diff --git a/jni/java/src/main/java/com/github/dart_lang/jni/PortProxy.java b/jni/java/src/main/java/com/github/dart_lang/jni/PortProxy.java
index a75a914f..bae1c325 100644
--- a/jni/java/src/main/java/com/github/dart_lang/jni/PortProxy.java
+++ b/jni/java/src/main/java/com/github/dart_lang/jni/PortProxy.java
@@ -62,21 +62,28 @@ private static void appendType(StringBuilder descriptor, Class<?> type) {
 
   public static Object newInstance(String binaryName, long port, long threadId, long functionPtr)
       throws ClassNotFoundException {
-    Class clazz = Class.forName(binaryName);
+    Class<?> clazz = Class.forName(binaryName);
     return Proxy.newProxyInstance(
         clazz.getClassLoader(), new Class[] {clazz}, new PortProxy(port, threadId, functionPtr));
   }
 
   @Override
-  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
-    return _invoke(port, threadId, functionPtr, proxy, getDescriptor(method), args);
+  public Object invoke(Object proxy, Method method, Object[] args) {
+    Object[] result = _invoke(port, threadId, functionPtr, proxy, getDescriptor(method), args);
+    _cleanUp((Long) result[0]);
+    return result[1];
   }
 
-  private native Object _invoke(
+  /// Returns an array with two objects:
+  /// [0]: The address of the result pointer used for the clean-up.
+  /// [1]: The result of the invocation.
+  private native Object[] _invoke(
       long port,
       long threadId,
       long functionPtr,
       Object proxy,
       String methodDescriptor,
       Object[] args);
+
+  private native void _cleanUp(long resultPtr);
 }
diff --git a/jni/src/dartjni.c b/jni/src/dartjni.c
index 6f0fa086..df8feb78 100644
--- a/jni/src/dartjni.c
+++ b/jni/src/dartjni.c
@@ -636,7 +636,12 @@ void resultFor(CallbackResult* result, jobject object) {
   release_lock(&result->lock);
 }
 
-JNIEXPORT jobject JNICALL
+jclass _c_Object = NULL;
+jclass _c_Long = NULL;
+
+jmethodID _m_Long_init = NULL;
+
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -645,9 +650,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args) {
-  attach_thread();
+  CallbackResult* result = (CallbackResult*)malloc(sizeof(CallbackResult));
   if (threadId != thread_id()) {
-    CallbackResult* result = (CallbackResult*)malloc(sizeof(CallbackResult));
     init_lock(&result->lock);
     init_cond(&result->cond);
     acquire_lock(&result->lock);
@@ -674,18 +678,40 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
     c_post.value.as_array.length = sizeof(c_post_arr) / sizeof(c_post_arr[0]);
 
     Dart_PostCObject_DL(port, &c_post);
+
     while (!result->ready) {
       wait_for(&result->cond, &result->lock);
     }
+
     release_lock(&result->lock);
     destroy_lock(&result->lock);
     destroy_cond(&result->cond);
-    jobject object = result->object;
-    free(result);
-    return object;
   } else {
-    return ((jobject(*)(uint64_t, jobject, jobject))functionPtr)(
+    result->object = ((jobject(*)(uint64_t, jobject, jobject))functionPtr)(
         port, (*env)->NewGlobalRef(env, methodDescriptor),
         (*env)->NewGlobalRef(env, args));
   }
-}
\ No newline at end of file
+  // Returning an array of length 2.
+  // [0]: The result pointer, used for cleaning up the global reference, and
+  //      freeing the memory since we passed the ownership to Java.
+  // [1]: The returned object.
+  attach_thread();
+  load_class_global_ref(&_c_Object, "java/lang/Object");
+  load_class_global_ref(&_c_Long, "java/lang/Long");
+  load_method(_c_Long, &_m_Long_init, "<init>", "(J)V");
+  jobject first = (*env)->NewObject(env, _c_Long, _m_Long_init, (jlong)result);
+  jobject second = result->object;
+  jobjectArray arr = (*env)->NewObjectArray(env, 2, _c_Object, NULL);
+  (*env)->SetObjectArrayElement(env, arr, 0, first);
+  (*env)->SetObjectArrayElement(env, arr, 1, second);
+  return arr;
+}
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr) {
+  CallbackResult* result = (CallbackResult*)resultPtr;
+  (*env)->DeleteGlobalRef(env, result->object);
+  free(result);
+}
diff --git a/jni/src/dartjni.h b/jni/src/dartjni.h
index cbb1f7f5..8a07d091 100644
--- a/jni/src/dartjni.h
+++ b/jni/src/dartjni.h
@@ -76,7 +76,7 @@ static inline void wait_for(ConditionVariable* cond, MutexLock* lock) {
 }
 
 static inline void destroy_cond(ConditionVariable* cond) {
-  DeleteCriticalSection(cond);
+  // Not available.
 }
 
 #elif defined __APPLE__ || defined __LINUX__ || defined __ANDROID__ ||         \
@@ -435,7 +435,7 @@ JniResult PortProxy__newInstance(jobject binaryName,
                                  int64_t port,
                                  int64_t functionPtr);
 
-JNIEXPORT jobject JNICALL
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -444,3 +444,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args);
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr);
diff --git a/jnigen/example/in_app_java/src/android_utils/dartjni.h b/jnigen/example/in_app_java/src/android_utils/dartjni.h
index cbb1f7f5..8a07d091 100644
--- a/jnigen/example/in_app_java/src/android_utils/dartjni.h
+++ b/jnigen/example/in_app_java/src/android_utils/dartjni.h
@@ -76,7 +76,7 @@ static inline void wait_for(ConditionVariable* cond, MutexLock* lock) {
 }
 
 static inline void destroy_cond(ConditionVariable* cond) {
-  DeleteCriticalSection(cond);
+  // Not available.
 }
 
 #elif defined __APPLE__ || defined __LINUX__ || defined __ANDROID__ ||         \
@@ -435,7 +435,7 @@ JniResult PortProxy__newInstance(jobject binaryName,
                                  int64_t port,
                                  int64_t functionPtr);
 
-JNIEXPORT jobject JNICALL
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -444,3 +444,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args);
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr);
diff --git a/jnigen/example/kotlin_plugin/src/dartjni.h b/jnigen/example/kotlin_plugin/src/dartjni.h
index cbb1f7f5..8a07d091 100644
--- a/jnigen/example/kotlin_plugin/src/dartjni.h
+++ b/jnigen/example/kotlin_plugin/src/dartjni.h
@@ -76,7 +76,7 @@ static inline void wait_for(ConditionVariable* cond, MutexLock* lock) {
 }
 
 static inline void destroy_cond(ConditionVariable* cond) {
-  DeleteCriticalSection(cond);
+  // Not available.
 }
 
 #elif defined __APPLE__ || defined __LINUX__ || defined __ANDROID__ ||         \
@@ -435,7 +435,7 @@ JniResult PortProxy__newInstance(jobject binaryName,
                                  int64_t port,
                                  int64_t functionPtr);
 
-JNIEXPORT jobject JNICALL
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -444,3 +444,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args);
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr);
diff --git a/jnigen/example/notification_plugin/src/dartjni.h b/jnigen/example/notification_plugin/src/dartjni.h
index cbb1f7f5..8a07d091 100644
--- a/jnigen/example/notification_plugin/src/dartjni.h
+++ b/jnigen/example/notification_plugin/src/dartjni.h
@@ -76,7 +76,7 @@ static inline void wait_for(ConditionVariable* cond, MutexLock* lock) {
 }
 
 static inline void destroy_cond(ConditionVariable* cond) {
-  DeleteCriticalSection(cond);
+  // Not available.
 }
 
 #elif defined __APPLE__ || defined __LINUX__ || defined __ANDROID__ ||         \
@@ -435,7 +435,7 @@ JniResult PortProxy__newInstance(jobject binaryName,
                                  int64_t port,
                                  int64_t functionPtr);
 
-JNIEXPORT jobject JNICALL
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -444,3 +444,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args);
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr);
diff --git a/jnigen/example/pdfbox_plugin/src/third_party/dartjni.h b/jnigen/example/pdfbox_plugin/src/third_party/dartjni.h
index cbb1f7f5..8a07d091 100644
--- a/jnigen/example/pdfbox_plugin/src/third_party/dartjni.h
+++ b/jnigen/example/pdfbox_plugin/src/third_party/dartjni.h
@@ -76,7 +76,7 @@ static inline void wait_for(ConditionVariable* cond, MutexLock* lock) {
 }
 
 static inline void destroy_cond(ConditionVariable* cond) {
-  DeleteCriticalSection(cond);
+  // Not available.
 }
 
 #elif defined __APPLE__ || defined __LINUX__ || defined __ANDROID__ ||         \
@@ -435,7 +435,7 @@ JniResult PortProxy__newInstance(jobject binaryName,
                                  int64_t port,
                                  int64_t functionPtr);
 
-JNIEXPORT jobject JNICALL
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -444,3 +444,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args);
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr);
diff --git a/jnigen/lib/src/bindings/dart_generator.dart b/jnigen/lib/src/bindings/dart_generator.dart
index 3db7c0bc..c78a3b25 100644
--- a/jnigen/lib/src/bindings/dart_generator.dart
+++ b/jnigen/lib/src/bindings/dart_generator.dart
@@ -1534,8 +1534,9 @@ class _InterfaceMethodIf extends Visitor<Method, void> {
 
   @override
   void visit(Method node) {
+    final isVoid = node.returnType.name == 'void';
     final signature = node.javaSig;
-    final saveResult = node.returnType.name == 'void' ? '' : 'final \$r = ';
+    final saveResult = isVoid ? '' : 'final \$r = ';
     s.write('''
       if (\$d == r"$signature") {
         ${saveResult}_\$methods[\$p]![\$d]!(
@@ -1588,13 +1589,25 @@ class _InterfaceParamCast extends Visitor<Param, void> {
 /// Only returns the reference for non primitive types.
 /// Returns null for void.
 ///
-/// For example `$r.toJInteger().reference` when the return type is `integer`.
+/// Since Dart doesn't know that this global reference is still used, it might
+/// garbage collect it via [NativeFinalizer] thus making it invalid.
+/// This passes the ownership to Java using [setAsDeleted].
+///
+/// `..setAsDeleted` detaches the object from the [NativeFinalizer] and Java
+/// will clean up the global reference afterwards.
+///
+/// For example `($r.toJInteger()..setAsDeleted()).reference` when the return
+/// type is `integer`.
 class _InterfaceReturnBox extends TypeVisitor<String> {
   const _InterfaceReturnBox();
 
   @override
   String visitNonPrimitiveType(ReferredType node) {
-    return '\$r.reference';
+    // Casting is done to create a new global reference. The user might
+    // use the original reference elsewhere and so the original object
+    // should not be [setAsDeleted].
+    return '((\$r as $_jObject).castTo(const ${_jObject}Type())'
+        '..setAsDeleted()).reference';
   }
 
   @override
@@ -1602,7 +1615,7 @@ class _InterfaceReturnBox extends TypeVisitor<String> {
     if (node.name == 'void') {
       return '$_jni.nullptr';
     }
-    return '$_jni.J${node.boxedName}(\$r).reference';
+    return '($_jni.J${node.boxedName}(\$r)..setAsDeleted()).reference';
   }
 }
 
diff --git a/jnigen/test/jackson_core_test/third_party/c_based/c_bindings/dartjni.h b/jnigen/test/jackson_core_test/third_party/c_based/c_bindings/dartjni.h
index cbb1f7f5..8a07d091 100644
--- a/jnigen/test/jackson_core_test/third_party/c_based/c_bindings/dartjni.h
+++ b/jnigen/test/jackson_core_test/third_party/c_based/c_bindings/dartjni.h
@@ -76,7 +76,7 @@ static inline void wait_for(ConditionVariable* cond, MutexLock* lock) {
 }
 
 static inline void destroy_cond(ConditionVariable* cond) {
-  DeleteCriticalSection(cond);
+  // Not available.
 }
 
 #elif defined __APPLE__ || defined __LINUX__ || defined __ANDROID__ ||         \
@@ -435,7 +435,7 @@ JniResult PortProxy__newInstance(jobject binaryName,
                                  int64_t port,
                                  int64_t functionPtr);
 
-JNIEXPORT jobject JNICALL
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -444,3 +444,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args);
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr);
diff --git a/jnigen/test/kotlin_test/c_based/c_bindings/dartjni.h b/jnigen/test/kotlin_test/c_based/c_bindings/dartjni.h
index cbb1f7f5..8a07d091 100644
--- a/jnigen/test/kotlin_test/c_based/c_bindings/dartjni.h
+++ b/jnigen/test/kotlin_test/c_based/c_bindings/dartjni.h
@@ -76,7 +76,7 @@ static inline void wait_for(ConditionVariable* cond, MutexLock* lock) {
 }
 
 static inline void destroy_cond(ConditionVariable* cond) {
-  DeleteCriticalSection(cond);
+  // Not available.
 }
 
 #elif defined __APPLE__ || defined __LINUX__ || defined __ANDROID__ ||         \
@@ -435,7 +435,7 @@ JniResult PortProxy__newInstance(jobject binaryName,
                                  int64_t port,
                                  int64_t functionPtr);
 
-JNIEXPORT jobject JNICALL
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -444,3 +444,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args);
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr);
diff --git a/jnigen/test/simple_package_test/c_based/c_bindings/dartjni.h b/jnigen/test/simple_package_test/c_based/c_bindings/dartjni.h
index cbb1f7f5..8a07d091 100644
--- a/jnigen/test/simple_package_test/c_based/c_bindings/dartjni.h
+++ b/jnigen/test/simple_package_test/c_based/c_bindings/dartjni.h
@@ -76,7 +76,7 @@ static inline void wait_for(ConditionVariable* cond, MutexLock* lock) {
 }
 
 static inline void destroy_cond(ConditionVariable* cond) {
-  DeleteCriticalSection(cond);
+  // Not available.
 }
 
 #elif defined __APPLE__ || defined __LINUX__ || defined __ANDROID__ ||         \
@@ -435,7 +435,7 @@ JniResult PortProxy__newInstance(jobject binaryName,
                                  int64_t port,
                                  int64_t functionPtr);
 
-JNIEXPORT jobject JNICALL
+JNIEXPORT jobjectArray JNICALL
 Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject thiz,
                                                   jlong port,
@@ -444,3 +444,8 @@ Java_com_github_dart_1lang_jni_PortProxy__1invoke(JNIEnv* env,
                                                   jobject proxy,
                                                   jstring methodDescriptor,
                                                   jobjectArray args);
+
+JNIEXPORT void JNICALL
+Java_com_github_dart_1lang_jni_PortProxy__1cleanUp(JNIEnv* env,
+                                                   jobject thiz,
+                                                   jlong resultPtr);
diff --git a/jnigen/test/simple_package_test/c_based/dart_bindings/simple_package.dart b/jnigen/test/simple_package_test/c_based/dart_bindings/simple_package.dart
index 7cb01108..11e873e5 100644
--- a/jnigen/test/simple_package_test/c_based/dart_bindings/simple_package.dart
+++ b/jnigen/test/simple_package_test/c_based/dart_bindings/simple_package.dart
@@ -2979,13 +2979,17 @@ class MyInterface<$T extends jni.JObject> extends jni.JObject {
       final $r = _$methods[$p]![$d]!(
         $a[0].castTo(const jni.JStringType(), deleteOriginal: true),
       );
-      return $r.reference;
+      return (($r as jni.JObject).castTo(const jni.JObjectType())
+            ..setAsDeleted())
+          .reference;
     }
     if ($d == r"varCallback(Ljava/lang/Object;)Ljava/lang/Object;") {
       final $r = _$methods[$p]![$d]!(
         $a[0].castTo(_$types[$p]!["T"]!, deleteOriginal: true),
       );
-      return $r.reference;
+      return (($r as jni.JObject).castTo(const jni.JObjectType())
+            ..setAsDeleted())
+          .reference;
     }
     if ($d == r"manyPrimitives(IZCD)J") {
       final $r = _$methods[$p]![$d]!(
@@ -3002,7 +3006,7 @@ class MyInterface<$T extends jni.JObject> extends jni.JObject {
             .castTo(const jni.JDoubleType(), deleteOriginal: true)
             .doubleValue(deleteOriginal: true),
       );
-      return jni.JLong($r).reference;
+      return (jni.JLong($r)..setAsDeleted()).reference;
     }
     return jni.nullptr;
   }
diff --git a/jnigen/test/simple_package_test/dart_only/dart_bindings/simple_package.dart b/jnigen/test/simple_package_test/dart_only/dart_bindings/simple_package.dart
index 7befe1b9..ef24ce3e 100644
--- a/jnigen/test/simple_package_test/dart_only/dart_bindings/simple_package.dart
+++ b/jnigen/test/simple_package_test/dart_only/dart_bindings/simple_package.dart
@@ -2818,13 +2818,17 @@ class MyInterface<$T extends jni.JObject> extends jni.JObject {
       final $r = _$methods[$p]![$d]!(
         $a[0].castTo(const jni.JStringType(), deleteOriginal: true),
       );
-      return $r.reference;
+      return (($r as jni.JObject).castTo(const jni.JObjectType())
+            ..setAsDeleted())
+          .reference;
     }
     if ($d == r"varCallback(Ljava/lang/Object;)Ljava/lang/Object;") {
       final $r = _$methods[$p]![$d]!(
         $a[0].castTo(_$types[$p]!["T"]!, deleteOriginal: true),
       );
-      return $r.reference;
+      return (($r as jni.JObject).castTo(const jni.JObjectType())
+            ..setAsDeleted())
+          .reference;
     }
     if ($d == r"manyPrimitives(IZCD)J") {
       final $r = _$methods[$p]![$d]!(
@@ -2841,7 +2845,7 @@ class MyInterface<$T extends jni.JObject> extends jni.JObject {
             .castTo(const jni.JDoubleType(), deleteOriginal: true)
             .doubleValue(deleteOriginal: true),
       );
-      return jni.JLong($r).reference;
+      return (jni.JLong($r)..setAsDeleted()).reference;
     }
     return jni.nullptr;
   }