用Go語言實現一個緩存器

Go語言的源碼閱讀

1、Cache的底層實現

package cache2go

import (
	"sync"
	"time"
)

// CacheItem is an individual cache item
// Parameter data contains the user-set value in the cache.
type CacheItem struct {
	sync.RWMutex

	// The item's key. 空interface(interface{})不包含任何的method，因此所有的類型都實現了空interface。
	// 空interface在我們需要存儲任意類型的數值的時候相當有用，有點類似於C語言的void*類型。
	key interface{}
	// The item's data.
	data interface{}
	// How long will the item live in the cache when not being accessed/kept alive.
	lifeSpan time.Duration

	// Creation timestamp.
	createdOn time.Time
	// Last access timestamp.
	accessedOn time.Time
	// How often the item was accessed.
	accessCount int64

	// Callback method triggered right before removing the item from the cache
	aboutToExpire []func(key interface{})
}

// NewCacheItem returns a newly created CacheItem.
// Parameter key is the item's cache-key.
// Parameter lifeSpan determines after which time period without an access the item
// will get removed from the cache.
// Parameter data is the item's value.
func NewCacheItem(key interface{}, lifeSpan time.Duration, data interface{}) *CacheItem {
	t := time.Now()
	return &CacheItem{
		key:           key,
		lifeSpan:      lifeSpan,
		createdOn:     t,
		accessedOn:    t,
		accessCount:   0,
		aboutToExpire: nil,
		data:          data,
	}
}

// KeepAlive marks an item to be kept for another expireDuration period. 過期週期
func (item *CacheItem) KeepAlive() {
	item.Lock()
	defer item.Unlock()
	item.accessedOn = time.Now()
	item.accessCount++
}

// LifeSpan returns this item's expiration duration.  返回緩存項的過期時間
func (item *CacheItem) LifeSpan() time.Duration {
	// immutable
	return item.lifeSpan
}

// AccessedOn returns when this item was last accessed. 返回該項的最後訪問時間
func (item *CacheItem) AccessedOn() time.Time {
	item.RLock()
	defer item.RUnlock()
	return item.accessedOn
}

// CreatedOn returns when this item was added to the cache.  返回該項的創建時間
func (item *CacheItem) CreatedOn() time.Time {
	// immutable
	return item.createdOn
}

// AccessCount returns how often this item has been accessed.  返回該項被訪問的次數
func (item *CacheItem) AccessCount() int64 {
	item.RLock()
	defer item.RUnlock()
	return item.accessCount
}

// Key returns the key of this cached item.  返回該事項的key值
func (item *CacheItem) Key() interface{} {
	// immutable
	return item.key
}

// Data returns the value of this cached item.   返回該項的數據
func (item *CacheItem) Data() interface{} {
	// immutable
	return item.data
}

// SetAboutToExpireCallback configures a callback, which will be called right
// before the item is about to be removed from the cache.  設置回調函數， 該項被移除之前調用這個回調函數
func (item *CacheItem) SetAboutToExpireCallback(f func(interface{})) {
	if len(item.aboutToExpire) > 0 {
		//清空回調函數的隊列
		item.RemoveAboutToExpireCallback()
	}
	item.Lock()
	defer item.Unlock()
	//然後追加一個回調函數到隊列中
	item.aboutToExpire = append(item.aboutToExpire, f)
}

// AddAboutToExpireCallback appends a new callback to the AboutToExpire queue 
// 把一個回調函數放進回調函數的隊列中
func (item *CacheItem) AddAboutToExpireCallback(f func(interface{})) {
	item.Lock()
	defer item.Unlock()
	item.aboutToExpire = append(item.aboutToExpire, f)
}

// RemoveAboutToExpireCallback empties the about to expire callback queue 清空回調函數的隊列
func (item *CacheItem) RemoveAboutToExpireCallback() {
	item.Lock()
	defer item.Unlock()
	item.aboutToExpire = nil
}

源碼分析點評： 經典的地方是使用了Go語言的空接口；