type Cache struct {
    *cache
    // If this is confusing, see the comment at the bottom of New()
    // 如果這令人困惑，請參閱New()底部的注釋。
}
type cache struct {
    defaultExpiration time.Duration
    items             map[string]Item  //存儲鍵值對
    mu                sync.RWMutex     // 讀寫鎖，并發(fā)安全
    onEvicted         func(string, interface{})  // 被清除時的回調函數
    janitor           *janitor         // 腳本，定期清理過期數據
}
type Item struct {
    Object     interface{} // 存儲的值
    Expiration int64   // 到期時間
}

// Return a new cache with a given default expiration duration and cleanup interval.
// 返回具有給定默認過期和清除時間的 new cache新緩存
// If the expiration duration is less than one (or NoExpiration),
// the items in the cache never expire (by default), 
// 假如 到期時間為-1,永不過期
// and must be deleted manually.
// 并且只能手動刪除。
// If the cleanup interval is less than one, expired items are not
// 如果清除時間小于1，過期item，在call之前是沒有被刪除的。
// deleted from the cache before calling c.DeleteExpired().
func New(defaultExpiration, cleanupInterval time.Duration) *Cache {
    items := make(map[string]Item)
    return newCacheWithJanitor(defaultExpiration, cleanupInterval, items)
}
func newCacheWithJanitor(de time.Duration, ci time.Duration, m map[string]Item) *Cache {
    c := newCache(de, m)
    // This trick ensures that the janitor goroutine (which--granted it
    // 這個代碼，確認是否啟動 janitor看門人goroutine
    // was enabled--is running DeleteExpired on c forever) does not keep
    // the returned C object from being garbage collected. When it is
    // garbage collected, the finalizer stops the janitor goroutine, after
    // which c can be collected.
    C := &Cache{c}
    if ci > 0 {
       runJanitor(c, ci)  // 調用守衛(wèi)進程來清理過期數據
       runtime.SetFinalizer(C, stopJanitor)
       // runtime.SetFinalizer(C, stopJanitor)會指定調用函數停止后臺 goroutine，
       // 當 GC 準備釋放對象時，會調用stopJanitor方法，
       // Run函數中j.stop通道會輸出一個信號，從而退出協程。
    }
    return C
}
func runJanitor(c *cache, ci time.Duration) {
    j := &janitor{
       Interval: ci,
       stop:     make(chan bool),
    }
    c.janitor = j
    go j.Run(c)   // 調用守衛(wèi)進程來清理過期數據
}
// 開啟一個定時器，來定時清理數據。
func (j *janitor) Run(c *cache) {
    // 創(chuàng)建了一個計時器,時間到時ticker.C通道會輸出一個值,調用DeleteExpired()函數
    // 該函數會通過遍歷cache中的map[string]Item的過期時間，過期則直接從map中刪除，
    // 如果該值有回調函數，則在刪除后執(zhí)行回調函數。
    ticker := time.NewTicker(j.Interval)
    for {
       select {
       case <-ticker.C:
          c.DeleteExpired()
       case <-j.stop:
          ticker.Stop()
          return
       }
    }
}
// Delete all expired items from the cache.
// 刪除cache中所有的過期items
// 此時會加鎖，如果定時清理的時間比較長，并且key比較多的話，
// 會導致一直被  清理協程鎖住。其他的協程沒法寫入。
func (c *cache) DeleteExpired() {
    var evictedItems []keyAndValue
    now := time.Now().UnixNano()
    c.mu.Lock()
    // 過期刪除的時候，需要上鎖。
    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()
    // 刪除完解鎖
    for _, v := range evictedItems {
       c.onEvicted(v.key, v.value)
    }
}
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
}

// Get an item from the cache. Returns the item or nil, and a bool indicating
// whether the key was found.
// 從cache中Get一個item.返回item或者nil，和一個bool值。
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
    }
    // 獲取到item，是否能返回還需要判斷過期時間
    if item.Expiration > 0 {
       if time.Now().UnixNano() > item.Expiration {
          c.mu.RUnlock()
          return nil, false
       }
    }
    c.mu.RUnlock()
    return item.Object, true
}

// Add an item to the cache, replacing any existing item. If the duration is 0
// 將item添加到緩存中，以更換任何現有item。如果duration持續(xù)時間為0
// (DefaultExpiration), the cache's default expiration time is used. If it is -1
// (DefaultExpiration），使用緩存的默認到期時間。如果是-1
// (NoExpiration), the item never expires.
// （否開發(fā)），該項目永遠不會到期。
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,
       Expiration: e,
    }
    // TODO: Calls to mu.Unlock are currently not deferred because defer
    // adds ~200 ns (as of go1.)
    c.mu.Unlock()
}