Introduction of cache interface and a file cacher.

cache.go

@@ -0,0 +1,58 @@
+// Copyright 2014 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package oauth2
+
+import (
+	"encoding/json"
+	"io/ioutil"
+)
+
+// Cache represents a token cacher.
+type Cache interface {
+	// Read reads a cache token from the specified file.
+	Read() (token *Token, err error)
+	// Write writes a token to the specified file.
+	Write(token *Token) (err error)
+}
+
+// NewFileCache creates a new file cache.
+func NewFileCache(filename string) *FileCache {
+	return &FileCache{filename: filename}
+}
+
+// FileCache represents a file based token cacher.
+type FileCache struct {
+	filename string
+}
+
+// Read reads a cache token from the specified file.
+func (f *FileCache) Read() (token *Token, err error) {
+	data, err := ioutil.ReadFile(f.filename)
+	if err != nil {
+		return nil, err
+	}
+	token = &Token{}
+	err = json.Unmarshal(data, &token)
+	return token, err
+}
+
+// Write writes a token to the specified file.
+func (f *FileCache) Write(token *Token) error {
+	data, err := json.Marshal(token)
+	if err != nil {
+		return err
+	}
+	return ioutil.WriteFile(f.filename, data, 0644)
+}

jwt.go

@@ -66,6 +66,7 @@ type JWTConfig struct {
 	opts      *JWTOptions
 	aud       string
 	signature []byte
+	cache     Cache
 }
 
 // Options returns JWT options.
@@ -153,3 +154,8 @@ func (c *JWTConfig) FetchToken(existing *Token) (token *Token, err error) {
 	token.Expiry = time.Now().Add(time.Duration(b.ExpiresIn) * time.Second)
 	return
 }
+
+// Cache returns a cache if specified, otherwise nil.
+func (c *JWTConfig) Cache() Cache {
+	return c.cache
+}

oauth2.go

@@ -74,6 +74,7 @@ type TokenFetcher interface {
 	// If the implementation doesn't know how to retrieve a new token,
 	// it returns an error.
 	FetchToken(existing *Token) (*Token, error)
+	Cache() Cache
 }
 
 // Options represents options to provide OAuth 2.0 client credentials
@@ -138,6 +139,8 @@ type Config struct {
 	authURL string
 	// TokenURL is the URL used to retrieve OAuth tokens.
 	tokenURL string
+
+	cache Cache
 }
 
 // Options returns options.
@@ -224,6 +227,11 @@ func (c *Config) FetchToken(existing *Token) (*Token, error) {
 	return existing, err
 }
 
+// Cache returns a cache if specified, otherwise nil.
+func (c *Config) Cache() Cache {
+	return c.cache
+}
+
 // Checks if all required configuration fields have non-zero values.
 func (c *Config) validate() error {
 	if c.opts.ClientID == "" {