findClass when calling init method

app/src/main/cpp/visualizer/frank_visualizer_jni.cpp

@@ -17,6 +17,8 @@
 
 struct fields_t {
     jfieldID context;
+    jclass visual_class;
+    jmethodID fft_method;
 };
 
 static fields_t fields;
@@ -41,11 +43,9 @@ FrankVisualizer *getCustomVisualizer(JNIEnv *env, jobject thiz) {
 }
 
 void fft_callback(JNIEnv *jniEnv, int8_t * arg, int samples) {
-    jclass visual_class = jniEnv->FindClass(className);
-    jmethodID fft_method = jniEnv->GetStaticMethodID(visual_class, "onFftCallback", "([B)V");
     jbyteArray dataArray = jniEnv->NewByteArray(samples);
     jniEnv->SetByteArrayRegion(dataArray, 0, samples, arg);
-    jniEnv->CallStaticVoidMethod(visual_class, fft_method, dataArray);
+    jniEnv->CallStaticVoidMethod(fields.visual_class, fields.fft_method, dataArray);
     jniEnv->DeleteLocalRef(dataArray);
 }
 
@@ -53,6 +53,9 @@ VISUALIZER_FUNC(int, nativeInitVisualizer) {
     setCustomVisualizer(env, thiz);
     FrankVisualizer *mVisualizer = getCustomVisualizer(env, thiz);
     if (!mVisualizer) return -2;
+    jclass mVisualClass = env->FindClass(className);
+    fields.visual_class = (jclass) env->NewGlobalRef(mVisualClass);
+    fields.fft_method = env->GetStaticMethodID(fields.visual_class, "onFftCallback", "([B)V");
     return mVisualizer->init_visualizer();
 }
 
@@ -76,5 +79,6 @@ VISUALIZER_FUNC(void, nativeReleaseVisualizer) {
     if (!mVisualizer) return;
     mVisualizer->release_visualizer();
     delete mVisualizer;
+    env->DeleteGlobalRef(fields.visual_class);
     env->SetLongField(thiz, fields.context, 0);
 }