Fix buffer overflow issue when converting strings from JNI to Dart

jni/lib/src/jni.dart

@@ -330,8 +330,7 @@ extension ProtectedJniExtensions on Jni {
 }
 
 extension AdditionalEnvMethods on GlobalJniEnv {
-  /// Convenience method for converting a [JStringPtr]
-  /// to dart string.
+  /// Convenience method for converting a [JStringPtr] to dart string.
   /// if [releaseOriginal] is specified, jstring passed will be deleted using
   /// DeleteGlobalRef.
   String toDartString(JStringPtr jstringPtr, {bool releaseOriginal = false}) {
@@ -342,7 +341,8 @@ extension AdditionalEnvMethods on GlobalJniEnv {
     if (chars == nullptr) {
       throw ArgumentError('Not a valid jstring pointer.');
     }
-    final result = chars.cast<Utf16>().toDartString();
+    final length = GetStringLength(jstringPtr);
+    final result = chars.cast<Utf16>().toDartString(length: length);
     ReleaseStringChars(jstringPtr, chars);
     if (releaseOriginal) {
       DeleteGlobalRef(jstringPtr);
@@ -377,7 +377,7 @@ extension StringMethodsForJni on String {
 
 extension CharPtrMethodsForJni on Pointer<Char> {
   /// Same as calling `cast<Utf8>` followed by `toDartString`.
-  String toDartString() {
-    return cast<Utf8>().toDartString();
+  String toDartString({int? length}) {
+    return cast<Utf8>().toDartString(length: length);
   }
 }

jni/lib/src/lang/jstring.dart

@@ -59,10 +59,7 @@ class JString extends JObject {
   /// after conversion and this object will be marked as released.
   String toDartString({bool releaseOriginal = false}) {
     ensureNotNull();
-    final length = Jni.env.GetStringLength(reference);
-    final chars = Jni.env.GetStringChars(reference, nullptr);
-    final result = chars.cast<Utf16>().toDartString(length: length);
-    Jni.env.ReleaseStringChars(reference, chars);
+    final result = Jni.env.toDartString(reference);
     if (releaseOriginal) {
       release();
     }

jni/test/global_env_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:io';
 
+import 'package:ffi/ffi.dart';
 import 'package:jni/jni.dart';
 import 'package:jni/src/jvalues.dart';
 import 'package:test/test.dart';
@@ -118,18 +119,31 @@ void run({required TestRunnerCallback testRunner}) {
           }));
 
   testRunner(
-      "Convert back & forth between Dart & Java strings",
+      "Convert back & forth between Dart & Java strings (UTF-8)",
       () => using((arena) {
             const str = "ABCD EFGH";
-            // This is what asJString and asDartString do internally
             final jstr = env.NewStringUTF(str.toNativeChars(arena));
             final jchars = env.GetStringUTFChars(jstr, nullptr);
-            final dstr = jchars.toDartString();
+            final jlen = env.GetStringUTFLength(jstr);
+            final dstr = jchars.toDartString(length: jlen);
             env.ReleaseStringUTFChars(jstr, jchars);
             expect(str, equals(dstr));
             env.DeleteGlobalRef(jstr);
           }));
 
+  testRunner(
+      "Convert back & forth between Dart & Java strings (UTF-16)",
+      () => using((arena) {
+            const str = "ABCD EFGH";
+            final jstr = env.NewString(str.toNativeUtf16().cast(), str.length);
+            final jchars = env.GetStringChars(jstr, nullptr);
+            final jlen = env.GetStringLength(jstr);
+            final dstr = jchars.cast<Utf16>().toDartString(length: jlen);
+            env.ReleaseStringChars(jstr, jchars);
+            expect(str, equals(dstr));
+            env.DeleteGlobalRef(jstr);
+          }));
+
   testRunner(
       "Print something from Java",
       () => using((arena) {