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size_t copy(JNIEnv *env, jobject assetManager, jstring srcName, jstring dstName) {
   std::string src_name =jstringToString(env,srcName);
   std::string dst_name =jstringToString(env,dstName);

   AAssetManager *mgr = AAssetManager_fromJava(env, assetManager);
   AAsset *asset = AAssetManager_open(mgr, src_name.c_str(), AASSET_MODE_UNKNOWN);
   ofstream dst(dst_name, ios::binary);

   //文件大小
   size_t totalSize = AAsset_getLength(asset);

   //这里分批次读取，以防大文件内存崩溃
   const size_t bufSize = 1024;
   char *buffer = (char *) malloc(sizeof(char) * bufSize);
   int readSize = 0;
   while ((readSize = AAsset_read(asset, buffer, bufSize))) {
       dst.write(buffer, readSize);
   }

   dst.flush();
   dst.close();

   AAsset_close(asset);
   free(buffer);


   return totalSize;
}