// Copyright 2024 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef JNI_ZERO_JNI_WRAPPERS_H_
#define JNI_ZERO_JNI_WRAPPERS_H_
#include <jni.h>
#include "third_party/jni_zero/java_refs.h"
#include "third_party/jni_zero/logging.h"
// Wrapper used to receive int when calling Java from native.
// The wrapper disallows automatic conversion of long to int.
// This is to avoid a common anti-pattern where a Java int is used
// to receive a native pointer. Please use a Java long to receive
// native pointers, so that the code works on both 32-bit and 64-bit
// platforms. Note the wrapper allows other lossy conversions into
// jint that could be consider anti-patterns, such as from size_t.
// Checking is only done in debugging builds.
#ifdef NDEBUG
typedef jint JniIntWrapper;
// This inline is sufficiently trivial that it does not change the
// final code generated by g++.
inline jint as_jint(JniIntWrapper wrapper) {
return wrapper;
}
#else
class JniIntWrapper {
public:
JniIntWrapper() : i_(0) {}
JniIntWrapper(int i) : i_(i) {}
JniIntWrapper(const JniIntWrapper& ji) : i_(ji.i_) {}
template <class T>
JniIntWrapper(const T& t) : i_(t) {}
jint as_jint() const { return i_; }
private:
// If you get an "is private" error at the line below it is because you used
// an implicit conversion to convert a long to an int when calling Java.
// We disallow this, as a common anti-pattern allows converting a native
// pointer (intptr_t) to a Java int. Please use a Java long to represent
// a native pointer. If you want a lossy conversion, please use an
// explicit conversion in your C++ code. Note an error is only seen when
// compiling on a 64-bit platform, as intptr_t is indistinguishable from
// int on 32-bit platforms.
JniIntWrapper(long);
jint i_;
};
inline jint as_jint(const JniIntWrapper& wrapper) {
return wrapper.as_jint();
}
#endif // NDEBUG
namespace jni_zero {
// 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 {
JNI_ZERO_DCHECK(reader_ == other.reader_);
return i_ == other.i_;
}
bool operator!=(const iterator& other) const {
JNI_ZERO_DCHECK(reader_ == other.reader_);
return i_ != other.i_;
}
reference operator*() const {
JNI_ZERO_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++() {
JNI_ZERO_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) {}
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;
};
// Use as: @JniType("jni_zero::ByteArrayView") byte[].
//
// This requests a direct pointer to the array data rather than a copy of it,
// so can be more efficient than std::vector<uint8_t> for large arrays.
//
// This helper needs to release the array via its destructor, and as a result
// has more binary size overhead than using std::vector<uint8_t>. As such, you
// should prefer std::vector for small arrays.
//
// Callers must ensure that the passed in array reference outlives this wrapper
// (always the case when used with @JniType).
class ByteArrayView {
public:
ByteArrayView(JNIEnv* env, jbyteArray array)
: env_(env),
array_(array),
length_(env->GetArrayLength(array)),
bytes_(env->GetByteArrayElements(array, nullptr)) {}
~ByteArrayView() {
env_->ReleaseByteArrayElements(array_, bytes_, JNI_ABORT);
}
ByteArrayView(const ByteArrayView&) = delete;
ByteArrayView(ByteArrayView&& other) = delete;
ByteArrayView& operator=(const ByteArrayView&) = delete;
size_t size() const { return static_cast<size_t>(length_); }
bool empty() const { return length_ == 0; }
const jbyte* bytes() const { return bytes_; }
const uint8_t* data() const { return reinterpret_cast<uint8_t*>(bytes_); }
const char* chars() const { return reinterpret_cast<char*>(bytes_); }
std::string_view string_view() const {
return std::string_view(chars(), size());
}
private:
JNIEnv* env_;
jbyteArray array_;
jsize length_;
jbyte* bytes_;
};
} // namespace jni_zero
#endif // JNI_ZERO_JNI_WRAPPERS_H_
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