[Java.Interop] Lazily allocate instance method cache

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: simonrozsival

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

@@ -2,6 +2,7 @@
 
 using System;
 using System.Collections.Concurrent;
+using System.Threading;
 
 namespace Java.Interop
 {
@@ -39,12 +40,12 @@ internal    JniType                                 JniPeerType {
 
 		readonly Type                                       DeclaringType;
 
-		readonly ConcurrentDictionary<string, JniMethodInfo>    InstanceMethods      = new ConcurrentDictionary<string, JniMethodInfo> (1, 3, StringComparer.Ordinal);
+		ConcurrentDictionary<string, JniMethodInfo>?             InstanceMethods;
 		readonly ConcurrentDictionary<Type, JniInstanceMethods> SubclassConstructors = new ConcurrentDictionary<Type, JniInstanceMethods> (1, 1);
 
 		internal void Dispose ()
 		{
-			InstanceMethods.Clear ();
+			Interlocked.Exchange (ref InstanceMethods, null)?.Clear ();
 			foreach (var p in SubclassConstructors.Values)
 				p.Dispose ();
 			SubclassConstructors.Clear ();
@@ -58,7 +59,7 @@ public JniMethodInfo GetConstructor (string signature)
 		{
 			if (signature == null)
 				throw new ArgumentNullException (nameof (signature));
-			return InstanceMethods.GetOrAdd (signature, static (member, methods) =>
+			return GetInstanceMethods ().GetOrAdd (signature, static (member, methods) =>
 					methods.JniPeerType.GetConstructor (member), this);
 		}
 
@@ -94,13 +95,23 @@ internal JniInstanceMethods GetConstructorsForType (Type declaringType)
 
 		public JniMethodInfo GetMethodInfo (string encodedMember)
 		{
-			return InstanceMethods.GetOrAdd (encodedMember, static (member, methods) => {
+			return GetInstanceMethods ().GetOrAdd (encodedMember, static (member, methods) => {
 				string method, signature;
 				JniPeerMembers.GetNameAndSignature (member, out method, out signature);
 				return methods.GetMethodInfo (method, signature);
 			}, this);
 		}
 
+		ConcurrentDictionary<string, JniMethodInfo> GetInstanceMethods ()
+		{
+			var methods = Volatile.Read (ref InstanceMethods);
+			if (methods != null)
+				return methods;
+
+			var candidate = new ConcurrentDictionary<string, JniMethodInfo> (1, 3, StringComparer.Ordinal);
+			return Interlocked.CompareExchange (ref InstanceMethods, candidate, null) ?? candidate;
+		}
+
 		JniMethodInfo GetMethodInfo (string method, string signature)
 		{
 			var m              = (JniMethodInfo?) null;

external/Java.Interop/tests/Java.Interop-Tests/Java.Interop/JniPeerMembersTests.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Reflection;
+using System.Threading.Tasks;
 
 using Java.Interop;
 using NUnit.Framework;
@@ -21,15 +22,35 @@ public void Ctor_CanReferenceNonexistentType ()
 		[Category ("TrimmableTypeMapUnsupported")]
 		public void VirtualInvokeOnBaseInvokesMostDerivedJavaMethod ()
 		{
-			var registered  = GetInstanceMethods (MyString._members.InstanceMethods);
-			Assert.AreEqual (0, registered.Count);
+			Assert.IsNull (GetInstanceMethods (MyString._members.InstanceMethods));
 			using (var s = new MyString ("hello!")) {
+				var registered = GetInstanceMethods (MyString._members.InstanceMethods);
 				Assert.AreEqual (1, registered.Count);  // for the constructor
 				Assert.AreEqual ("hello!", s.ToString ());
 				Assert.AreEqual (1, registered.Count);
 			}
 		}
 
+		[Test]
+		[Category ("TrimmableTypeMapUnsupported")]
+		public void ConcurrentFirstUsePublishesSingleInstanceMethodCache ()
+		{
+			var members = new JniPeerMembers (MyString.JniTypeName, typeof (MyString));
+			var methods = members.InstanceMethods;
+			var constructors = new JniMethodInfo [16];
+
+			Assert.IsNull (GetInstanceMethods (methods));
+			Parallel.For (0, constructors.Length, i => constructors [i] = methods.GetConstructor ("()V"));
+
+			var registered = GetInstanceMethods (methods);
+			Assert.AreEqual (1, registered.Count);
+			foreach (var constructor in constructors)
+				Assert.AreSame (constructors [0], constructor);
+			Assert.AreSame (registered ["()V"], constructors [0]);
+
+			JniPeerMembers.Dispose (members);
+		}
+
 		static ConcurrentDictionary<string, JniMethodInfo> GetInstanceMethods (JniPeerMembers.JniInstanceMethods methods)
 		{
 			var f   = typeof (JniPeerMembers.JniInstanceMethods).GetField ("InstanceMethods", BindingFlags.NonPublic | BindingFlags.Instance);