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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package google
import (
// Credentials holds Google credentials, including "Application Default Credentials".
// For more details, see:
type Credentials struct {
ProjectID string // may be empty
TokenSource oauth2.TokenSource
// JSON contains the raw bytes from a JSON credentials file.
// This field may be nil if authentication is provided by the
// environment and not with a credentials file, e.g. when code is
// running on Google Cloud Platform.
JSON []byte
// DefaultCredentials is the old name of Credentials.
// Deprecated: use Credentials instead.
type DefaultCredentials = Credentials
// DefaultClient returns an HTTP Client that uses the
// DefaultTokenSource to obtain authentication credentials.
func DefaultClient(ctx context.Context, scope ...string) (*http.Client, error) {
ts, err := DefaultTokenSource(ctx, scope...)
if err != nil {
return nil, err
return oauth2.NewClient(ctx, ts), nil
// DefaultTokenSource returns the token source for
// "Application Default Credentials".
// It is a shortcut for FindDefaultCredentials(ctx, scope).TokenSource.
func DefaultTokenSource(ctx context.Context, scope ...string) (oauth2.TokenSource, error) {
creds, err := FindDefaultCredentials(ctx, scope...)
if err != nil {
return nil, err
return creds.TokenSource, nil
// FindDefaultCredentials searches for "Application Default Credentials".
// It looks for credentials in the following places,
// preferring the first location found:
// 1. A JSON file whose path is specified by the
// 2. A JSON file in a location known to the gcloud command-line tool.
// On Windows, this is %APPDATA%/gcloud/application_default_credentials.json.
// On other systems, $HOME/.config/gcloud/application_default_credentials.json.
// 3. On Google App Engine standard first generation runtimes (<= Go 1.9) it uses
// the appengine.AccessToken function.
// 4. On Google Compute Engine, Google App Engine standard second generation runtimes
// (>= Go 1.11), and Google App Engine flexible environment, it fetches
// credentials from the metadata server.
// (In this final case any provided scopes are ignored.)
func FindDefaultCredentials(ctx context.Context, scopes ...string) (*Credentials, error) {
// First, try the environment variable.
if filename := os.Getenv(envVar); filename != "" {
creds, err := readCredentialsFile(ctx, filename, scopes)
if err != nil {
return nil, fmt.Errorf("google: error getting credentials using %v environment variable: %v", envVar, err)
return creds, nil
// Second, try a well-known file.
filename := wellKnownFile()
if creds, err := readCredentialsFile(ctx, filename, scopes); err == nil {
return creds, nil
} else if !os.IsNotExist(err) {
return nil, fmt.Errorf("google: error getting credentials using well-known file (%v): %v", filename, err)
// Third, if we're on a Google App Engine standard first generation runtime (<= Go 1.9)
// use those credentials. App Engine standard second generation runtimes (>= Go 1.11)
// and App Engine flexible use ComputeTokenSource and the metadata server.
if appengineTokenFunc != nil {
return &DefaultCredentials{
ProjectID: appengineAppIDFunc(ctx),
TokenSource: AppEngineTokenSource(ctx, scopes...),
}, nil
// Fourth, if we're on Google Compute Engine, an App Engine standard second generation runtime,
// or App Engine flexible, use the metadata server.
if metadata.OnGCE() {
id, _ := metadata.ProjectID()
return &DefaultCredentials{
ProjectID: id,
TokenSource: ComputeTokenSource(""),
}, nil
// None are found; return helpful error.
const url = ""
return nil, fmt.Errorf("google: could not find default credentials. See %v for more information.", url)
// CredentialsFromJSON obtains Google credentials from a JSON value. The JSON can
// represent either a Google Developers Console client_credentials.json file (as in
// ConfigFromJSON) or a Google Developers service account key file (as in
// JWTConfigFromJSON).
func CredentialsFromJSON(ctx context.Context, jsonData []byte, scopes ...string) (*Credentials, error) {
var f credentialsFile
if err := json.Unmarshal(jsonData, &f); err != nil {
return nil, err
ts, err := f.tokenSource(ctx, append([]string(nil), scopes...))
if err != nil {
return nil, err
return &DefaultCredentials{
ProjectID: f.ProjectID,
TokenSource: ts,
JSON: jsonData,
}, nil
func wellKnownFile() string {
const f = "application_default_credentials.json"
if runtime.GOOS == "windows" {
return filepath.Join(os.Getenv("APPDATA"), "gcloud", f)
return filepath.Join(guessUnixHomeDir(), ".config", "gcloud", f)
func readCredentialsFile(ctx context.Context, filename string, scopes []string) (*DefaultCredentials, error) {
b, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
return CredentialsFromJSON(ctx, b, scopes...)