JNI: Add WeakGlobalRef helper type

modules/juce_core/native/juce_JNIHelpers_android.cpp

@@ -248,7 +248,7 @@ struct SystemJavaClassComparator
 
 //==============================================================================
 JNIClassBase::JNIClassBase (const char* cp, int classMinSDK, const uint8* bc, size_t n)
-    : classPath (cp), byteCode (bc), byteCodeSize (n), minSDK (classMinSDK), classRef (nullptr)
+    : classPath (cp), byteCode (bc), byteCodeSize (n), minSDK (classMinSDK)
 {
     SystemJavaClassComparator comparator;
 
@@ -375,7 +375,7 @@ void JNIClassBase::initialise (JNIEnv* env, jobject context)
         }
 
         if (classRef == nullptr)
-            classRef = (jclass) env->NewGlobalRef (LocalRef<jobject> (env->FindClass (classPath)));
+            classRef = GlobalRefImpl { LocalRef { env->FindClass (classPath) } };
 
         jassert (classRef != nullptr);
         initialiseFields (env);
@@ -388,23 +388,22 @@ void JNIClassBase::tryLoadingClassWithClassLoader (JNIEnv* env, jobject classLoa
 
     // Android SDK <= 19 has a bug where the class loader might throw an exception but still return
     // a non-nullptr. So don't assign the result of this call to a jobject just yet...
-    auto classObj = env->CallObjectMethod (classLoader, JavaClassLoader.loadClass, classNameAndPackage.get(), (jboolean) true);
+    LocalRef classObj { (jclass) env->CallObjectMethod (classLoader, JavaClassLoader.loadClass, classNameAndPackage.get(), (jboolean) true) };
 
     if (jthrowable exception = env->ExceptionOccurred())
     {
         env->ExceptionClear();
-        classObj = nullptr;
+        classObj = {};
     }
 
     // later versions of Android don't throw at all, so re-check the object
     if (classObj != nullptr)
-        classRef = (jclass) env->NewGlobalRef (LocalRef<jobject> (classObj));
+        classRef = GlobalRefImpl { classObj };
 }
 
 void JNIClassBase::release (JNIEnv* env)
 {
-    if (classRef != nullptr)
-        env->DeleteGlobalRef (classRef);
+    classRef.clear (env);
 }
 
 void JNIClassBase::initialiseAllClasses (JNIEnv* env, jobject context)

modules/juce_core/native/juce_JNIHelpers_android.h

@@ -270,7 +270,7 @@ private:
     size_t byteCodeSize;
 
     int minSDK;
-    jclass classRef = nullptr;
+    GlobalRefImpl<jclass> classRef;
 
     static Array<JNIClassBase*>& getClasses();
     void initialise (JNIEnv*, jobject context);

modules/juce_core/native/juce_Threads_android.cpp

@@ -48,7 +48,7 @@ DECLARE_JNI_CLASS (AndroidResolveInfo, "android/content/pm/ResolveInfo")
 
 //==============================================================================
 JavaVM* androidJNIJavaVM = nullptr;
-jobject androidApkContext = nullptr;
+GlobalRef androidApkContext;
 
 //==============================================================================
 JNIEnv* getEnv() noexcept
@@ -105,6 +105,56 @@ extern "C" jint JNIEXPORT JNI_OnLoad (JavaVM* vm, void*)
     return JNI_VERSION_1_2;
 }
 
+//==============================================================================
+class WeakGlobalRef
+{
+public:
+    WeakGlobalRef() = default;
+    explicit WeakGlobalRef (jobject o) : WeakGlobalRef (o, getEnv()) {}
+    WeakGlobalRef (jobject o, JNIEnv* env) : obj (retain (o, env)) {}
+
+    WeakGlobalRef (const WeakGlobalRef& o) : obj (retain (o.obj)) {}
+    WeakGlobalRef (WeakGlobalRef&& o) noexcept : obj (std::exchange (o.obj, nullptr)) {}
+
+    WeakGlobalRef& operator= (const WeakGlobalRef& o)
+    {
+        WeakGlobalRef { o }.swap (*this);
+        return *this;
+    }
+
+    WeakGlobalRef& operator= (WeakGlobalRef&& o) noexcept
+    {
+        WeakGlobalRef { std::move (o) }.swap (*this);
+        return *this;
+    }
+
+    ~WeakGlobalRef() noexcept { clear(); }
+
+    void clear (JNIEnv* env = getEnv())
+    {
+        if (obj != nullptr)
+            env->DeleteWeakGlobalRef (obj);
+    }
+
+    auto lock (JNIEnv* env = getEnv()) const
+    {
+        return LocalRef { env->NewLocalRef (obj) };
+    }
+
+private:
+    void swap (WeakGlobalRef& other) noexcept
+    {
+        std::swap (other.obj, obj);
+    }
+
+    static jweak retain (jweak o, JNIEnv* env = getEnv())
+    {
+        return env->NewWeakGlobalRef (o);
+    }
+
+    jweak obj = nullptr;
+};
+
 //==============================================================================
 class JuceActivityWatcher final : public ActivityLifecycleCallbacks
 {
@@ -122,21 +172,16 @@ public:
 
         ScopedLock lock (currentActivityLock);
 
-        if (currentActivity != nullptr)
+        if (auto locked = currentActivity.lock (env))
         {
-            // see Clarification June 2001 in JNI reference for why this is
-            // necessary
-            LocalRef<jobject> localStorage (env->NewLocalRef (currentActivity));
-
-            if (env->IsSameObject (localStorage.get(), activity) != 0)
+            if (env->IsSameObject (locked, activity) != 0)
                 return;
 
-            env->DeleteWeakGlobalRef (currentActivity);
-            currentActivity = nullptr;
+            currentActivity = {};
         }
 
         if (activity != nullptr)
-            currentActivity = env->NewWeakGlobalRef (activity);
+            currentActivity = WeakGlobalRef { activity };
     }
 
     void onActivityStopped (jobject activity) override
@@ -145,16 +190,15 @@ public:
 
         ScopedLock lock (currentActivityLock);
 
-        if (currentActivity != nullptr)
+        if (auto locked = currentActivity.lock (env))
         {
             // important that the comparison happens in this order
             // to avoid race condition where the weak reference becomes null
             // just after the first check
-            if (env->IsSameObject (currentActivity, activity) != 0
-                || env->IsSameObject (currentActivity, nullptr) != 0)
+            if (env->IsSameObject (locked, activity) != 0
+                || env->IsSameObject (locked, nullptr) != 0)
             {
-                env->DeleteWeakGlobalRef (currentActivity);
-                currentActivity = nullptr;
+                currentActivity = {};
             }
         }
     }
@@ -162,13 +206,13 @@ public:
     LocalRef<jobject> getCurrent()
     {
         ScopedLock lock (currentActivityLock);
-        return LocalRef<jobject> (getEnv()->NewLocalRef (currentActivity));
+        return currentActivity.lock();
     }
 
     LocalRef<jobject> getMain()
     {
         ScopedLock lock (currentActivityLock);
-        return LocalRef<jobject> (getEnv()->NewLocalRef (mainActivity));
+        return mainActivity.lock();
     }
 
     static JuceActivityWatcher& getInstance()
@@ -184,16 +228,13 @@ private:
 
         ScopedLock lock (currentActivityLock);
 
-        if (mainActivity != nullptr)
+        if (auto locked = mainActivity.lock (env))
         {
-            if (env->IsSameObject (mainActivity, nullptr) != 0)
-            {
-                env->DeleteWeakGlobalRef (mainActivity);
-                mainActivity = nullptr;
-            }
+            if (env->IsSameObject (locked, nullptr) != 0)
+                mainActivity = {};
         }
 
-        if (mainActivity == nullptr)
+        if (mainActivity.lock() == nullptr)
         {
             auto appContext = getAppContext();
             auto mainActivityPath = getMainActivityClassPath();
@@ -206,7 +247,7 @@ private:
                 // This may be problematic for apps which use several activities with the same type. We just
                 // assume that the very first activity of this type is the main one
                 if (activityPath == mainActivityPath)
-                    mainActivity = env->NewWeakGlobalRef (context);
+                    mainActivity = WeakGlobalRef { context };
             }
         }
     }
@@ -250,8 +291,7 @@ private:
     }
 
     CriticalSection currentActivityLock;
-    jweak currentActivity = nullptr;
-    jweak mainActivity    = nullptr;
+    WeakGlobalRef currentActivity, mainActivity;
     ActivityLifecycleCallbackForwarder forwarder { GlobalRef { getAppContext() }, this };
 };
 
@@ -287,7 +327,7 @@ void Thread::initialiseJUCE (void* jniEnv, void* context)
         firstCall = false;
 
         // if we ever support unloading then this should probably be a weak reference
-        androidApkContext = env->NewGlobalRef (static_cast<jobject> (context));
+        androidApkContext = GlobalRef { LocalRef { (jobject) context } };
         JuceActivityWatcher::getInstance();
 
        #if JUCE_MODULE_AVAILABLE_juce_events && JUCE_ANDROID