Implement efficient access to java.nio.ByteBuffer

jni/lib/src/jni.dart

@@ -5,6 +5,7 @@
 import 'dart:ffi';
 import 'dart:io';
 import 'dart:isolate';
+import 'dart:typed_data' as typed_data;
 
 import 'package:ffi/ffi.dart';
 import 'package:path/path.dart';
@@ -350,6 +351,28 @@ extension AdditionalEnvMethods on GlobalJniEnv {
     return result;
   }
 
+  typed_data.Uint8List _asUInt8List(JObjectPtr jbuf) {
+    final addr = GetDirectBufferAddress(jbuf);
+    if (addr == nullptr) {
+      throw ArgumentError();
+    }
+
+    final capacity = GetDirectBufferCapacity(jbuf);
+    if (capacity == -1) {
+      throw ArgumentError();
+    }
+
+    return addr.cast<Uint8>().asTypedList(capacity);
+  }
+
+  typed_data.Uint8List asUInt8List(JObjectPtr jbuf, [int start = 0, int? end]) {
+    return _asUInt8List(jbuf).sublist(start, end);
+  }
+
+  void setRange(JObjectPtr jbuf, List<int> l, int start, int end) {
+    _asUInt8List(jbuf).setRange(start, end, l);
+  }
+
   /// Returns a new [JStringPtr] from contents of [s].
   JStringPtr toJStringPtr(String s) => using((arena) {
         final utf = s.toNativeUtf16(allocator: arena).cast<Uint16>();

jni/pubspec.yaml

@@ -28,6 +28,7 @@ dependencies:
 
 dev_dependencies:
   ## Pin ffigen version because we are depending on internal APIs.
+  benchmarking: ^0.6.1
   ffigen: 8.0.2
   flutter_lints: ^2.0.0
   test: ^1.21.1

jni/test/global_env_test.dart

@@ -3,7 +3,9 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
+import 'dart:typed_data';
 
+import 'package:benchmarking/benchmarking.dart';
 import 'package:jni/jni.dart';
 import 'package:jni/src/jvalues.dart';
 import 'package:test/test.dart';
@@ -12,6 +14,34 @@ import 'test_util/test_util.dart';
 
 const maxLongInJava = 9223372036854775807;
 
+abstract class A extends SyncBenchmark {
+  late JObject buffer;
+  @override
+  void setup() {
+    buffer = Jni.invokeStaticMethod<JObject>("java/nio/ByteBuffer",
+        'allocateDirect', '(I)Ljava/nio/ByteBuffer;', [JValueInt(1000000)]);
+  }
+}
+
+class B extends A {
+  @override
+  void run() {
+    Jni.env.asUInt8List(buffer.reference);
+  }
+}
+
+class C extends A {
+  @override
+  void run() {
+    final length = buffer.callMethodByName<int>('capacity', '()I', []);
+
+    final l = Uint8List(length);
+    for (var i = 0; i < length; ++i) {
+      l[i] = buffer.callMethodByName<int>('get', '(I)B', [JValueInt(i)]);
+    }
+  }
+}
+
 void main() {
   // Running on Android through flutter, this plugin
   // will bind to Android runtime's JVM.
@@ -28,6 +58,8 @@ void main() {
     checkDylibIsUpToDate();
     Jni.spawnIfNotExists(dylibDir: "build/jni_libs", jvmOptions: ["-Xmx128m"]);
   }
+  B().measure().report();
+  C().measure().report();
   run(testRunner: test);
 }
 
@@ -50,6 +82,25 @@ void run({required TestRunnerCallback testRunner}) {
     Jni.initDLApi();
   });
 
+  testRunner('test', () {
+    final m = Jni.invokeStaticMethod<JObject>("java/nio/ByteBuffer",
+        'allocateDirect', '(I)Ljava/nio/ByteBuffer;', [JValueInt(4)]);
+
+    m.callMethodByName<JObject>(
+        'put', '(IB)Ljava/nio/ByteBuffer;', [JValueInt(2), JValueByte(29)]);
+    expect(Jni.env.asUInt8List(m.reference), equals([0, 0, 29, 0]));
+  });
+
+  testRunner('test2', () {
+    final m = Jni.invokeStaticMethod<JObject>("java/nio/ByteBuffer",
+        'allocateDirect', '(I)Ljava/nio/ByteBuffer;', [JValueInt(4)]);
+
+    m.callMethodByName<JObject>(
+        'put', '(IB)Ljava/nio/ByteBuffer;', [JValueInt(2), JValueByte(29)]);
+    Jni.env.setRange(m.reference, [1, 2, 3], 0, 2);
+    expect(Jni.env.asUInt8List(m.reference), equals([1, 2, 29, 0]));
+  });
+
   testRunner('get JNI Version', () {
     expect(Jni.env.GetVersion(), isNot(equals(0)));
   });