一、先说结论 User Data 路径优先级如下：

1、注册表中策略配置的路径。

   计算机\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Chromium

    UserDataDir="xx_path"

2、命令行中的路径。

   --user-data-dir="xx_path"

3、默认用户路径

   %LOCALAPPDATA%/Chromium/User Data

二、接下来上源码分析过程

一、chrome\app\chrome_crash_reporter_client_win.cc

    //1、开始初始化user data目录
    std::wstring user_data_dir;
    if (process_type.empty())
      install_static::GetUserDataDirectory(&user_data_dir, nullptr);

// static
void ChromeCrashReporterClient::InitializeCrashReportingForProcess() {static ChromeCrashReporterClient* instance = nullptr;if (instance)return;instance = new ChromeCrashReporterClient();ANNOTATE_LEAKING_OBJECT_PTR(instance);std::wstring process_type = install_static::GetSwitchValueFromCommandLine(::GetCommandLine(), install_static::kProcessType);// Don't set up Crashpad crash reporting in the Crashpad handler itself, nor// in the fallback crash handler for the Crashpad handler process.if (process_type != install_static::kCrashpadHandler &&process_type != install_static::kFallbackHandler) {crash_reporter::SetCrashReporterClient(instance);//1、开始初始化user data目录std::wstring user_data_dir;if (process_type.empty())install_static::GetUserDataDirectory(&user_data_dir, nullptr);// TODO(wfh): Add a DCHECK for success. See https://crbug.com/1329269.std::ignore = crash_reporter::InitializeCrashpadWithEmbeddedHandler(/*initial_client=*/process_type.empty(),install_static::WideToUTF8(process_type),install_static::WideToUTF8(user_data_dir), base::FilePath());}
}

二、第二步看install_static::GetUserDataDirectory(&user_data_dir, nullptr);实现

 在chrome\install_static\user_data_dir.cc中

namespace install_static {namespace {std::wstring* g_user_data_dir;
std::wstring* g_invalid_user_data_dir;// Retrieves a registry policy for the user data directory from the registry, if
// one is set. If there's none set in either HKLM or HKCU, |user_data_dir| will
// be unmodified.
void GetUserDataDirFromRegistryPolicyIfSet(const InstallConstants& mode,std::wstring* user_data_dir) {assert(user_data_dir);std::wstring policies_path = L"SOFTWARE\\Policies\\";AppendChromeInstallSubDirectory(mode, false /* !include_suffix */,&policies_path);std::wstring value;constexpr wchar_t kUserDataDirRegistryKeyName[] = L"UserDataDir";// First, try HKLM.if (nt::QueryRegValueSZ(nt::HKLM, nt::NONE, policies_path.c_str(),kUserDataDirRegistryKeyName, &value)) {*user_data_dir = ExpandPathVariables(value);return;}// Second, try HKCU.if (nt::QueryRegValueSZ(nt::HKCU, nt::NONE, policies_path.c_str(),kUserDataDirRegistryKeyName, &value)) {*user_data_dir = ExpandPathVariables(value);return;}
}std::wstring MakeAbsoluteFilePath(const std::wstring& input) {wchar_t file_path[MAX_PATH];if (!_wfullpath(file_path, input.c_str(), _countof(file_path)))return std::wstring();return file_path;
}// The same as GetUserDataDirectory(), but directly queries the global command
// line object for the --user-data-dir flag. This is the more commonly used
// function, where GetUserDataDirectory() is used primiarily for testing.
bool GetUserDataDirectoryUsingProcessCommandLine(const InstallConstants& mode,std::wstring* result,std::wstring* invalid_supplied_directory) {return GetUserDataDirectoryImpl(GetSwitchValueFromCommandLine(::GetCommandLine(), kUserDataDirSwitch),mode, result, invalid_supplied_directory);
}// Populates |result| with the default User Data directory for the current
// user. Returns false if all attempts at locating a User Data directory fail.
// TODO(ananta)
// http://crbug.com/604923
// Unify this with the Browser Distribution code.
bool GetDefaultUserDataDirectory(const InstallConstants& mode,std::wstring* result) {// This environment variable should be set on Windows Vista and later// (https://msdn.microsoft.com/library/windows/desktop/dd378457.aspx).std::wstring user_data_dir = GetEnvironmentString(L"LOCALAPPDATA");if (user_data_dir.empty()) {// LOCALAPPDATA was not set; fallback to the temporary files path.DWORD size = ::GetTempPath(0, nullptr);if (!size)return false;user_data_dir.resize(size + 1);size = ::GetTempPath(size + 1, &user_data_dir[0]);if (!size || size >= user_data_dir.size())return false;user_data_dir.resize(size);}result->swap(user_data_dir);if ((*result)[result->length() - 1] != L'\\')result->push_back(L'\\');AppendChromeInstallSubDirectory(mode, true /* include_suffix */, result);result->push_back(L'\\');result->append(L"User Data");return true;
}}  // namespacebool GetUserDataDirectoryImpl(const std::wstring& user_data_dir_from_command_line,const InstallConstants& mode,std::wstring* result,std::wstring* invalid_supplied_directory) {std::wstring user_data_dir = user_data_dir_from_command_line;//从注册表中读取路径GetUserDataDirFromRegistryPolicyIfSet(mode, &user_data_dir);// On Windows, trailing separators leave Chrome in a bad state. See// crbug.com/464616.while (!user_data_dir.empty() &&(user_data_dir.back() == '\\' || user_data_dir.back() == '/')) {user_data_dir.pop_back();}bool got_valid_directory =!user_data_dir.empty() && RecursiveDirectoryCreate(user_data_dir);if (!got_valid_directory) {*invalid_supplied_directory = user_data_dir;//从%LOCALAPPDATA%/Chromium/User Data 中读取got_valid_directory = GetDefaultUserDataDirectory(mode, &user_data_dir);}// The Chrome implementation CHECKs() here in the browser process. We// don't as this function is used to initialize crash reporting, so// we would get no report of this failure.assert(got_valid_directory);if (!got_valid_directory)return false;*result = MakeAbsoluteFilePath(user_data_dir);return true;
}bool GetUserDataDirectory(std::wstring* user_data_dir,std::wstring* invalid_user_data_dir) {if (!g_user_data_dir) {g_user_data_dir = new std::wstring();g_invalid_user_data_dir = new std::wstring();//从命令行中解析路径if (!GetUserDataDirectoryUsingProcessCommandLine(InstallDetails::Get().mode(), g_user_data_dir,g_invalid_user_data_dir)) {return false;}assert(!g_user_data_dir->empty());}*user_data_dir = *g_user_data_dir;if (invalid_user_data_dir)*invalid_user_data_dir = *g_invalid_user_data_dir;return true;
}}  // namespace install_static

调用堆栈:

GetUserDataDirectory->GetUserDataDirectoryUsingProcessCommandLine->

GetUserDataDirectoryImpl->GetSwitchValueFromCommandLine->GetUserDataDirFromRegistryPolicyIfSet->GetDefaultUserDataDirectory

   

三、最后将初始化的g_user_data_dir路径设置到   

chrome\app\chrome_main_delegate.cc

CHECK(base::PathService::OverrideAndCreateIfNeeded(
        chrome::DIR_USER_DATA, user_data_dir, false, true));中，

// Initializes the user data dir. Must be called before InitializeLocalState().
void InitializeUserDataDir(base::CommandLine* command_line) {
#if BUILDFLAG(IS_CHROMEOS_LACROS)// In debug builds of Lacros, we keep track of when the user data dir// is initialized, to ensure the cryptohome is not accessed before login// when prelaunching at login screen.chromeos::lacros_paths::SetInitializedUserDataDir();
#endif
#if BUILDFLAG(IS_WIN)// Reach out to chrome_elf for the truth on the user data directory.// Note that in tests, this links to chrome_elf_test_stubs.wchar_t user_data_dir_buf[MAX_PATH], invalid_user_data_dir_buf[MAX_PATH];// In tests this may return false, implying the user data dir should be unset.if (GetUserDataDirectoryThunk(user_data_dir_buf, std::size(user_data_dir_buf),invalid_user_data_dir_buf,std::size(invalid_user_data_dir_buf))) {base::FilePath user_data_dir(user_data_dir_buf);if (invalid_user_data_dir_buf[0] != 0) {chrome::SetInvalidSpecifiedUserDataDir(base::FilePath(invalid_user_data_dir_buf));command_line->AppendSwitchPath(switches::kUserDataDir, user_data_dir);}//将user data路径存储到path缓存中CHECK(base::PathService::OverrideAndCreateIfNeeded(chrome::DIR_USER_DATA, user_data_dir, false, true));}

这样

  base::FilePath user_data_dir;

  base::PathService::Get(chrome::DIR_USER_DATA, &user_data_dir);

就可以读取其中的路径了。