go项目源码分析与使用--内存缓存go-cache
1.项目介绍
地址: https://github.com/patrickmn/go-cache
一个基于内存的key-value存储/缓存项目，类似于Memcached，并且可选择定期的垃圾回收，适合单机程序。代码量不多，也不难懂。
2.关键源码
cache.go
package cache
import (
"encoding/gob"
"fmt"
"io"
"os"
"runtime"
"sync"
"time"
)
//key对应的item
type Item struct {
Object interface{} //value
Expiration int64 //过期时间戳
}
//判断item是否过期
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默认的过期时间
items map[string]Item //一个cache实例内部有多个Item
mu sync.RWMutex
onEvicted func(string, interface{}) //删除item时的回调函数
janitor *janitor //用于定期的清除回收，它会检查过期item，并调用删除函数
}
//把key对应的item存储到cache，如果key存在则覆盖原有的
//如果参数d为0（DefaultExpiration），则使用cache实例的默认到期时间
//如果参数d是-1（NoExpiration），则该item永不过期，除非手动删除
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()
}
//跟上面一样，少了锁
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,
}
}
//使用默认到期时间set一个item
func (c *cache) SetDefault(k string, x interface{}) {
c.Set(k, x, DefaultExpiration)
}
//key对应的item不存在或已过期时，才能将item添加到缓存中，否则返回错误
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
}
//仅当key已经存在并且对应的item尚未过期时，才为其设置新值，否则返回错误
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对应的item存在则返回item，否则返回nil
//布尔值表示是否存在
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
}
//从缓存中返回一个item及其到期时间
//一个布尔值表示是否找到
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存在但是过期了
if time.Now().UnixNano() > item.Expiration {
c.mu.RUnlock()
return nil, time.Time{}, false
}
////item存在还没过期
c.mu.RUnlock()
return item.Object, time.Unix(0, item.Expiration), true
}
//永不过期
// 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
}
//获取值
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，传入int64类型，内部适配不同的类型
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
}
//下面的部分代码都是针对不同类型的增加或减操作，省略....
// Delete an item from the cache. Does nothing if the key is not in the cache.
//删除item，根据情况判断否调用回调函数
func (c *cache) Delete(k string) {
c.mu.Lock()
v, evicted := c.delete(k)
c.mu.Unlock()
if evicted {
c.onEvicted(k, v)
}
}
//判断删除item时触发的回调函数是否为nil
//不为nil的话，删除存在的item的同时把值返回去，回调函数要用到
//为nil的话直接删除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{}
}
// 删除所有的过期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.
//设置删除item时的回调函数
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包自带的一个数据结构序列化的编码/解码工具
//编码保存
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().)
//把缓存里的数据用gob编码并保存到文件中
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().)
//根据文件里的内容解码存回缓存里
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.
//得到所有未过期的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()
}
//用于定期清除过期的item
type janitor struct {
Interval time.Duration //回收间隔
stop chan bool
}
//起一个ticker，定期调用删除函数
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
}
//起一个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 {
//不过期
if de == 0 {
de = -1
}
c := &cache{
defaultExpiration: de,
items: m,
}
return c
}
//带有定期清除过期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被垃圾回收时，确保c也能被回收，即回收时要把该c.janitor所在的goroutine停掉，这样c才能被回收
//runtime.SetFinalizer(obj, func(obj *typeObj))
//golang提供了runtime.SetFinalizer函数，当GC准备释放对象时，会回调该函数指定的方法
runtime.SetFinalizer(C, stopJanitor)
}
return C
}
//传入cache的默认过期时间和定期清除时间
func New(defaultExpiration, cleanupInterval time.Duration) *Cache {
items := make(map[string]Item)
return newCacheWithJanitor(defaultExpiration, cleanupInterval, items)
}
//创建实例的同时把现有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实例应用于item的默认过期时间为5分钟，定期清除时间是每隔10分钟
c := cache.New(5*time.Minute, 10*time.Minute)
//该key使用cache的默认过期时间
c.Set("foo", "bar", cache.DefaultExpiration)
//永不过期除非手动删除
// c.Delete("baz")
c.Set("baz", 42, cache.NoExpiration)
foo, found := c.Get("foo")
if found {
fmt.Println(foo.(string))
}
}