OatFileAssistant::ResultOfAttemptToUpdate
OatFileAssistant::MakeUpToDate(bool profile_changed, std::string* error_msg) {
  CompilerFilter::Filter target;
  if (!GetRuntimeCompilerFilterOption(&target, error_msg)) {
    return kUpdateNotAttempted;
  }
  OatFileInfo& info = GetBestInfo();
  switch (info.GetDexOptNeeded(target, profile_changed)) {
    case kNoDexOptNeeded:
      return kUpdateSucceeded;
    // TODO: 现在，不要为我们打电话的各种方式而烦恼
    // dex2oat生成oat文件。始终生成oat文件，就像它
    // KDEX2O是从头开始的。
    case kDex2OatFromScratch:
    case kDex2OatForBootImage:
    case kDex2OatForRelocation:
    case kDex2OatForFilter:
      // ★ 核心跳转 
      return GenerateOatFileNoChecks(info, target, error_msg);
  }
  UNREACHABLE();
}

!runtime->IsDex2OatEnabled()

OatFileAssistant::ResultOfAttemptToUpdate OatFileAssistant::GenerateOatFileNoChecks(
      OatFileAssistant::OatFileInfo& info, CompilerFilter::Filter filter, std::string* error_msg) {
  CHECK(error_msg != nullptr);
  Runtime* runtime = Runtime::Current();
  // 判断 Dex2Oat 当前是否可用 , 如果不可用 , 直接返回 
  if (!runtime->IsDex2OatEnabled()) {
    *error_msg = "Generation of oat file for dex location " + dex_location_
      + " not attempted because dex2oat is disabled.";
    return kUpdateNotAttempted;
  }
  if (info.Filename() == nullptr) {
    *error_msg = "Generation of oat file for dex location " + dex_location_
      + " not attempted because the oat file name could not be determined.";
    return kUpdateNotAttempted;
  }
  const std::string& oat_file_name = *info.Filename();
  const std::string& vdex_file_name = GetVdexFilename(oat_file_name);
  // dex2oat ignores missing dex files and doesn't report an error.
  // Check explicitly here so we can detect the error properly.
  // TODO: Why does dex2oat behave that way?
  struct stat dex_path_stat;
  if (TEMP_FAILURE_RETRY(stat(dex_location_.c_str(), &dex_path_stat)) != 0) {
    *error_msg = "Could not access dex location " + dex_location_ + ":" + strerror(errno);
    return kUpdateNotAttempted;
  }
  // If this is the odex location, we need to create the odex file layout (../oat/isa/..)
  if (!info.IsOatLocation()) {
    if (!PrepareOdexDirectories(dex_location_, oat_file_name, isa_, error_msg)) {
      return kUpdateNotAttempted;
    }
  }
  // Set the permissions for the oat and the vdex files.
  // The user always gets read and write while the group and others propagate
  // the reading access of the original dex file.
  mode_t file_mode = S_IRUSR | S_IWUSR |
      (dex_path_stat.st_mode & S_IRGRP) |
      (dex_path_stat.st_mode & S_IROTH);
  std::unique_ptr<File> vdex_file(OS::CreateEmptyFile(vdex_file_name.c_str()));
  if (vdex_file.get() == nullptr) {
    *error_msg = "Generation of oat file " + oat_file_name
      + " not attempted because the vdex file " + vdex_file_name
      + " could not be opened.";
    return kUpdateNotAttempted;
  }
  if (fchmod(vdex_file->Fd(), file_mode) != 0) {
    *error_msg = "Generation of oat file " + oat_file_name
      + " not attempted because the vdex file " + vdex_file_name
      + " could not be made world readable.";
    return kUpdateNotAttempted;
  }
  std::unique_ptr<File> oat_file(OS::CreateEmptyFile(oat_file_name.c_str()));
  if (oat_file.get() == nullptr) {
    *error_msg = "Generation of oat file " + oat_file_name
      + " not attempted because the oat file could not be created.";
    return kUpdateNotAttempted;
  }
  if (fchmod(oat_file->Fd(), file_mode) != 0) {
    *error_msg = "Generation of oat file " + oat_file_name
      + " not attempted because the oat file could not be made world readable.";
    oat_file->Erase();
    return kUpdateNotAttempted;
  }
  std::vector<std::string> args;
  args.push_back("--dex-file=" + dex_location_);
  args.push_back("--output-vdex-fd=" + std::to_string(vdex_file->Fd()));
  args.push_back("--oat-fd=" + std::to_string(oat_file->Fd()));
  args.push_back("--oat-location=" + oat_file_name);
  args.push_back("--compiler-filter=" + CompilerFilter::NameOfFilter(filter));
  // ★ 核心跳转 
  if (!Dex2Oat(args, error_msg)) {
    // Manually delete the oat and vdex files. This ensures there is no garbage
    // left over if the process unexpectedly died.
    vdex_file->Erase();
    unlink(vdex_file_name.c_str());
    oat_file->Erase();
    unlink(oat_file_name.c_str());
    return kUpdateFailed;
  }
  if (vdex_file->FlushCloseOrErase() != 0) {
    *error_msg = "Unable to close vdex file " + vdex_file_name;
    unlink(vdex_file_name.c_str());
    return kUpdateFailed;
  }
  if (oat_file->FlushCloseOrErase() != 0) {
    *error_msg = "Unable to close oat file " + oat_file_name;
    unlink(oat_file_name.c_str());
    return kUpdateFailed;
  }
  // Mark that the odex file has changed and we should try to reload.
  info.Reset();
  return kUpdateSucceeded;
}

Exec(argv, error_msg)

bool OatFileAssistant::Dex2Oat(const std::vector<std::string>& args,
                               std::string* error_msg) {
  Runtime* runtime = Runtime::Current();
  std::string image_location = ImageLocation();
  if (image_location.empty()) {
    *error_msg = "No image location found for Dex2Oat.";
    return false;
  }
  std::vector<std::string> argv;
  argv.push_back(runtime->GetCompilerExecutable());
  argv.push_back("--runtime-arg");
  argv.push_back("-classpath");
  argv.push_back("--runtime-arg");
  std::string class_path = runtime->GetClassPathString();
  if (class_path == "") {
    class_path = OatFile::kSpecialSharedLibrary;
  }
  argv.push_back(class_path);
  if (runtime->IsJavaDebuggable()) {
    argv.push_back("--debuggable");
  }
  runtime->AddCurrentRuntimeFeaturesAsDex2OatArguments(&argv);
  if (!runtime->IsVerificationEnabled()) {
    argv.push_back("--compiler-filter=verify-none");
  }
  if (runtime->MustRelocateIfPossible()) {
    argv.push_back("--runtime-arg");
    argv.push_back("-Xrelocate");
  } else {
    argv.push_back("--runtime-arg");
    argv.push_back("-Xnorelocate");
  }
  if (!kIsTargetBuild) {
    argv.push_back("--host");
  }
  argv.push_back("--boot-image=" + image_location);
  std::vector<std::string> compiler_options = runtime->GetCompilerOptions();
  argv.insert(argv.end(), compiler_options.begin(), compiler_options.end());
  argv.insert(argv.end(), args.begin(), args.end());
  std::string command_line(android::base::Join(argv, ' '));
  // ★ 核心跳转
  return Exec(argv, error_msg);
}