Add java.lang.ref.WeakReference wrapper

Author: jfirebaugh

include/jni/jni.hpp

@@ -21,3 +21,4 @@
 #include <jni/native_method.hpp>
 #include <jni/boxing.hpp>
 #include <jni/advanced_ownership.hpp>
+#include <jni/weak_reference.hpp>

include/jni/unique_pointerlike.hpp

@@ -129,6 +129,10 @@ namespace jni
 
     // Attempt to promote a weak reference to a strong one. Returns an empty result
     // if the weak reference has expired.
+    //
+    // Beware that the semantics of JNI weak references are weaker than is typically
+    // desired: a JNI weak reference may still be promoted to a non-null strong reference
+    // even during finalization. Consider using jni::WeakReference<T> instead.
     template < template < RefDeletionMethod > class Deleter, class T, template < RefDeletionMethod > class WeakDeleter >
     Global<T, Deleter> NewGlobal(JNIEnv& env, const Weak<T, WeakDeleter>& t)
        {
@@ -146,6 +150,10 @@ namespace jni
 
     // Attempt to promote a weak reference to a strong one. Returns an empty result
     // if the weak reference has expired.
+    //
+    // Beware that the semantics of JNI weak references are weaker than is typically
+    // desired: a JNI weak reference may still be promoted to a non-null strong reference
+    // even during finalization. Consider using jni::WeakReference<T> instead.
     template < class T, template < RefDeletionMethod > class WeakDeleter >
     Local<T> NewLocal(JNIEnv& env, const Weak<T, WeakDeleter>& t)
        {

include/jni/weak_reference.hpp

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <jni/class.hpp>
+#include <jni/object.hpp>
+
+namespace jni
+   {
+    struct WeakReferenceTag { static constexpr auto Name() { return "java/lang/ref/WeakReference"; } };
+
+    // Wraps a JNI global reference to a java.lang.ref.WeakReference, producing an ownership class
+    // similar to jni::Weak<T> (JNI's weak global reference), but with more reliable promotion semantics.
+    // Whereas a JNI weak global reference may still be promoted to a strong reference even during
+    // finalization, leading to potential use-after-free errors, a WeakReference cannot.
+    template < class T, template < RefDeletionMethod > class Deleter = DefaultRefDeleter >
+    class WeakReference
+       {
+        private:
+            Global<Object<WeakReferenceTag>, Deleter> reference;
+
+        public:
+            WeakReference(JNIEnv& env, T referent)
+               {
+                static auto klass = Class<WeakReferenceTag>::Singleton(env);
+                static auto constructor = klass.GetConstructor<Object<>>(env);
+                reference = klass.New(env, constructor, Object<>(referent.Get())).template NewGlobalRef<Deleter>(env);
+               }
+
+            Local<T> get(JNIEnv& env)
+               {
+                if (!reference)
+                   {
+                    return Local<T>();
+                   }
+
+                static auto klass = Class<WeakReferenceTag>::Singleton(env);
+                static auto get = klass.template GetMethod<Object<> ()>(env, "get");
+                return SeizeLocal(env, T(reinterpret_cast<UntaggedType<T>>(reference->Call(env, get).Get())));
+               }
+       };
+   }