media/jni/soundpool/android_media_SoundPool.cpp

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <stdio.h>

//#define LOG_NDEBUG 0
#define LOG_TAG "SoundPool-JNI"

#include <utils/Log.h>
#include <audio_utils/string.h>
#include <jni.h>
#include <nativehelper/JNIPlatformHelp.h>
#include <nativehelper/ScopedUtfChars.h>
#include <android_runtime/AndroidRuntime.h>
#include "SoundPool.h"

using namespace android;

static struct fields_t {
    jfieldID    mNativeContext;
    jmethodID   mPostEvent;
    jclass      mSoundPoolClass;
} fields;

namespace {

/**
 * ObjectManager creates a native "object" on the heap and stores
 * its pointer in a long field in a Java object.
 *
 * The type T must have 3 properties in the current implementation.
 *    1) A T{} default constructor which represents a nullValue.
 *    2) T::operator bool() const efficient detection of such a nullValue.
 *    3) T must be copyable.
 *
 * Some examples of such a type T are std::shared_ptr<>, android::sp<>,
 * std::optional, std::function<>, etc.
 *
 * Using set() with a nullValue T results in destroying the underlying native
 * "object" if it exists.  A nullValue T is returned by get() if there is
 * no underlying native Object.
 *
 * This class is thread safe for multiple access.
 *
 * Design notes:
 * 1) For objects of type T that do not naturally have an "nullValue",
 *    wrapping with
 *           a) TOpt, where TOpt = std::optional<T>
 *           b) TShared, where TShared = std::shared_ptr<T>
 *
 * 2) An overload for an explicit equality comparable nullValue such as
 *    get(..., const T& nullValue) or set(..., const T& nullValue)
 *    is omitted.  An alternative is to pass a fixed nullValue in the constructor.
 */
template <typename T>
class ObjectManager
{
// Can a jlong hold a pointer?
static_assert(sizeof(jlong) >= sizeof(void*));

public:
    // fieldId is associated with a Java long member variable in the object.
    // ObjectManager will store the native pointer in that field.
    //
    // If a native object is set() in that field, it
    explicit ObjectManager(jfieldID fieldId) : mFieldId(fieldId) {}
    ~ObjectManager() {
        ALOGE_IF(mObjectCount != 0, "%s: mObjectCount: %d should be zero on destruction",
                __func__, mObjectCount.load());
        // Design note: it would be possible to keep a map of the outstanding allocated
        // objects and force a delete on them on ObjectManager destruction.
        // The consequences of that is probably worse than keeping them alive.
    }

    // Retrieves the associated object, returns nullValue T if not available.
    T get(JNIEnv *env, jobject thiz) const {
        std::lock_guard lg(mLock);
        // NOLINTNEXTLINE(performance-no-int-to-ptr)
        auto ptr = reinterpret_cast<T*>(env->GetLongField(thiz, mFieldId));
        if (ptr != nullptr) {
            return *ptr;
        }
        return {};
    }

    // Sets the object and returns the old one.
    //
    // If the old object doesn't exist, then nullValue T is returned.
    // If the new object is false by operator bool(), the internal object is destroyed.
    // Note: The old object is returned so if T is a smart pointer, it can be held
    // by the caller to be deleted outside of any external lock.
    //
    // Remember to call set(env, thiz, {}) to destroy the object in the Java
    // object finalize to avoid orphaned objects on the heap.
    T set(JNIEnv *env, jobject thiz, const T& newObject) {
        std::lock_guard lg(mLock);
        // NOLINTNEXTLINE(performance-no-int-to-ptr)
        auto ptr = reinterpret_cast<T*>(env->GetLongField(thiz, mFieldId));
        if (ptr != nullptr) {
            T old = std::move(*ptr);  // *ptr will be replaced or deleted.
            if (newObject) {
                env->SetLongField(thiz, mFieldId, (jlong)0);
                delete ptr;
                --mObjectCount;
            } else {
                *ptr = newObject;
            }
            return old;
        } else {
             if (newObject) {
                 env->SetLongField(thiz, mFieldId, (jlong)new T(newObject));
                 ++mObjectCount;
             }
             return {};
        }
    }

    // Returns the number of outstanding objects.
    //
    // This is purely for debugging purposes and tracks the number of active Java
    // objects that have native T objects; hence represents the number of
    // T heap allocations we have made.
    //
    // When all those Java objects have been finalized we expect this to go to 0.
    int32_t getObjectCount() const {
        return mObjectCount;
    }

private:
    // NOLINTNEXTLINE(misc-misplaced-const)
    const jfieldID mFieldId;  // '_jfieldID *const'

    // mObjectCount is the number of outstanding native T heap allocations we have
    // made (and thus the number of active Java objects which are associated with them).
    std::atomic_int32_t mObjectCount{};

    mutable std::mutex mLock;
};

// We use SoundPoolManager to associate a native std::shared_ptr<SoundPool>
// object with a field in the Java object.
//
// We can then retrieve the std::shared_ptr<SoundPool> from the object.
//
// Design notes:
// 1) This is based on ObjectManager class.
// 2) An alternative that does not require a field in the Java object
//    is to create an associative map using as a key a NewWeakGlobalRef
//    to the Java object.
//    The problem of this method is that lookup is O(N) because comparison
//    between the WeakGlobalRef to a JNI jobject LocalRef must be done
//    through the JNI IsSameObject() call, hence iterative through the map.
//    One advantage of this method is that manual garbage collection
//    is possible by checking if the WeakGlobalRef is null equivalent.

auto& getSoundPoolManager() {
    // never-delete singleton
    static auto soundPoolManager =
            new ObjectManager<std::shared_ptr<SoundPool>>(fields.mNativeContext);
    return *soundPoolManager;
}

inline auto getSoundPool(JNIEnv *env, jobject thiz) {
    return getSoundPoolManager().get(env, thiz);
}

// Note: one must call setSoundPool(env, thiz, nullptr) to release any native resources
// somewhere in the Java object finalize().
inline auto setSoundPool(
        JNIEnv *env, jobject thiz, const std::shared_ptr<SoundPool>& soundPool) {
    return getSoundPoolManager().set(env, thiz, soundPool);
}

/**
 * ConcurrentHashMap is a locked hash map
 *
 * As from the name, this class is thread_safe.
 *
 * The type V must have 3 properties in the current implementation.
 *    1) A V{} default constructor which represents a nullValue.
 *    2) V::operator bool() const efficient detection of such a nullValue.
 *    3) V must be copyable.
 *
 * Note: The Key cannot be a Java LocalRef, as those change between JNI calls.
 * The Key could be the raw native object pointer if one wanted to associate
 * extra data with a native object.
 *
 * Using set() with a nullValue V results in erasing the key entry.
 * A nullValue V is returned by get() if there is no underlying entry.
 *
 * Design notes:
 * 1) For objects of type V that do not naturally have a "nullValue",
 *    wrapping VOpt = std::optional<V> or VShared = std::shared<V> is recommended.
 *
 * 2) An overload for an explicit equality comparable nullValue such as
 *    get(..., const V& nullValue) or set(..., const V& nullValue)
 *    is omitted.  An alternative is to pass a fixed nullValue into a special
 *    constructor (omitted) for equality comparisons and return value.
 *
 * 3) This ConcurrentHashMap currently allows only one thread at a time.
 *    It is not optimized for heavy multi-threaded use.
 */
template <typename K, typename V>
class ConcurrentHashMap
{
public:

    // Sets the value and returns the old one.
    //
    // If the old value doesn't exist, then nullValue V is returned.
    // If the new value is false by operator bool(), the internal value is destroyed.
    // Note: The old value is returned so if V is a smart pointer, it can be held
    // by the caller to be deleted outside of any external lock.

    V set(const K& key, const V& value) {
        std::lock_guard lg(mLock);
        auto it = mMap.find(key);
        if (it == mMap.end()) {
            if (value) {
                mMap[key] = value;
            }
            return {};
        }
        V oldValue = std::move(it->second);
        if (value) {
            it->second = value;
        } else {
            mMap.erase(it);
        }
        return oldValue;
    }

    // Retrieves the associated object, returns nullValue V if not available.
    V get(const K& key) const {
        std::lock_guard lg(mLock);
        auto it = mMap.find(key);
        return it != mMap.end() ? it->second : V{};
    }
private:
    mutable std::mutex mLock;
    std::unordered_map<K, V> mMap GUARDED_BY(mLock);
};

// *jobject is needed to fit the jobject into a std::shared_ptr.
// This is the Android type _jobject, but we derive this type as JObjectValue.
using JObjectValue = std::remove_pointer_t<jobject>; // _jobject

// Check that jobject is really a pointer to JObjectValue.
// The JNI contract is that jobject is NULL comparable,
// so jobject is pointer equivalent; we check here to be sure.
// Note std::remove_ptr_t<NonPointerType> == NonPointerType.
static_assert(std::is_same_v<JObjectValue*, jobject>);

// *jweak is needed to fit the jweak into a std::shared_ptr.
// This is the Android type _jobject, but we derive this type as JWeakValue.
using JWeakValue = std::remove_pointer_t<jweak>; // this is just _jobject

// Check that jweak is really a pointer to JWeakValue.
static_assert(std::is_same_v<JWeakValue*, jweak>);

// We store the ancillary data associated with a SoundPool object in a concurrent
// hash map indexed on the SoundPool native object pointer.
auto& getSoundPoolJavaRefManager() {
    // Note this can store shared_ptrs to either jweak and jobject,
    // as the underlying type is identical.
    static auto concurrentHashMap =
            new ConcurrentHashMap<SoundPool *, std::shared_ptr<JWeakValue>>();
    return *concurrentHashMap;
}

// make_shared_globalref_from_localref() creates a sharable Java global
// reference from a Java local reference. The equivalent type is
// std::shared_ptr<_jobject> (where _jobject is JObjectValue,
// and _jobject * is jobject),
// and the jobject may be retrieved by .get() or pointer dereference.
// This encapsulation gives the benefit of std::shared_ptr
// ref counting, weak_ptr, etc.
//
// The Java global reference should be stable between JNI calls.  It is a limited
// quantity so sparingly use global references.
//
// The Android JNI implementation is described here:
// https://developer.android.com/training/articles/perf-jni
// https://android-developers.googleblog.com/2011/11/jni-local-reference-changes-in-ics.html
//
// Consider using a weak reference if this is self-referential.
[[maybe_unused]]
inline auto make_shared_globalref_from_localref(JNIEnv *env, jobject localRef) {
    return std::shared_ptr<JObjectValue>(
            env->NewGlobalRef(localRef),
            [](JObjectValue* object) { // cannot cache env as don't know which thread we're on.
                if (object != nullptr) AndroidRuntime::getJNIEnv()->DeleteGlobalRef(object);
            });
}

// Create a weak global reference from local ref.
inline auto make_shared_weakglobalref_from_localref(JNIEnv *env, jobject localRef) {
    return std::shared_ptr<JWeakValue>(
            env->NewWeakGlobalRef(localRef),
            [](JWeakValue* weak) { // cannot cache env as don't know which thread we're on.
                if (weak != nullptr) AndroidRuntime::getJNIEnv()->DeleteWeakGlobalRef(weak);
            });
}

// std::unique_ptr<> does not store a type-erased deleter like std::shared_ptr<>.
// Define a lambda here to use for the std::unique_ptr<> type definition.
auto LocalRefDeleter = [](JObjectValue* object) {
    if (object != nullptr) AndroidRuntime::getJNIEnv()->DeleteLocalRef(object);
};

// Create a local reference from another reference.
// This is a unique_ptr to avoid the temptation of sharing with other threads.
//
// This can be used to promote a WeakGlobalRef jweak into a stable LocalRef jobject.
//
inline auto make_unique_localref_from_ref(JNIEnv *env, jobject object) {
    return std::unique_ptr<JObjectValue, decltype(LocalRefDeleter)>(
            env->NewLocalRef(object),
            LocalRefDeleter);
}

} // namespace

static const char* const kAudioAttributesClassPathName = "android/media/AudioAttributes";
struct audio_attributes_fields_t {
    jfieldID  fieldUsage;        // AudioAttributes.mUsage
    jfieldID  fieldContentType;  // AudioAttributes.mContentType
    jfieldID  fieldFlags;        // AudioAttributes.mFlags
    jfieldID  fieldFormattedTags;// AudioAttributes.mFormattedTags
};
static audio_attributes_fields_t javaAudioAttrFields;

// ----------------------------------------------------------------------------

static jint
android_media_SoundPool_load_FD(JNIEnv *env, jobject thiz, jobject fileDescriptor,
        jlong offset, jlong length, jint priority)
{
    ALOGV("android_media_SoundPool_load_FD");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return 0;
    return (jint) soundPool->load(jniGetFDFromFileDescriptor(env, fileDescriptor),
            int64_t(offset), int64_t(length), int(priority));
}

static jboolean
android_media_SoundPool_unload(JNIEnv *env, jobject thiz, jint sampleID) {
    ALOGV("android_media_SoundPool_unload\n");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return JNI_FALSE;
    return soundPool->unload(sampleID) ? JNI_TRUE : JNI_FALSE;
}

static jint
android_media_SoundPool_play(JNIEnv *env, jobject thiz, jint sampleID,
        jfloat leftVolume, jfloat rightVolume, jint priority, jint loop,
        jfloat rate, jint playerIId)
{
    ALOGV("android_media_SoundPool_play\n");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return 0;

    return (jint) soundPool->play(sampleID,
                                  leftVolume,
                                  rightVolume,
                                  priority,
                                  loop,
                                  rate,
                                  playerIId);
}

static void
android_media_SoundPool_pause(JNIEnv *env, jobject thiz, jint channelID)
{
    ALOGV("android_media_SoundPool_pause");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->pause(channelID);
}

static void
android_media_SoundPool_resume(JNIEnv *env, jobject thiz, jint channelID)
{
    ALOGV("android_media_SoundPool_resume");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->resume(channelID);
}

static void
android_media_SoundPool_autoPause(JNIEnv *env, jobject thiz)
{
    ALOGV("android_media_SoundPool_autoPause");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->autoPause();
}

static void
android_media_SoundPool_autoResume(JNIEnv *env, jobject thiz)
{
    ALOGV("android_media_SoundPool_autoResume");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->autoResume();
}

static void
android_media_SoundPool_stop(JNIEnv *env, jobject thiz, jint channelID)
{
    ALOGV("android_media_SoundPool_stop");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->stop(channelID);
}

static void
android_media_SoundPool_setVolume(JNIEnv *env, jobject thiz, jint channelID,
        jfloat leftVolume, jfloat rightVolume)
{
    ALOGV("android_media_SoundPool_setVolume");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->setVolume(channelID, (float) leftVolume, (float) rightVolume);
}

static void
android_media_SoundPool_mute(JNIEnv *env, jobject thiz, jboolean muting)
{
    ALOGV("android_media_SoundPool_mute(%d)", muting);
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->mute(muting == JNI_TRUE);
}

static void
android_media_SoundPool_setPriority(JNIEnv *env, jobject thiz, jint channelID,
        jint priority)
{
    ALOGV("android_media_SoundPool_setPriority");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->setPriority(channelID, (int) priority);
}

static void
android_media_SoundPool_setLoop(JNIEnv *env, jobject thiz, jint channelID,
        int loop)
{
    ALOGV("android_media_SoundPool_setLoop");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->setLoop(channelID, loop);
}

static void
android_media_SoundPool_setRate(JNIEnv *env, jobject thiz, jint channelID,
       jfloat rate)
{
    ALOGV("android_media_SoundPool_setRate");
    auto soundPool = getSoundPool(env, thiz);
    if (soundPool == nullptr) return;
    soundPool->setRate(channelID, (float) rate);
}

static void android_media_callback(SoundPoolEvent event, SoundPool* soundPool, void* user)
{
    ALOGV("callback: (%d, %d, %d, %p, %p)", event.mMsg, event.mArg1, event.mArg2, soundPool, user);
    auto weakRef = getSoundPoolJavaRefManager().get(soundPool); // shared_ptr to WeakRef
    if (weakRef == nullptr) {
        ALOGD("%s: no weak ref, object released, ignoring callback", __func__);
        return;
    }
    JNIEnv *env = AndroidRuntime::getJNIEnv();
    // "promote" the WeakGlobalRef into a LocalRef.
    auto javaSoundPool = make_unique_localref_from_ref(env, weakRef.get());
    if (!javaSoundPool) {
        ALOGW("%s: weak reference promotes to null (release() not called?), "
                "ignoring callback", __func__);
        return;
    }
    env->CallVoidMethod(javaSoundPool.get(), fields.mPostEvent,
            event.mMsg, event.mArg1, event.mArg2, nullptr /* object */);

    if (env->ExceptionCheck() != JNI_FALSE) {
        ALOGE("%s: Uncaught exception returned from Java callback", __func__);
        env->ExceptionDescribe();
        env->ExceptionClear(); // Just clear it, hopefully all is ok.
    }
}

static jint
android_media_SoundPool_native_setup(JNIEnv *env, jobject thiz,
        jint maxChannels, jobject jaa, jstring opPackageName)
{
    ALOGV("android_media_SoundPool_native_setup");
    if (jaa == nullptr) {
        ALOGE("Error creating SoundPool: invalid audio attributes");
        return -1;
    }

    // Use the AUDIO_ATTRIBUTES_INITIALIZER here to ensure all non-relevant fields are
    // initialized properly. (note that .source is not explicitly initialized here).
    audio_attributes_t audioAttributes = AUDIO_ATTRIBUTES_INITIALIZER;
    // read the AudioAttributes values
    const auto jtags =
            (jstring) env->GetObjectField(jaa, javaAudioAttrFields.fieldFormattedTags);
    const char* tags = env->GetStringUTFChars(jtags, nullptr);
    // infers array size and guarantees zero termination (does not zero fill to the end).
    audio_utils_strlcpy(audioAttributes.tags, tags);
    env->ReleaseStringUTFChars(jtags, tags);
    audioAttributes.usage =
            (audio_usage_t) env->GetIntField(jaa, javaAudioAttrFields.fieldUsage);
    audioAttributes.content_type =
            (audio_content_type_t) env->GetIntField(jaa, javaAudioAttrFields.fieldContentType);
    audioAttributes.flags =
            (audio_flags_mask_t) env->GetIntField(jaa, javaAudioAttrFields.fieldFlags);
    ScopedUtfChars opPackageNameStr(env, opPackageName);
    auto soundPool = std::make_shared<SoundPool>(
            maxChannels, audioAttributes, opPackageNameStr.c_str());
    soundPool->setCallback(android_media_callback, nullptr /* user */);

    // register with SoundPoolManager.
    auto oldSoundPool = setSoundPool(env, thiz, soundPool);
    // register Java SoundPool WeakRef using native SoundPool * as the key, for the callback.
    auto oldSoundPoolJavaRef = getSoundPoolJavaRefManager().set(
            soundPool.get(), make_shared_weakglobalref_from_localref(env, thiz));

    ALOGW_IF(oldSoundPool != nullptr, "%s: Aliased SoundPool object %p",
            __func__, oldSoundPool.get());
    return 0;
}

static void
android_media_SoundPool_release(JNIEnv *env, jobject thiz)
{
    ALOGV("android_media_SoundPool_release");

    // Remove us from SoundPoolManager.

    auto oldSoundPool = setSoundPool(env, thiz, nullptr);
    if (oldSoundPool != nullptr) {
        // Note: setting the weak ref is thread safe in case there is a callback
        // simultaneously occurring.
        auto oldSoundPoolJavaRef = getSoundPoolJavaRefManager().set(oldSoundPool.get(), nullptr);
    }
    // destructor to oldSoundPool should occur at exit.
}

// ----------------------------------------------------------------------------

// Dalvik VM type signatures
static JNINativeMethod gMethods[] = {
    {   "_load",
        "(Ljava/io/FileDescriptor;JJI)I",
        (void *)android_media_SoundPool_load_FD
    },
    {   "unload",
        "(I)Z",
        (void *)android_media_SoundPool_unload
    },
    {   "_play",
        "(IFFIIFI)I",
        (void *)android_media_SoundPool_play
    },
    {   "pause",
        "(I)V",
        (void *)android_media_SoundPool_pause
    },
    {   "resume",
        "(I)V",
        (void *)android_media_SoundPool_resume
    },
    {   "autoPause",
        "()V",
        (void *)android_media_SoundPool_autoPause
    },
    {   "autoResume",
        "()V",
        (void *)android_media_SoundPool_autoResume
    },
    {   "stop",
        "(I)V",
        (void *)android_media_SoundPool_stop
    },
    {   "_setVolume",
        "(IFF)V",
        (void *)android_media_SoundPool_setVolume
    },
    {   "_mute",
        "(Z)V",
        (void *)android_media_SoundPool_mute
    },
    {   "setPriority",
        "(II)V",
        (void *)android_media_SoundPool_setPriority
    },
    {   "setLoop",
        "(II)V",
        (void *)android_media_SoundPool_setLoop
    },
    {   "setRate",
        "(IF)V",
        (void *)android_media_SoundPool_setRate
    },
    {   "native_setup",
        "(ILjava/lang/Object;Ljava/lang/String;)I",
        (void*)android_media_SoundPool_native_setup
    },
    {   "native_release",
        "()V",
        (void*)android_media_SoundPool_release
    }
};

static const char* const kClassPathName = "android/media/SoundPool";

jint JNI_OnLoad(JavaVM* vm, void* /* reserved */)
{
    JNIEnv* env = nullptr;
    jint result = -1;
    jclass clazz;

    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        ALOGE("ERROR: GetEnv failed\n");
        return result;
    }
    assert(env != nullptr);

    clazz = env->FindClass(kClassPathName);
    if (clazz == nullptr) {
        ALOGE("Can't find %s", kClassPathName);
        return result;
    }

    fields.mNativeContext = env->GetFieldID(clazz, "mNativeContext", "J");
    if (fields.mNativeContext == nullptr) {
        ALOGE("Can't find SoundPool.mNativeContext");
        return result;
    }

    fields.mPostEvent = env->GetMethodID(
            clazz, "postEventFromNative", "(IIILjava/lang/Object;)V");
    if (fields.mPostEvent == nullptr) {
        ALOGE("Can't find android/media/SoundPool.postEventFromNative");
        return result;
    }

    // create a reference to class. Technically, we're leaking this reference
    // since it's a static object.
    fields.mSoundPoolClass = (jclass) env->NewGlobalRef(clazz);

    if (AndroidRuntime::registerNativeMethods(
                env, kClassPathName, gMethods, NELEM(gMethods)) < 0) {
        return result;
    }

    // Get the AudioAttributes class and fields
    jclass audioAttrClass = env->FindClass(kAudioAttributesClassPathName);
    if (audioAttrClass == nullptr) {
        ALOGE("Can't find %s", kAudioAttributesClassPathName);
        return result;
    }
    auto audioAttributesClassRef = (jclass)env->NewGlobalRef(audioAttrClass);
    javaAudioAttrFields.fieldUsage = env->GetFieldID(audioAttributesClassRef, "mUsage", "I");
    javaAudioAttrFields.fieldContentType
                                   = env->GetFieldID(audioAttributesClassRef, "mContentType", "I");
    javaAudioAttrFields.fieldFlags = env->GetFieldID(audioAttributesClassRef, "mFlags", "I");
    javaAudioAttrFields.fieldFormattedTags =
            env->GetFieldID(audioAttributesClassRef, "mFormattedTags", "Ljava/lang/String;");
    env->DeleteGlobalRef(audioAttributesClassRef);
    if (javaAudioAttrFields.fieldUsage == nullptr
            || javaAudioAttrFields.fieldContentType == nullptr
            || javaAudioAttrFields.fieldFlags == nullptr
            || javaAudioAttrFields.fieldFormattedTags == nullptr) {
        ALOGE("Can't initialize AudioAttributes fields");
        return result;
    }

    /* success -- return valid version number */
    return JNI_VERSION_1_4;
}