gecko-dev/widget/android/jni/Natives.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef mozilla_jni_Natives_h__
#define mozilla_jni_Natives_h__

#include <jni.h>

#include <type_traits>
#include <utility>

#include "mozilla/RefPtr.h"
#include "mozilla/Tuple.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/UniquePtr.h"
#include "mozilla/Unused.h"
#include "mozilla/WeakPtr.h"
#include "mozilla/jni/Accessors.h"
#include "mozilla/jni/Refs.h"
#include "mozilla/jni/Types.h"
#include "mozilla/jni/Utils.h"
#include "nsThreadUtils.h"

struct NativeException {
  const char* str;
};

template <class T>
static NativeException NullHandle() {
  return {__func__};
}

template <class T>
static NativeException NullWeakPtr() {
  return {__func__};
}

namespace mozilla {
namespace jni {

/**
 * C++ classes implementing instance (non-static) native methods can choose
 * from one of two ownership models, when associating a C++ object with a Java
 * instance.
 *
 * * If the C++ class inherits from mozilla::SupportsWeakPtr, weak pointers
 *   will be used. The Java instance will store and own the pointer to a
 *   WeakPtr object. The C++ class itself is otherwise not owned or directly
 *   referenced. Note that mozilla::SupportsWeakPtr only supports being used on
 *   a single thread. To attach a Java instance to a C++ instance, pass in a
 *   mozilla::SupportsWeakPtr pointer to the C++ class (i.e. MyClass*).
 *
 *   class MyClass : public SupportsWeakPtr<MyClass>
 *                 , public MyJavaClass::Natives<MyClass>
 *   {
 *       // ...
 *
 *   public:
 *       MOZ_DECLARE_WEAKREFERENCE_TYPENAME(MyClass)
 *       using MyJavaClass::Natives<MyClass>::DisposeNative;
 *
 *       void AttachTo(const MyJavaClass::LocalRef& instance)
 *       {
 *           MyJavaClass::Natives<MyClass>::AttachNative(
 *                   instance, static_cast<SupportsWeakPtr<MyClass>*>(this));
 *
 *           // "instance" does NOT own "this", so the C++ object
 *           // lifetime is separate from the Java object lifetime.
 *       }
 *   };
 *
 * * If the C++ class contains public members AddRef() and Release(), the Java
 *   instance will store and own the pointer to a RefPtr object, which holds a
 *   strong reference on the C++ instance. Normal ref-counting considerations
 *   apply in this case; for example, disposing may cause the C++ instance to
 *   be deleted and the destructor to be run on the current thread, which may
 *   not be desirable. To attach a Java instance to a C++ instance, pass in a
 *   pointer to the C++ class (i.e. MyClass*).
 *
 *   class MyClass : public RefCounted<MyClass>
 *                 , public MyJavaClass::Natives<MyClass>
 *   {
 *       // ...
 *
 *   public:
 *       using MyJavaClass::Natives<MyClass>::DisposeNative;
 *
 *       void AttachTo(const MyJavaClass::LocalRef& instance)
 *       {
 *           MyJavaClass::Natives<MyClass>::AttachNative(instance, this);
 *
 *           // "instance" owns "this" through the RefPtr, so the C++ object
 *           // may be destroyed as soon as instance.disposeNative() is called.
 *       }
 *   };
 *
 * * In other cases, the Java instance will store and own a pointer to the C++
 *   object itself. This pointer must not be stored or deleted elsewhere. To
 *   attach a Java instance to a C++ instance, pass in a reference to a
 *   UniquePtr of the C++ class (i.e. UniquePtr<MyClass>).
 *
 *   class MyClass : public MyJavaClass::Natives<MyClass>
 *   {
 *       // ...
 *
 *   public:
 *       using MyJavaClass::Natives<MyClass>::DisposeNative;
 *
 *       static void AttachTo(const MyJavaClass::LocalRef& instance)
 *       {
 *           MyJavaClass::Natives<MyClass>::AttachNative(
 *                   instance, mozilla::MakeUnique<MyClass>());
 *
 *           // "instance" owns the newly created C++ object, so the C++
 *           // object is destroyed as soon as instance.disposeNative() is
 *           // called.
 *       }
 *   };
 */

namespace detail {

enum NativePtrType { OWNING, WEAK, REFPTR };

template <class Impl>
class NativePtrPicker {
  template <class I>
  static std::enable_if_t<std::is_base_of<SupportsWeakPtr<I>, I>::value,
                          char (&)[NativePtrType::WEAK]>
  Test(char);

  template <class I, typename = decltype(&I::AddRef, &I::Release)>
  static char (&Test(int))[NativePtrType::REFPTR];

  template <class>
  static char (&Test(...))[NativePtrType::OWNING];

 public:
  static const int value = sizeof(Test<Impl>('\0')) / sizeof(char);
};

template <class Impl>
inline uintptr_t CheckNativeHandle(JNIEnv* env, uintptr_t handle) {
  if (!handle) {
    if (!env->ExceptionCheck()) {
      ThrowException(env, "java/lang/NullPointerException",
                     NullHandle<Impl>().str);
    }
    return 0;
  }
  return handle;
}

template <class Impl, int Type = NativePtrPicker<Impl>::value>
struct NativePtr;

template <class Impl>
struct NativePtr<Impl, /* Type = */ NativePtrType::OWNING> {
  static Impl* Get(JNIEnv* env, jobject instance) {
    return reinterpret_cast<Impl*>(
        CheckNativeHandle<Impl>(env, GetNativeHandle(env, instance)));
  }

  template <class LocalRef>
  static Impl* Get(const LocalRef& instance) {
    return Get(instance.Env(), instance.Get());
  }

  template <class LocalRef>
  static void Set(const LocalRef& instance, UniquePtr<Impl>&& ptr) {
    Clear(instance);
    SetNativeHandle(instance.Env(), instance.Get(),
                    reinterpret_cast<uintptr_t>(ptr.release()));
    MOZ_CATCH_JNI_EXCEPTION(instance.Env());
  }

  template <class LocalRef>
  static void Clear(const LocalRef& instance) {
    UniquePtr<Impl> ptr(reinterpret_cast<Impl*>(
        GetNativeHandle(instance.Env(), instance.Get())));
    MOZ_CATCH_JNI_EXCEPTION(instance.Env());

    if (ptr) {
      SetNativeHandle(instance.Env(), instance.Get(), 0);
      MOZ_CATCH_JNI_EXCEPTION(instance.Env());
    }
  }
};

template <class Impl>
struct NativePtr<Impl, /* Type = */ NativePtrType::WEAK> {
  static Impl* Get(JNIEnv* env, jobject instance) {
    const auto ptr = reinterpret_cast<WeakPtr<Impl>*>(
        CheckNativeHandle<Impl>(env, GetNativeHandle(env, instance)));
    if (!ptr) {
      return nullptr;
    }

    Impl* const impl = *ptr;
    if (!impl) {
      ThrowException(env, "java/lang/NullPointerException",
                     NullWeakPtr<Impl>().str);
    }
    return impl;
  }

  template <class LocalRef>
  static Impl* Get(const LocalRef& instance) {
    return Get(instance.Env(), instance.Get());
  }

  template <class LocalRef>
  static void Set(const LocalRef& instance, Impl* ptr) {
    // Create the new handle first before clearing any old handle, so the
    // new handle is guaranteed to have different value than any old handle.
    const uintptr_t handle =
        reinterpret_cast<uintptr_t>(new WeakPtr<Impl>(ptr));
    Clear(instance);
    SetNativeHandle(instance.Env(), instance.Get(), handle);
    MOZ_CATCH_JNI_EXCEPTION(instance.Env());
  }

  template <class LocalRef>
  static void Clear(const LocalRef& instance) {
    const auto ptr = reinterpret_cast<WeakPtr<Impl>*>(
        GetNativeHandle(instance.Env(), instance.Get()));
    MOZ_CATCH_JNI_EXCEPTION(instance.Env());

    if (ptr) {
      SetNativeHandle(instance.Env(), instance.Get(), 0);
      MOZ_CATCH_JNI_EXCEPTION(instance.Env());
      delete ptr;
    }
  }
};

template <class Impl>
struct NativePtr<Impl, /* Type = */ NativePtrType::REFPTR> {
  static Impl* Get(JNIEnv* env, jobject instance) {
    const auto ptr = reinterpret_cast<RefPtr<Impl>*>(
        CheckNativeHandle<Impl>(env, GetNativeHandle(env, instance)));
    if (!ptr) {
      return nullptr;
    }

    MOZ_ASSERT(*ptr);
    return *ptr;
  }

  template <class LocalRef>
  static Impl* Get(const LocalRef& instance) {
    return Get(instance.Env(), instance.Get());
  }

  template <class LocalRef>
  static void Set(const LocalRef& instance, Impl* ptr) {
    // Create the new handle first before clearing any old handle, so the
    // new handle is guaranteed to have different value than any old handle.
    const uintptr_t handle = reinterpret_cast<uintptr_t>(new RefPtr<Impl>(ptr));
    Clear(instance);
    SetNativeHandle(instance.Env(), instance.Get(), handle);
    MOZ_CATCH_JNI_EXCEPTION(instance.Env());
  }

  template <class LocalRef>
  static void Clear(const LocalRef& instance) {
    const auto ptr = reinterpret_cast<RefPtr<Impl>*>(
        GetNativeHandle(instance.Env(), instance.Get()));
    MOZ_CATCH_JNI_EXCEPTION(instance.Env());

    if (ptr) {
      SetNativeHandle(instance.Env(), instance.Get(), 0);
      MOZ_CATCH_JNI_EXCEPTION(instance.Env());
      delete ptr;
    }
  }
};

}  // namespace detail

using namespace detail;

/**
 * For JNI native methods that are dispatched to a proxy, i.e. using
 * @WrapForJNI(dispatchTo = "proxy"), the implementing C++ class must provide a
 * OnNativeCall member. Subsequently, every native call is automatically
 * wrapped in a functor object, and the object is passed to OnNativeCall. The
 * OnNativeCall implementation can choose to invoke the call, save it, dispatch
 * it to a different thread, etc. Each copy of functor may only be invoked
 * once.
 *
 * class MyClass : public MyJavaClass::Natives<MyClass>
 * {
 *     // ...
 *
 *     template<class Functor>
 *     class ProxyRunnable final : public Runnable
 *     {
 *         Functor mCall;
 *     public:
 *         ProxyRunnable(Functor&& call) : mCall(std::move(call)) {}
 *         virtual void run() override { mCall(); }
 *     };
 *
 * public:
 *     template<class Functor>
 *     static void OnNativeCall(Functor&& call)
 *     {
 *         RunOnAnotherThread(new ProxyRunnable(std::move(call)));
 *     }
 * };
 */

namespace detail {

// ProxyArg is used to handle JNI ref arguments for proxies. Because a proxied
// call may happen outside of the original JNI native call, we must save all
// JNI ref arguments as global refs to avoid the arguments going out of scope.
template <typename T>
struct ProxyArg {
  static_assert(mozilla::IsPod<T>::value, "T must be primitive type");

  // Primitive types can be saved by value.
  typedef T Type;
  typedef typename TypeAdapter<T>::JNIType JNIType;

  static void Clear(JNIEnv* env, Type&) {}

  static Type From(JNIEnv* env, JNIType val) {
    return TypeAdapter<T>::ToNative(env, val);
  }
};

template <class C, typename T>
struct ProxyArg<Ref<C, T>> {
  // Ref types need to be saved by global ref.
  typedef typename C::GlobalRef Type;
  typedef typename TypeAdapter<Ref<C, T>>::JNIType JNIType;

  static void Clear(JNIEnv* env, Type& ref) { ref.Clear(env); }

  static Type From(JNIEnv* env, JNIType val) {
    return Type(env, C::Ref::From(val));
  }
};

template <typename C>
struct ProxyArg<const C&> : ProxyArg<C> {};
template <>
struct ProxyArg<StringParam> : ProxyArg<String::Ref> {};
template <class C>
struct ProxyArg<LocalRef<C>> : ProxyArg<typename C::Ref> {};

// ProxyNativeCall implements the functor object that is passed to OnNativeCall
template <class Impl, class Owner, bool IsStatic,
          bool HasThisArg /* has instance/class local ref in the call */,
          typename... Args>
class ProxyNativeCall {
  // "this arg" refers to the Class::LocalRef (for static methods) or
  // Owner::LocalRef (for instance methods) that we optionally (as indicated
  // by HasThisArg) pass into the destination C++ function.
  using ThisArgClass = std::conditional_t<IsStatic, Class, Owner>;
  using ThisArgJNIType = std::conditional_t<IsStatic, jclass, jobject>;

  // Type signature of the destination C++ function, which matches the
  // Method template parameter in NativeStubImpl::Wrap.
  using NativeCallType = std::conditional_t<
      IsStatic,
      std::conditional_t<HasThisArg, void (*)(const Class::LocalRef&, Args...),
                         void (*)(Args...)>,
      std::conditional_t<
          HasThisArg, void (Impl::*)(const typename Owner::LocalRef&, Args...),
          void (Impl::*)(Args...)>>;

  // Destination C++ function.
  NativeCallType mNativeCall;
  // Saved this arg.
  typename ThisArgClass::GlobalRef mThisArg;
  // Saved arguments.
  mozilla::Tuple<typename ProxyArg<Args>::Type...> mArgs;

  // We cannot use IsStatic and HasThisArg directly (without going through
  // extra hoops) because GCC complains about invalid overloads, so we use
  // another pair of template parameters, Static and ThisArg.

  template <bool Static, bool ThisArg, size_t... Indices>
  std::enable_if_t<Static && ThisArg, void> Call(
      const Class::LocalRef& cls, std::index_sequence<Indices...>) const {
    (*mNativeCall)(cls, mozilla::Get<Indices>(mArgs)...);
  }

  template <bool Static, bool ThisArg, size_t... Indices>
  std::enable_if_t<Static && !ThisArg, void> Call(
      const Class::LocalRef& cls, std::index_sequence<Indices...>) const {
    (*mNativeCall)(mozilla::Get<Indices>(mArgs)...);
  }

  template <bool Static, bool ThisArg, size_t... Indices>
  std::enable_if_t<!Static && ThisArg, void> Call(
      const typename Owner::LocalRef& inst,
      std::index_sequence<Indices...>) const {
    Impl* const impl = NativePtr<Impl>::Get(inst);
    MOZ_CATCH_JNI_EXCEPTION(inst.Env());
    (impl->*mNativeCall)(inst, mozilla::Get<Indices>(mArgs)...);
  }

  template <bool Static, bool ThisArg, size_t... Indices>
  std::enable_if_t<!Static && !ThisArg, void> Call(
      const typename Owner::LocalRef& inst,
      std::index_sequence<Indices...>) const {
    Impl* const impl = NativePtr<Impl>::Get(inst);
    MOZ_CATCH_JNI_EXCEPTION(inst.Env());
    (impl->*mNativeCall)(mozilla::Get<Indices>(mArgs)...);
  }

  template <size_t... Indices>
  void Clear(JNIEnv* env, std::index_sequence<Indices...>) {
    int dummy[] = {(ProxyArg<Args>::Clear(env, Get<Indices>(mArgs)), 0)...};
    mozilla::Unused << dummy;
  }

  static Impl* GetNativeObject(Class::Param thisArg) { return nullptr; }

  static Impl* GetNativeObject(typename Owner::Param thisArg) {
    return NativePtr<Impl>::Get(GetEnvForThread(), thisArg.Get());
  }

 public:
  // The class that implements the call target.
  typedef Impl TargetClass;
  typedef typename ThisArgClass::Param ThisArgType;

  static const bool isStatic = IsStatic;

  ProxyNativeCall(ThisArgJNIType thisArg, NativeCallType nativeCall,
                  JNIEnv* env, typename ProxyArg<Args>::JNIType... args)
      : mNativeCall(nativeCall),
        mThisArg(env, ThisArgClass::Ref::From(thisArg)),
        mArgs(ProxyArg<Args>::From(env, args)...) {}

  ProxyNativeCall(ProxyNativeCall&&) = default;
  ProxyNativeCall(const ProxyNativeCall&) = default;

  // Get class ref for static calls or object ref for instance calls.
  typename ThisArgClass::Param GetThisArg() const { return mThisArg; }

  // Get the native object targeted by this call.
  // Returns nullptr for static calls.
  Impl* GetNativeObject() const { return GetNativeObject(mThisArg); }

  // Return if target is the given function pointer / pointer-to-member.
  // Because we can only compare pointers of the same type, we use a
  // templated overload that is chosen only if given a different type of
  // pointer than our target pointer type.
  bool IsTarget(NativeCallType call) const { return call == mNativeCall; }
  template <typename T>
  bool IsTarget(T&&) const {
    return false;
  }

  // Redirect the call to another function / class member with the same
  // signature as the original target. Crash if given a wrong signature.
  void SetTarget(NativeCallType call) { mNativeCall = call; }
  template <typename T>
  void SetTarget(T&&) const {
    MOZ_CRASH();
  }

  void operator()() {
    JNIEnv* const env = GetEnvForThread();
    typename ThisArgClass::LocalRef thisArg(env, mThisArg);
    Call<IsStatic, HasThisArg>(thisArg, std::index_sequence_for<Args...>{});

    // Clear all saved global refs. We do this after the call is invoked,
    // and not inside the destructor because we already have a JNIEnv here,
    // so it's more efficient to clear out the saved args here. The
    // downside is that the call can only be invoked once.
    Clear(env, std::index_sequence_for<Args...>{});
    mThisArg.Clear(env);
  }
};

template <class Impl, bool HasThisArg, typename... Args>
struct Dispatcher {
  template <class Traits, bool IsStatic = Traits::isStatic,
            typename... ProxyArgs>
  static std::enable_if_t<Traits::dispatchTarget == DispatchTarget::PROXY, void>
  Run(ProxyArgs&&... args) {
    Impl::OnNativeCall(
        ProxyNativeCall<Impl, typename Traits::Owner, IsStatic, HasThisArg,
                        Args...>(std::forward<ProxyArgs>(args)...));
  }

  template <class Traits, bool IsStatic = Traits::isStatic, typename ThisArg,
            typename... ProxyArgs>
  static std::enable_if_t<
      Traits::dispatchTarget == DispatchTarget::GECKO_PRIORITY, void>
  Run(ThisArg thisArg, ProxyArgs&&... args) {
    // For a static method, do not forward the "this arg" (i.e. the class
    // local ref) if the implementation does not request it. This saves us
    // a pair of calls to add/delete global ref.
    auto proxy =
        ProxyNativeCall<Impl, typename Traits::Owner, IsStatic, HasThisArg,
                        Args...>((HasThisArg || !IsStatic) ? thisArg : nullptr,
                                 std::forward<ProxyArgs>(args)...);
    DispatchToGeckoPriorityQueue(
        NS_NewRunnableFunction("PriorityNativeCall", std::move(proxy)));
  }

  template <class Traits, bool IsStatic = Traits::isStatic, typename ThisArg,
            typename... ProxyArgs>
  static std::enable_if_t<Traits::dispatchTarget == DispatchTarget::GECKO, void>
  Run(ThisArg thisArg, ProxyArgs&&... args) {
    // For a static method, do not forward the "this arg" (i.e. the class
    // local ref) if the implementation does not request it. This saves us
    // a pair of calls to add/delete global ref.
    auto proxy =
        ProxyNativeCall<Impl, typename Traits::Owner, IsStatic, HasThisArg,
                        Args...>((HasThisArg || !IsStatic) ? thisArg : nullptr,
                                 std::forward<ProxyArgs>(args)...);
    NS_DispatchToMainThread(
        NS_NewRunnableFunction("GeckoNativeCall", std::move(proxy)));
  }

  template <class Traits, bool IsStatic = false, typename... ProxyArgs>
  static std::enable_if_t<Traits::dispatchTarget == DispatchTarget::CURRENT,
                          void>
  Run(ProxyArgs&&... args) {
    MOZ_CRASH("Unreachable code");
  }
};

}  // namespace detail

// Wrapper methods that convert arguments from the JNI types to the native
// types, e.g. from jobject to jni::Object::Ref. For instance methods, the
// wrapper methods also convert calls to calls on objects.
//
// We need specialization for static/non-static because the two have different
// signatures (jobject vs jclass and Impl::*Method vs *Method).
// We need specialization for return type, because void return type requires
// us to not deal with the return value.

// Bug 1207642 - Work around Dalvik bug by realigning stack on JNI entry
#ifdef __i386__
#  define MOZ_JNICALL JNICALL __attribute__((force_align_arg_pointer))
#else
#  define MOZ_JNICALL JNICALL
#endif

template <class Traits, class Impl, class Args = typename Traits::Args>
class NativeStub;

template <class Traits, class Impl, typename... Args>
class NativeStub<Traits, Impl, jni::Args<Args...>> {
  using Owner = typename Traits::Owner;
  using ReturnType = typename Traits::ReturnType;

  static constexpr bool isStatic = Traits::isStatic;
  static constexpr bool isVoid = std::is_void_v<ReturnType>;

  struct VoidType {
    using JNIType = void;
  };
  using ReturnJNIType =
      typename std::conditional_t<isVoid, VoidType,
                                  TypeAdapter<ReturnType>>::JNIType;

  using ReturnTypeForNonVoidInstance =
      std::conditional_t<!isStatic && !isVoid, ReturnType, VoidType>;
  using ReturnTypeForVoidInstance =
      std::conditional_t<!isStatic && isVoid, ReturnType, VoidType&>;
  using ReturnTypeForNonVoidStatic =
      std::conditional_t<isStatic && !isVoid, ReturnType, VoidType>;
  using ReturnTypeForVoidStatic =
      std::conditional_t<isStatic && isVoid, ReturnType, VoidType&>;

  static_assert(Traits::dispatchTarget == DispatchTarget::CURRENT || isVoid,
                "Dispatched calls must have void return type");

 public:
  // Non-void instance method
  template <ReturnTypeForNonVoidInstance (Impl::*Method)(Args...)>
  static MOZ_JNICALL ReturnJNIType
  Wrap(JNIEnv* env, jobject instance,
       typename TypeAdapter<Args>::JNIType... args) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    Impl* const impl = NativePtr<Impl>::Get(env, instance);
    if (!impl) {
      // There is a pending JNI exception at this point.
      return ReturnJNIType();
    }
    return TypeAdapter<ReturnType>::FromNative(
        env, (impl->*Method)(TypeAdapter<Args>::ToNative(env, args)...));
  }

  // Non-void instance method with instance reference
  template <ReturnTypeForNonVoidInstance (Impl::*Method)(
      const typename Owner::LocalRef&, Args...)>
  static MOZ_JNICALL ReturnJNIType
  Wrap(JNIEnv* env, jobject instance,
       typename TypeAdapter<Args>::JNIType... args) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    Impl* const impl = NativePtr<Impl>::Get(env, instance);
    if (!impl) {
      // There is a pending JNI exception at this point.
      return ReturnJNIType();
    }
    auto self = Owner::LocalRef::Adopt(env, instance);
    const auto res = TypeAdapter<ReturnType>::FromNative(
        env, (impl->*Method)(self, TypeAdapter<Args>::ToNative(env, args)...));
    self.Forget();
    return res;
  }

  // Void instance method
  template <ReturnTypeForVoidInstance (Impl::*Method)(Args...)>
  static MOZ_JNICALL void Wrap(JNIEnv* env, jobject instance,
                               typename TypeAdapter<Args>::JNIType... args) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    if (Traits::dispatchTarget != DispatchTarget::CURRENT) {
      Dispatcher<Impl, /* HasThisArg */ false, Args...>::template Run<Traits>(
          instance, Method, env, args...);
      return;
    }

    Impl* const impl = NativePtr<Impl>::Get(env, instance);
    if (!impl) {
      // There is a pending JNI exception at this point.
      return;
    }
    (impl->*Method)(TypeAdapter<Args>::ToNative(env, args)...);
  }

  // Void instance method with instance reference
  template <ReturnTypeForVoidInstance (Impl::*Method)(
      const typename Owner::LocalRef&, Args...)>
  static MOZ_JNICALL void Wrap(JNIEnv* env, jobject instance,
                               typename TypeAdapter<Args>::JNIType... args) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    if (Traits::dispatchTarget != DispatchTarget::CURRENT) {
      Dispatcher<Impl, /* HasThisArg */ true, Args...>::template Run<Traits>(
          instance, Method, env, args...);
      return;
    }

    Impl* const impl = NativePtr<Impl>::Get(env, instance);
    if (!impl) {
      // There is a pending JNI exception at this point.
      return;
    }
    auto self = Owner::LocalRef::Adopt(env, instance);
    (impl->*Method)(self, TypeAdapter<Args>::ToNative(env, args)...);
    self.Forget();
  }

  // Overload for DisposeNative
  template <ReturnTypeForVoidInstance (*DisposeNative)(
      const typename Owner::LocalRef&)>
  static MOZ_JNICALL void Wrap(JNIEnv* env, jobject instance) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    if (Traits::dispatchTarget != DispatchTarget::CURRENT) {
      using LocalRef = typename Owner::LocalRef;
      Dispatcher<Impl, /* HasThisArg */ false, const LocalRef&>::template Run<
          Traits, /* IsStatic */ true>(
          /* ThisArg */ nullptr, DisposeNative, env, instance);
      return;
    }

    auto self = Owner::LocalRef::Adopt(env, instance);
    DisposeNative(self);
    self.Forget();
  }

  // Non-void static method
  template <ReturnTypeForNonVoidStatic (*Method)(Args...)>
  static MOZ_JNICALL ReturnJNIType
  Wrap(JNIEnv* env, jclass, typename TypeAdapter<Args>::JNIType... args) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    return TypeAdapter<ReturnType>::FromNative(
        env, (*Method)(TypeAdapter<Args>::ToNative(env, args)...));
  }

  // Non-void static method with class reference
  template <ReturnTypeForNonVoidStatic (*Method)(const Class::LocalRef&,
                                                 Args...)>
  static MOZ_JNICALL ReturnJNIType
  Wrap(JNIEnv* env, jclass cls, typename TypeAdapter<Args>::JNIType... args) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    auto clazz = Class::LocalRef::Adopt(env, cls);
    const auto res = TypeAdapter<ReturnType>::FromNative(
        env, (*Method)(clazz, TypeAdapter<Args>::ToNative(env, args)...));
    clazz.Forget();
    return res;
  }

  // Void static method
  template <ReturnTypeForVoidStatic (*Method)(Args...)>
  static MOZ_JNICALL void Wrap(JNIEnv* env, jclass cls,
                               typename TypeAdapter<Args>::JNIType... args) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    if (Traits::dispatchTarget != DispatchTarget::CURRENT) {
      Dispatcher<Impl, /* HasThisArg */ false, Args...>::template Run<Traits>(
          cls, Method, env, args...);
      return;
    }

    (*Method)(TypeAdapter<Args>::ToNative(env, args)...);
  }

  // Void static method with class reference
  template <ReturnTypeForVoidStatic (*Method)(const Class::LocalRef&, Args...)>
  static MOZ_JNICALL void Wrap(JNIEnv* env, jclass cls,
                               typename TypeAdapter<Args>::JNIType... args) {
    MOZ_ASSERT_JNI_THREAD(Traits::callingThread);

    if (Traits::dispatchTarget != DispatchTarget::CURRENT) {
      Dispatcher<Impl, /* HasThisArg */ true, Args...>::template Run<Traits>(
          cls, Method, env, args...);
      return;
    }

    auto clazz = Class::LocalRef::Adopt(env, cls);
    (*Method)(clazz, TypeAdapter<Args>::ToNative(env, args)...);
    clazz.Forget();
  }
};

// Generate a JNINativeMethod from a native
// method's traits class and a wrapped stub.
template <class Traits, typename Ret, typename... Args>
constexpr JNINativeMethod MakeNativeMethod(MOZ_JNICALL Ret (*stub)(JNIEnv*,
                                                                   Args...)) {
  return {Traits::name, Traits::signature, reinterpret_cast<void*>(stub)};
}

// Class inherited by implementing class.
template <class Cls, class Impl>
class NativeImpl {
  typedef typename Cls::template Natives<Impl> Natives;

  static bool sInited;

 public:
  static void Init() {
    if (sInited) {
      return;
    }
    const auto& ctx = typename Cls::Context();
    ctx.Env()->RegisterNatives(
        ctx.ClassRef(), Natives::methods,
        sizeof(Natives::methods) / sizeof(Natives::methods[0]));
    MOZ_CATCH_JNI_EXCEPTION(ctx.Env());
    sInited = true;
  }

 protected:
  // Associate a C++ instance with a Java instance.
  static void AttachNative(const typename Cls::LocalRef& instance,
                           SupportsWeakPtr<Impl>* ptr) {
    static_assert(NativePtrPicker<Impl>::value == NativePtrType::WEAK,
                  "Use another AttachNative for non-WeakPtr usage");
    return NativePtr<Impl>::Set(instance, static_cast<Impl*>(ptr));
  }

  static void AttachNative(const typename Cls::LocalRef& instance,
                           UniquePtr<Impl>&& ptr) {
    static_assert(NativePtrPicker<Impl>::value == NativePtrType::OWNING,
                  "Use another AttachNative for WeakPtr or RefPtr usage");
    return NativePtr<Impl>::Set(instance, std::move(ptr));
  }

  static void AttachNative(const typename Cls::LocalRef& instance, Impl* ptr) {
    static_assert(NativePtrPicker<Impl>::value == NativePtrType::REFPTR,
                  "Use another AttachNative for non-RefPtr usage");
    return NativePtr<Impl>::Set(instance, ptr);
  }

  // Get the C++ instance associated with a Java instance.
  // There is always a pending exception if the return value is nullptr.
  static Impl* GetNative(const typename Cls::LocalRef& instance) {
    return NativePtr<Impl>::Get(instance);
  }

  static void DisposeNative(const typename Cls::LocalRef& instance) {
    NativePtr<Impl>::Clear(instance);
  }

  NativeImpl() {
    // Initialize on creation if not already initialized.
    Init();
  }
};

// Define static member.
template <class C, class I>
bool NativeImpl<C, I>::sInited;

}  // namespace jni
}  // namespace mozilla

#endif  // mozilla_jni_Natives_h__