Go-Cache Source Analysis
一、简介
基于内存的K/V存储/缓存：（类似Memcacheed）,适用于单机应用程序，支持删除，过期，默认Cache共享锁
主要优点：本质上是一个map[string]interface{}具有过期时间的安全线程，它不需要序列化或通过网络传输其内容。
大量Key的情况下会造成锁竞争严重。
go-cache可以存储任何对象（在给定的持续时间内或永久存储），并且缓存可以由多个goroutine安全地使用。
虽然go-cache并不打算用作持久性数据存储，但整个缓存可以保存到文件中并从文件中加载（c.Items()用于检索条目映射以进行序列化，NewFrom()从反序列化的缓存中创建一个缓存）快速从停机时间恢复
二、示例
import (
"fmt"
"github.com/patrickmn/go-cache"
"time"
)
type MyStruct struct {
Age int
}
func main() {
// 设置超时时间、清理时间
// Create a cache with a default expiration time of 5 minutes, and which
// purges expired items every 10 minutes
c := cache.New(5*time.Minute, 10*time.Minute)
// 设置缓存值，默认过期时间:永久不过期
// Set the value of the key "foo" to "bar", with the default expiration time
c.Set("foo", "bar", cache.DefaultExpiration)
// 设置没有过期时间的KEY，这个KEY不会被自动清除，清除使用：c.Delete("baz")
// Set the value of the key "baz" to 42, with no expiration time
// (the item won't be removed until it is re-set, or removed using
// c.Delete("baz")
c.Set("baz", 42, cache.NoExpiration)
// 获取值， 并断言
// This gets tedious if the value is used several times in the same function.
// You might do either of the following instead:
if x, found := c.Get("foo"); found {
foo := x.(string)
// ...
}
// 对结构体指针进行操作
// Want performance? Store pointers!
c.Set("foo", &MyStruct, cache.DefaultExpiration)
if x, found := c.Get("foo"); found {
foo := x.(*MyStruct)
// ...
}
}
三、源码分析
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：设置的时间+缓存时长
}
// 判断是否过期
func (item Item) Expired() bool {
if item.Expiration == 0 {
return false
}
return time.Now().UnixNano() > item.Expiration
}
const (
// 永不过期
NoExpiration time.Duration = -1
// 默认时间
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默认过期时间
items map[string]Item // 一个cache实例内存中多个Item
mu sync.RWMutex // 读写锁
onEvicted func(string, interface{}) // 删除Key时的CallBack函数
janitor *janitor // 定时清理回收，检查过期的Item，并调用c.Delete()
}
//添加key对应的Item存储到cache,如果key存在则覆盖，如果d为0，使用默认时间，d为-1，永不过期
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, // 实际数据
Expiration: e, // 过期时间
}
// TODO: Calls to mu.Unlock are currently not deferred because defer
// adds ~200 ns (as of go1.)
c.mu.Unlock()
}
//与上述一样，不带锁
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,
}
}
// Set缓存使用默认过期时间
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，添加Item到cache中，否则error
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
}
// 如果key已存在且对应的Item未过期，可设置new value,否则error
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
}
// 从缓存中获取key对应的Item，否则返回nil，bool表示返回是否存在
func (c *cache) Get(k string) (interface{}, bool) {
c.mu.RLock() // 加锁，限制并发读写
// "Inlining" of get and Expired
item, found := c.items[k] // 查询cache中k的Item
if !found {
c.mu.RUnlock()
return nil, false
}
if item.Expiration > 0 {
if time.Now().UnixNano() > item.Expiration { //已过期返回nil
c.mu.RUnlock()
return nil, false
}
}
c.mu.RUnlock()
return item.Object, true
}
// 从缓存中获取key对应的Item的过期时间，bool表示返回是否存在
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 {
if time.Now().UnixNano() > item.Expiration {
c.mu.RUnlock()
return nil, time.Time{}, false
}
// Return the item and the expiration time
c.mu.RUnlock()
return item.Object, time.Unix(0, item.Expiration), true
}
//
// 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
}
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
}
// 增加操作，针对不同的类型进行适配
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
}
... ... 省略部分代码（针对不同类型代码操作）
// 删除Item
func (c *cache) Delete(k string) {
c.mu.Lock()
v, evicted := c.delete(k)
c.mu.Unlock()
if evicted {
c.onEvicted(k, v) // 删除k的CallBack
}
}
// 判断删除Item触发的Callback是否为nil，不为nil删除存在的Item，并返回value，反之直接删除Item
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{}
}
// 删除缓存中所有过期的Item
func (c *cache) DeleteExpired() {
var evictedItems []keyAndValue
now := time.Now().UnixNano()
c.mu.Lock()
for k, v := range c.items { // 加锁遍历整个Items列表
// "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)
}
}
// 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()
}
// 使用gob（序列化编/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
}
// 把cache中的数据使用gob编码保存到文件中
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
}
// 把文件中的内容 编码存到cache中
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()
}
// 获取cache中所有未过期的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()
}
//定时清空缓存结构
type janitor struct {
Interval time.Duration // 回收间隔时长
stop chan bool // 是否停止
}
// 定时调用删除函数，设置一个Interval
func (j *janitor) Run(c *cache) {
ticker := time.NewTicker(j.Interval)
for {
select {
case <-ticker.C: // 每到一个周期就全部遍历一次
c.DeleteExpired() // 实际的删除逻辑
case <-j.stop:
ticker.Stop()
return
}
}
}
// 停止清除
func stopJanitor(c *Cache) {
c.janitor.stop <- true
}
// 设置一个goroutine定时清理
func runJanitor(c *cache, ci time.Duration) {
j := &janitor{
Interval: ci,
stop: make(chan bool),
}
c.janitor = j
go j.Run(c) // 新的协程做过期删除逻辑
}
func newCache(de time.Duration, m map[string]Item) *cache {
// 永不过期
if de == 0 {
de = -1
}
c := &cache{
defaultExpiration: de,
items: m,
}
return c
}
// 定期清理cache中过期的Item
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) // 定时清理过期的key
// C被垃圾回收时，确保c也能被回收，回收时把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) //当C被GC回收时，停止runJanitor中的协程
// 对象可以关联一个SetFinalizer函数， 当gc检测到unreachable对象有关联的SetFinalizer函数时，会执行关联的SetFinalizer函数， 同时取消关联。 当下一次gc的时候，对象重新处于unreachable状态并且没有SetFinalizer关联， 就会被回收
}
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的items存储到cache中
func NewFrom(defaultExpiration, cleanupInterval time.Duration, items map[string]Item) *Cache {
return newCacheWithJanitor(defaultExpiration, cleanupInterval, items)
}
