go項目源碼分析與使用--內存緩存go-cache

1.項目介紹

地址: https://github.com/patrickmn/go-cache
一個基於內存的key-value存儲/緩存項目，類似於Memcached，並且可選擇定期的垃圾回收，適合單機程序。代碼量不多，也不難懂。

2.關鍵源碼

cache.go

package cache

import (
	"encoding/gob"
	"fmt"
	"io"
	"os"
	"runtime"
	"sync"
	"time"
)

//key對應的item
type Item struct {
	Object     interface{} //value
	Expiration int64       //過期時間戳
}

//判斷item是否過期
func (item Item) Expired() bool {
	if item.Expiration == 0 {
		return false
	}
	return time.Now().UnixNano() > item.Expiration
}

const (
	NoExpiration time.Duration = -1
	DefaultExpiration time.Duration = 0
)

type Cache struct {
	*cache
	// If this is confusing, see the comment at the bottom of New()
}

type cache struct {
	defaultExpiration time.Duration   //item默認的過期時間
	items             map[string]Item         //一個cache實例內部有多個Item
	mu                sync.RWMutex
	onEvicted         func(string, interface{})   //刪除item時的回調函數
	janitor           *janitor                      //用於定期的清除回收，它會檢查過期item，並調用刪除函數
}

//把key對應的item存儲到cache，如果key存在則覆蓋原有的
//如果參數d爲0（DefaultExpiration），則使用cache實例的默認到期時間
//如果參數d是-1（NoExpiration），則該item永不過期，除非手動刪除

func (c *cache) Set(k string, x interface{}, d time.Duration) {
	// "Inlining" of set
	var e int64
	if d == DefaultExpiration {
		d = c.defaultExpiration
	}
	if d > 0 {
		e = time.Now().Add(d).UnixNano()
	}
	c.mu.Lock()
	c.items[k] = Item{
		Object:     x,
		Expiration: e,
	}
	// TODO: Calls to mu.Unlock are currently not deferred because defer
	// adds ~200 ns (as of go1.)
	c.mu.Unlock()
}

//跟上面一樣，少了鎖
func (c *cache) set(k string, x interface{}, d time.Duration) {
	var e int64
	if d == DefaultExpiration {
		d = c.defaultExpiration
	}
	if d > 0 {
		e = time.Now().Add(d).UnixNano()
	}
	c.items[k] = Item{
		Object:     x,
		Expiration: e,
	}
}

//使用默認到期時間set一個item
func (c *cache) SetDefault(k string, x interface{}) {
	c.Set(k, x, DefaultExpiration)
}

//key對應的item不存在或已過期時，才能將item添加到緩存中，否則返回錯誤
func (c *cache) Add(k string, x interface{}, d time.Duration) error {
	c.mu.Lock()
	_, found := c.get(k)
	if found {
		c.mu.Unlock()
		return fmt.Errorf("Item %s already exists", k)
	}
	c.set(k, x, d)
	c.mu.Unlock()
	return nil
}

//僅當key已經存在並且對應的item尚未過期時，才爲其設置新值，否則返回錯誤
func (c *cache) Replace(k string, x interface{}, d time.Duration) error {
	c.mu.Lock()
	_, found := c.get(k)
	if !found {
		c.mu.Unlock()
		return fmt.Errorf("Item %s doesn't exist", k)
	}
	c.set(k, x, d)
	c.mu.Unlock()
	return nil
}

//key對應的item存在則返回item，否則返回nil
//布爾值表示是否存在
func (c *cache) Get(k string) (interface{}, bool) {
	c.mu.RLock()
	// "Inlining" of get and Expired
	item, found := c.items[k]
	if !found {
		c.mu.RUnlock()
		return nil, false
	}
	if item.Expiration > 0 {
		if time.Now().UnixNano() > item.Expiration {
			c.mu.RUnlock()
			return nil, false
		}
	}
	c.mu.RUnlock()
	return item.Object, true
}

//從緩存中返回一個item及其到期時間
//一個布爾值表示是否找到
func (c *cache) GetWithExpiration(k string) (interface{}, time.Time, bool) {
	c.mu.RLock()
	// "Inlining" of get and Expired
	item, found := c.items[k]
	if !found {
		c.mu.RUnlock()
		return nil, time.Time{}, false
	}

	if item.Expiration > 0 {
		//item存在但是過期了
		if time.Now().UnixNano() > item.Expiration {
			c.mu.RUnlock()
			return nil, time.Time{}, false
		}
		////item存在還沒過期
		c.mu.RUnlock()
		return item.Object, time.Unix(0, item.Expiration), true
	}
	//永不過期
	// If expiration <= 0 (i.e. no expiration time set) then return the item
	// and a zeroed time.Time
	c.mu.RUnlock()
	return item.Object, time.Time{}, true
}

//獲取值
func (c *cache) get(k string) (interface{}, bool) {
	item, found := c.items[k]
	if !found {
		return nil, false
	}
	// "Inlining" of Expired
	if item.Expiration > 0 {
		if time.Now().UnixNano() > item.Expiration {
			return nil, false
		}
	}
	return item.Object, true
}

// Increment an item of type int, int8, int16, int32, int64, uintptr, uint,
// uint8, uint32, or uint64, float32 or float64 by n. Returns an error if the
// item's value is not an integer, if it was not found, or if it is not
// possible to increment it by n. To retrieve the incremented value, use one
// of the specialized methods, e.g. IncrementInt64.
//增加操作，傳入int64類型，內部適配不同的類型
func (c *cache) Increment(k string, n int64) error {
	c.mu.Lock()
	v, found := c.items[k]
	if !found || v.Expired() {
		c.mu.Unlock()
		return fmt.Errorf("Item %s not found", k)
	}
	switch v.Object.(type) {
	case int:
		v.Object = v.Object.(int) + int(n)
	case int8:
		v.Object = v.Object.(int8) + int8(n)
	case int16:
		v.Object = v.Object.(int16) + int16(n)
	case int32:
		v.Object = v.Object.(int32) + int32(n)
	case int64:
		v.Object = v.Object.(int64) + n
	case uint:
		v.Object = v.Object.(uint) + uint(n)
	case uintptr:
		v.Object = v.Object.(uintptr) + uintptr(n)
	case uint8:
		v.Object = v.Object.(uint8) + uint8(n)
	case uint16:
		v.Object = v.Object.(uint16) + uint16(n)
	case uint32:
		v.Object = v.Object.(uint32) + uint32(n)
	case uint64:
		v.Object = v.Object.(uint64) + uint64(n)
	case float32:
		v.Object = v.Object.(float32) + float32(n)
	case float64:
		v.Object = v.Object.(float64) + float64(n)
	default:
		c.mu.Unlock()
		return fmt.Errorf("The value for %s is not an integer", k)
	}
	c.items[k] = v
	c.mu.Unlock()
	return nil
}

//下面的部分代碼都是針對不同類型的增加或減操作，省略....

// Delete an item from the cache. Does nothing if the key is not in the cache.
//刪除item，根據情況判斷否調用回調函數
func (c *cache) Delete(k string) {
	c.mu.Lock()
	v, evicted := c.delete(k)
	c.mu.Unlock()
	if evicted {
		c.onEvicted(k, v)
	}
}

//判斷刪除item時觸發的回調函數是否爲nil
//不爲nil的話，刪除存在的item的同時把值返回去，回調函數要用到
//爲nil的話直接刪除item就行啦
func (c *cache) delete(k string) (interface{}, bool) {
	if c.onEvicted != nil {
		if v, found := c.items[k]; found {
			delete(c.items, k)
			return v.Object, true
		}
	}
	delete(c.items, k)
	return nil, false
}

type keyAndValue struct {
	key   string
	value interface{}
}

// 刪除所有的過期item
func (c *cache) DeleteExpired() {
	var evictedItems []keyAndValue
	now := time.Now().UnixNano()
	c.mu.Lock()
	for k, v := range c.items {
		// "Inlining" of expired
		if v.Expiration > 0 && now > v.Expiration {
			ov, evicted := c.delete(k)
			if evicted {
				evictedItems = append(evictedItems, keyAndValue{k, ov})
			}
		}
	}
	c.mu.Unlock()
	for _, v := range evictedItems {
		c.onEvicted(v.key, v.value)
	}
}

// Sets an (optional) function that is called with the key and value when an
// item is evicted from the cache. (Including when it is deleted manually, but
// not when it is overwritten.) Set to nil to disable.
//設置刪除item時的回調函數
func (c *cache) OnEvicted(f func(string, interface{})) {
	c.mu.Lock()
	c.onEvicted = f
	c.mu.Unlock()
}

// Write the cache's items (using Gob) to an io.Writer.
//
// NOTE: This method is deprecated in favor of c.Items() and NewFrom() (see the
// documentation for NewFrom().)
//gob是Golang包自帶的一個數據結構序列化的編碼/解碼工具
//編碼保存
func (c *cache) Save(w io.Writer) (err error) {
	enc := gob.NewEncoder(w)
	defer func() {
		if x := recover(); x != nil {
			err = fmt.Errorf("Error registering item types with Gob library")
		}
	}()
	c.mu.RLock()
	defer c.mu.RUnlock()
	for _, v := range c.items {
		gob.Register(v.Object)
	}
	err = enc.Encode(&c.items)
	return
}

// Save the cache's items to the given filename, creating the file if it
// doesn't exist, and overwriting it if it does.
//
// NOTE: This method is deprecated in favor of c.Items() and NewFrom() (see the
// documentation for NewFrom().)
//把緩存裏的數據用gob編碼並保存到文件中
func (c *cache) SaveFile(fname string) error {
	fp, err := os.Create(fname)
	if err != nil {
		return err
	}
	err = c.Save(fp)
	if err != nil {
		fp.Close()
		return err
	}
	return fp.Close()
}

// Add (Gob-serialized) cache items from an io.Reader, excluding any items with
// keys that already exist (and haven't expired) in the current cache.
//
// NOTE: This method is deprecated in favor of c.Items() and NewFrom() (see the
// documentation for NewFrom().)
func (c *cache) Load(r io.Reader) error {
	dec := gob.NewDecoder(r)
	items := map[string]Item{}
	err := dec.Decode(&items)
	if err == nil {
		c.mu.Lock()
		defer c.mu.Unlock()
		for k, v := range items {
			ov, found := c.items[k]
			if !found || ov.Expired() {
				c.items[k] = v
			}
		}
	}
	return err
}

// Load and add cache items from the given filename, excluding any items with
// keys that already exist in the current cache.
//
// NOTE: This method is deprecated in favor of c.Items() and NewFrom() (see the
// documentation for NewFrom().)
//根據文件裏的內容解碼存回緩存裏
func (c *cache) LoadFile(fname string) error {
	fp, err := os.Open(fname)
	if err != nil {
		return err
	}
	err = c.Load(fp)
	if err != nil {
		fp.Close()
		return err
	}
	return fp.Close()
}

// Copies all unexpired items in the cache into a new map and returns it.
//得到所有未過期的item
func (c *cache) Items() map[string]Item {
	c.mu.RLock()
	defer c.mu.RUnlock()
	m := make(map[string]Item, len(c.items))
	now := time.Now().UnixNano()
	for k, v := range c.items {
		// "Inlining" of Expired
		if v.Expiration > 0 {
			if now > v.Expiration {
				continue
			}
		}
		m[k] = v
	}
	return m
}

// Returns the number of items in the cache. This may include items that have
// expired, but have not yet been cleaned up.
func (c *cache) ItemCount() int {
	c.mu.RLock()
	n := len(c.items)
	c.mu.RUnlock()
	return n
}

// 清空cache
func (c *cache) Flush() {
	c.mu.Lock()
	c.items = map[string]Item{}
	c.mu.Unlock()
}

//用於定期清除過期的item
type janitor struct {
	Interval time.Duration //回收間隔
	stop     chan bool
}

//起一個ticker，定期調用刪除函數
func (j *janitor) Run(c *cache) {
	ticker := time.NewTicker(j.Interval)
	for {
		select {
		case <-ticker.C:
			c.DeleteExpired()
		case <-j.stop:
			ticker.Stop()
			return
		}
	}
}

//停止清除
func stopJanitor(c *Cache) {
	c.janitor.stop <- true
}

//起一個goroutine去定期清除
func runJanitor(c *cache, ci time.Duration) {
	j := &janitor{
		Interval: ci,
		stop:     make(chan bool),
	}
	c.janitor = j
	go j.Run(c)
}

func newCache(de time.Duration, m map[string]Item) *cache {
	//不過期
	if de == 0 {
		de = -1
	}
	c := &cache{
		defaultExpiration: de,
		items:             m,
	}
	return c
}

//帶有定期清除過期item的cache
func newCacheWithJanitor(de time.Duration, ci time.Duration, m map[string]Item) *Cache {
	c := newCache(de, m)
	C := &Cache{c}
	if ci > 0 {
		runJanitor(c, ci)
		//C被垃圾回收時，確保c也能被回收，即回收時要把該c.janitor所在的goroutine停掉，這樣c才能被回收
		//runtime.SetFinalizer(obj, func(obj *typeObj))
		//golang提供了runtime.SetFinalizer函數，當GC準備釋放對象時，會回調該函數指定的方法
		runtime.SetFinalizer(C, stopJanitor)
	}
	return C
}

//傳入cache的默認過期時間和定期清除時間
func New(defaultExpiration, cleanupInterval time.Duration) *Cache {
	items := make(map[string]Item)
	return newCacheWithJanitor(defaultExpiration, cleanupInterval, items)
}

//創建實例的同時把現有items存到cache中
func NewFrom(defaultExpiration, cleanupInterval time.Duration, items map[string]Item) *Cache {
	return newCacheWithJanitor(defaultExpiration, cleanupInterval, items)
}

3.例子

package main

import (
	"fmt"
	"time"

	"github.com/patrickmn/go-cache"
)

func main() {

	//cache實例應用於item的默認過期時間爲5分鐘，定期清除時間是每隔10分鐘
	c := cache.New(5*time.Minute, 10*time.Minute)

	//該key使用cache的默認過期時間
	c.Set("foo", "bar", cache.DefaultExpiration)

	//永不過期除非手動刪除
	// c.Delete("baz")
	c.Set("baz", 42, cache.NoExpiration)

	foo, found := c.Get("foo")
	if found {
		fmt.Println(foo.(string))
	}
}