lokinet/jni/lokinet_jni_common.hpp

#pragma once

#include <jni.h>
#include <string_view>
#include <functional>

/// visit string as native bytes
/// jvm uses some unholy encoding internally so we convert it to utf-8
template <typename T, typename V>
static T
VisitStringAsStringView(JNIEnv* env, jobject str, V visit)
{
  const jclass stringClass = env->GetObjectClass(str);
  const jmethodID getBytes = env->GetMethodID(stringClass, "getBytes", "(Ljava/lang/String;)[B");

  const jstring charsetName = env->NewStringUTF("UTF-8");
  const jbyteArray stringJbytes = (jbyteArray)env->CallObjectMethod(str, getBytes, charsetName);
  env->DeleteLocalRef(charsetName);

  const size_t length = env->GetArrayLength(stringJbytes);
  jbyte* pBytes = env->GetByteArrayElements(stringJbytes, NULL);

  T result = visit(std::string_view((const char*)pBytes, length));

  env->ReleaseByteArrayElements(stringJbytes, pBytes, JNI_ABORT);
  env->DeleteLocalRef(stringJbytes);

  return result;
}

/// cast jni buffer to T *
template <typename T>
static T*
FromBuffer(JNIEnv* env, jobject o)
{
  if (o == nullptr)
    return nullptr;
  return static_cast<T*>(env->GetDirectBufferAddress(o));
}

/// get T * from object member called membername
template <typename T>
static T*
FromObjectMember(JNIEnv* env, jobject self, const char* membername)
{
  jclass cl = env->GetObjectClass(self);
  jfieldID name = env->GetFieldID(cl, membername, "Ljava/nio/ByteBuffer;");
  jobject buffer = env->GetObjectField(self, name);
  return FromBuffer<T>(env, buffer);
}

/// visit object string member called membername as bytes
template <typename T, typename V>
static T
VisitObjectMemberStringAsStringView(JNIEnv* env, jobject self, const char* membername, V v)
{
  jclass cl = env->GetObjectClass(self);
  jfieldID name = env->GetFieldID(cl, membername, "Ljava/lang/String;");
  jobject str = env->GetObjectField(self, name);
  return VisitStringAsStringView<T, V>(env, str, v);
}

/// get object member int called membername
template <typename Int_t>
Int_t
GetObjectMemberAsInt(JNIEnv* env, jobject self, const char* membername)
{
  jclass cl = env->GetObjectClass(self);
  jfieldID name = env->GetFieldID(cl, membername, "I");
  return env->GetIntField(self, name);
}

/// get implementation on jni type
template <typename T>
T*
GetImpl(JNIEnv* env, jobject self)
{
  return FromObjectMember<T>(env, self, "impl");
}