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// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_ANDROID_SCOPED_JAVA_REF_H_
#define BASE_ANDROID_SCOPED_JAVA_REF_H_
#include <jni.h>
#include <stddef.h>
#include <type_traits>
#include <utility>
#include "base/base_export.h"
#include "base/check_op.h"
#include "base/memory/raw_ptr.h"
namespace base {
namespace android {
// Creates a new local reference frame, in which at least a given number of
// local references can be created. Note that local references already created
// in previous local frames are still valid in the current local frame.
class BASE_EXPORT ScopedJavaLocalFrame {
public:
explicit ScopedJavaLocalFrame(JNIEnv* env);
ScopedJavaLocalFrame(JNIEnv* env, int capacity);
ScopedJavaLocalFrame(const ScopedJavaLocalFrame&) = delete;
ScopedJavaLocalFrame& operator=(const ScopedJavaLocalFrame&) = delete;
~ScopedJavaLocalFrame();
private:
// This class is only good for use on the thread it was created on so
// it's safe to cache the non-threadsafe JNIEnv* inside this object.
raw_ptr<JNIEnv> env_;
};
// Forward declare the generic java reference template class.
template <typename T>
class JavaRef;
// Template specialization of JavaRef, which acts as the base class for all
// other JavaRef<> template types. This allows you to e.g. pass
// ScopedJavaLocalRef<jstring> into a function taking const JavaRef<jobject>&
template <>
class BASE_EXPORT JavaRef<jobject> {
public:
// Initializes a null reference.
constexpr JavaRef() {}
// Allow nullptr to be converted to JavaRef. This avoids having to declare an
// empty JavaRef just to pass null to a function, and makes C++ "nullptr" and
// Java "null" equivalent.
constexpr JavaRef(std::nullptr_t) {}
JavaRef(const JavaRef&) = delete;
JavaRef& operator=(const JavaRef&) = delete;
// Public to allow destruction of null JavaRef objects.
~JavaRef() {}
// TODO(torne): maybe rename this to get() for consistency with unique_ptr
// once there's fewer unnecessary uses of it in the codebase.
jobject obj() const { return obj_; }
explicit operator bool() const { return obj_ != nullptr; }
// Deprecated. Just use bool conversion.
// TODO(torne): replace usage and remove this.
bool is_null() const { return obj_ == nullptr; }
protected:
// Takes ownership of the |obj| reference passed; requires it to be a local
// reference type.
#if DCHECK_IS_ON()
// Implementation contains a DCHECK; implement out-of-line when DCHECK_IS_ON.
JavaRef(JNIEnv* env, jobject obj);
#else
JavaRef(JNIEnv* env, jobject obj) : obj_(obj) {}
#endif
// Used for move semantics. obj_ must have been released first if non-null.
void steal(JavaRef&& other) {
obj_ = other.obj_;
other.obj_ = nullptr;
}
// The following are implementation detail convenience methods, for
// use by the sub-classes.
JNIEnv* SetNewLocalRef(JNIEnv* env, jobject obj);
void SetNewGlobalRef(JNIEnv* env, jobject obj);
void ResetLocalRef(JNIEnv* env);
void ResetGlobalRef();
jobject ReleaseInternal();
private:
jobject obj_ = nullptr;
};
// Forward declare the object array reader for the convenience function.
template <typename T>
class JavaObjectArrayReader;
// Generic base class for ScopedJavaLocalRef and ScopedJavaGlobalRef. Useful
// for allowing functions to accept a reference without having to mandate
// whether it is a local or global type.
template <typename T>
class JavaRef : public JavaRef<jobject> {
public:
constexpr JavaRef() {}
constexpr JavaRef(std::nullptr_t) {}
JavaRef(const JavaRef&) = delete;
JavaRef& operator=(const JavaRef&) = delete;
~JavaRef() {}
T obj() const { return static_cast<T>(JavaRef<jobject>::obj()); }
// Get a JavaObjectArrayReader for the array pointed to by this reference.
// Only defined for JavaRef<jobjectArray>.
// You must pass the type of the array elements (usually jobject) as the
// template parameter.
template <typename ElementType,
typename T_ = T,
typename = std::enable_if_t<std::is_same_v<T_, jobjectArray>>>
JavaObjectArrayReader<ElementType> ReadElements() const {
return JavaObjectArrayReader<ElementType>(*this);
}
protected:
JavaRef(JNIEnv* env, T obj) : JavaRef<jobject>(env, obj) {}
};
// Holds a local reference to a JNI method parameter.
// Method parameters should not be deleted, and so this class exists purely to
// wrap them as a JavaRef<T> in the JNI binding generator. Do not create
// instances manually.
template <typename T>
class JavaParamRef : public JavaRef<T> {
public:
// Assumes that |obj| is a parameter passed to a JNI method from Java.
// Does not assume ownership as parameters should not be deleted.
JavaParamRef(JNIEnv* env, T obj) : JavaRef<T>(env, obj) {}
// Allow nullptr to be converted to JavaParamRef. Some unit tests call JNI
// methods directly from C++ and pass null for objects which are not actually
// used by the implementation (e.g. the caller object); allow this to keep
// working.
JavaParamRef(std::nullptr_t) {}
JavaParamRef(const JavaParamRef&) = delete;
JavaParamRef& operator=(const JavaParamRef&) = delete;
~JavaParamRef() {}
// TODO(torne): remove this cast once we're using JavaRef consistently.
// http://crbug.com/506850
operator T() const { return JavaRef<T>::obj(); }
};
// Holds a local reference to a Java object. The local reference is scoped
// to the lifetime of this object.
// Instances of this class may hold onto any JNIEnv passed into it until
// destroyed. Therefore, since a JNIEnv is only suitable for use on a single
// thread, objects of this class must be created, used, and destroyed, on a
// single thread.
// Therefore, this class should only be used as a stack-based object and from a
// single thread. If you wish to have the reference outlive the current
// callstack (e.g. as a class member) or you wish to pass it across threads,
// use a ScopedJavaGlobalRef instead.
template <typename T>
class ScopedJavaLocalRef : public JavaRef<T> {
public:
// Take ownership of a bare jobject. This does not create a new reference.
// This should only be used by JNI helper functions, or in cases where code
// must call JNIEnv methods directly.
static ScopedJavaLocalRef Adopt(JNIEnv* env, T obj) {
return ScopedJavaLocalRef(env, obj);
}
constexpr ScopedJavaLocalRef() {}
constexpr ScopedJavaLocalRef(std::nullptr_t) {}
// Copy constructor. This is required in addition to the copy conversion
// constructor below.
ScopedJavaLocalRef(const ScopedJavaLocalRef& other) : env_(other.env_) {
JavaRef<T>::SetNewLocalRef(env_, other.obj());
}
// Copy conversion constructor.
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
ScopedJavaLocalRef(const ScopedJavaLocalRef<U>& other) : env_(other.env_) {
JavaRef<T>::SetNewLocalRef(env_, other.obj());
}
// Move constructor. This is required in addition to the move conversion
// constructor below.
ScopedJavaLocalRef(ScopedJavaLocalRef&& other) : env_(other.env_) {
JavaRef<T>::steal(std::move(other));
}
// Move conversion constructor.
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
ScopedJavaLocalRef(ScopedJavaLocalRef<U>&& other) : env_(other.env_) {
JavaRef<T>::steal(std::move(other));
}
// Constructor for other JavaRef types.
explicit ScopedJavaLocalRef(const JavaRef<T>& other) { Reset(other); }
// Assumes that |obj| is a local reference to a Java object and takes
// ownership of this local reference.
// TODO(torne): make legitimate uses call Adopt() instead, and make this
// private.
ScopedJavaLocalRef(JNIEnv* env, T obj) : JavaRef<T>(env, obj), env_(env) {}
~ScopedJavaLocalRef() { Reset(); }
// Null assignment, for disambiguation.
ScopedJavaLocalRef& operator=(std::nullptr_t) {
Reset();
return *this;
}
// Copy assignment.
ScopedJavaLocalRef& operator=(const ScopedJavaLocalRef& other) {
Reset(other);
return *this;
}
// Copy conversion assignment.
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
ScopedJavaLocalRef& operator=(const ScopedJavaLocalRef<U>& other) {
Reset(other);
return *this;
}
// Move assignment.
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
ScopedJavaLocalRef& operator=(ScopedJavaLocalRef<U>&& other) {
env_ = other.env_;
Reset();
JavaRef<T>::steal(std::move(other));
return *this;
}
// Assignment for other JavaRef types.
ScopedJavaLocalRef& operator=(const JavaRef<T>& other) {
Reset(other);
return *this;
}
void Reset() { JavaRef<T>::ResetLocalRef(env_); }
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
void Reset(const ScopedJavaLocalRef<U>& other) {
// We can copy over env_ here as |other| instance must be from the same
// thread as |this| local ref. (See class comment for multi-threading
// limitations, and alternatives).
env_ = JavaRef<T>::SetNewLocalRef(other.env_, other.obj());
}
void Reset(const JavaRef<T>& other) {
// If |env_| was not yet set (is still null) it will be attached to the
// current thread in SetNewLocalRef().
env_ = JavaRef<T>::SetNewLocalRef(env_, other.obj());
}
// Releases the local reference to the caller. The caller *must* delete the
// local reference when it is done with it. Note that calling a Java method
// is *not* a transfer of ownership and Release() should not be used.
T Release() { return static_cast<T>(JavaRef<T>::ReleaseInternal()); }
private:
// This class is only good for use on the thread it was created on so
// it's safe to cache the non-threadsafe JNIEnv* inside this object.
raw_ptr<JNIEnv> env_ = nullptr;
// Prevent ScopedJavaLocalRef(JNIEnv*, T obj) from being used to take
// ownership of a JavaParamRef's underlying object - parameters are not
// allowed to be deleted and so should not be owned by ScopedJavaLocalRef.
// TODO(torne): this can be removed once JavaParamRef no longer has an
// implicit conversion back to T.
ScopedJavaLocalRef(JNIEnv* env, const JavaParamRef<T>& other);
// Friend required to get env_ from conversions.
template <typename U>
friend class ScopedJavaLocalRef;
// Avoids JavaObjectArrayReader having to accept and store its own env.
template <typename U>
friend class JavaObjectArrayReader;
};
// Holds a global reference to a Java object. The global reference is scoped
// to the lifetime of this object. This class does not hold onto any JNIEnv*
// passed to it, hence it is safe to use across threads (within the constraints
// imposed by the underlying Java object that it references).
template <typename T>
class ScopedJavaGlobalRef : public JavaRef<T> {
public:
constexpr ScopedJavaGlobalRef() {}
constexpr ScopedJavaGlobalRef(std::nullptr_t) {}
// Copy constructor. This is required in addition to the copy conversion
// constructor below.
ScopedJavaGlobalRef(const ScopedJavaGlobalRef& other) { Reset(other); }
// Copy conversion constructor.
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
ScopedJavaGlobalRef(const ScopedJavaGlobalRef<U>& other) {
Reset(other);
}
// Move constructor. This is required in addition to the move conversion
// constructor below.
ScopedJavaGlobalRef(ScopedJavaGlobalRef&& other) {
JavaRef<T>::steal(std::move(other));
}
// Move conversion constructor.
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
ScopedJavaGlobalRef(ScopedJavaGlobalRef<U>&& other) {
JavaRef<T>::steal(std::move(other));
}
// Conversion constructor for other JavaRef types.
explicit ScopedJavaGlobalRef(const JavaRef<T>& other) { Reset(other); }
// Create a new global reference to the object.
// Deprecated. Don't use bare jobjects; use a JavaRef as the input.
ScopedJavaGlobalRef(JNIEnv* env, T obj) { Reset(env, obj); }
~ScopedJavaGlobalRef() { Reset(); }
// Null assignment, for disambiguation.
ScopedJavaGlobalRef& operator=(std::nullptr_t) {
Reset();
return *this;
}
// Copy assignment.
ScopedJavaGlobalRef& operator=(const ScopedJavaGlobalRef& other) {
Reset(other);
return *this;
}
// Copy conversion assignment.
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
ScopedJavaGlobalRef& operator=(const ScopedJavaGlobalRef<U>& other) {
Reset(other);
return *this;
}
// Move assignment.
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
ScopedJavaGlobalRef& operator=(ScopedJavaGlobalRef<U>&& other) {
Reset();
JavaRef<T>::steal(std::move(other));
return *this;
}
// Assignment for other JavaRef types.
ScopedJavaGlobalRef& operator=(const JavaRef<T>& other) {
Reset(other);
return *this;
}
void Reset() { JavaRef<T>::ResetGlobalRef(); }
template <typename U,
typename = std::enable_if_t<std::is_convertible_v<U, T>>>
void Reset(const ScopedJavaGlobalRef<U>& other) {
Reset(nullptr, other.obj());
}
void Reset(const JavaRef<T>& other) { Reset(nullptr, other.obj()); }
// Deprecated. You can just use Reset(const JavaRef&).
void Reset(JNIEnv* env, const JavaParamRef<T>& other) {
Reset(env, other.obj());
}
// Deprecated. Don't use bare jobjects; use a JavaRef as the input.
void Reset(JNIEnv* env, T obj) { JavaRef<T>::SetNewGlobalRef(env, obj); }
// Releases the global reference to the caller. The caller *must* delete the
// global reference when it is done with it. Note that calling a Java method
// is *not* a transfer of ownership and Release() should not be used.
T Release() { return static_cast<T>(JavaRef<T>::ReleaseInternal()); }
};
// Wrapper for a jobjectArray which supports input iteration, allowing Java
// arrays to be iterated over with a range-based for loop, or used with
// <algorithm> functions that accept input iterators.
//
// The iterator returns each object in the array in turn, wrapped in a
// ScopedJavaLocalRef<T>. T will usually be jobject, but if you know that the
// array contains a more specific type (such as jstring) you can use that
// instead. This does not check the type at runtime!
//
// The wrapper holds a local reference to the array and only queries the size of
// the array once, so must only be used as a stack-based object from the current
// thread.
//
// Note that this does *not* update the contents of the array if you mutate the
// returned ScopedJavaLocalRef.
template <typename T>
class JavaObjectArrayReader {
public:
class iterator {
public:
// We can only be an input iterator, as all richer iterator types must
// implement the multipass guarantee (always returning the same object for
// the same iterator position), which is not practical when returning
// temporary objects.
using iterator_category = std::input_iterator_tag;
using difference_type = ptrdiff_t;
using value_type = ScopedJavaLocalRef<T>;
// It doesn't make sense to return a reference type as the iterator creates
// temporary wrapper objects when dereferenced. Fortunately, it's not
// required that input iterators actually use references, and defining it
// as value_type is valid.
using reference = value_type;
// This exists to make operator-> work as expected: its return value must
// resolve to an actual pointer (otherwise the compiler just keeps calling
// operator-> on the return value until it does), so we need an extra level
// of indirection. This is sometimes called an "arrow proxy" or similar, and
// this version is adapted from base/value_iterators.h.
class pointer {
public:
explicit pointer(const reference& ref) : ref_(ref) {}
pointer(const pointer& ptr) = default;
pointer& operator=(const pointer& ptr) = delete;
reference* operator->() { return &ref_; }
private:
reference ref_;
};
iterator(const iterator&) = default;
~iterator() = default;
iterator& operator=(const iterator&) = default;
bool operator==(const iterator& other) const {
DCHECK(reader_ == other.reader_);
return i_ == other.i_;
}
bool operator!=(const iterator& other) const {
DCHECK(reader_ == other.reader_);
return i_ != other.i_;
}
reference operator*() const {
DCHECK(i_ < reader_->size_);
// JNIEnv functions return unowned local references; take ownership with
// Adopt so that ~ScopedJavaLocalRef will release it automatically later.
return value_type::Adopt(
reader_->array_.env_,
static_cast<T>(reader_->array_.env_->GetObjectArrayElement(
reader_->array_.obj(), i_)));
}
pointer operator->() const { return pointer(operator*()); }
iterator& operator++() {
DCHECK(i_ < reader_->size_);
++i_;
return *this;
}
iterator operator++(int) {
iterator old = *this;
++*this;
return old;
}
private:
iterator(const JavaObjectArrayReader* reader, jsize i)
: reader_(reader), i_(i) {}
raw_ptr<const JavaObjectArrayReader<T>> reader_;
jsize i_;
friend JavaObjectArrayReader;
};
JavaObjectArrayReader(const JavaRef<jobjectArray>& array) : array_(array) {
size_ = array_.env_->GetArrayLength(array_.obj());
}
// Copy constructor to allow returning it from JavaRef::ReadElements().
JavaObjectArrayReader(const JavaObjectArrayReader& other) = default;
// Assignment operator for consistency with copy constructor.
JavaObjectArrayReader& operator=(const JavaObjectArrayReader& other) =
default;
// Allow move constructor and assignment since this owns a local ref.
JavaObjectArrayReader(JavaObjectArrayReader&& other) = default;
JavaObjectArrayReader& operator=(JavaObjectArrayReader&& other) = default;
bool empty() const { return size_ == 0; }
jsize size() const { return size_; }
iterator begin() const { return iterator(this, 0); }
iterator end() const { return iterator(this, size_); }
private:
ScopedJavaLocalRef<jobjectArray> array_;
jsize size_;
friend iterator;
};
} // namespace android
} // namespace base
#endif // BASE_ANDROID_SCOPED_JAVA_REF_H_
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