange (jars);
+			builder.AddSystemProperty ("java.class.path", string.Join (":", _jars));
 			return builder;
 		}