Java 对象缓存数据结构,带有;“对象过期”;
Java中哪种数据结构最适合实现内存中的对象缓存,其中对象有各自的过期时间Java 对象缓存数据结构,带有;“对象过期”;,java,caching,data-structures,collections,Java,Caching,Data Structures,Collections,Java中哪种数据结构最适合实现内存中的对象缓存,其中对象有各自的过期时间 基本上,对于缓存,我可以使用提供put和get方法的Map(其中key可以是字符串),并使用“timestamp”+“object”对的有序列表来管理过期时间。因此,清理线程可以检查第一个列表条目,并在过期时删除该对象。(删除第一个元素应该在O(1)时间内)我认为你的决定是正确的。 我将使用HasMead是精确的。 < P>我会考虑使用一个现有的库,比如 EHCACHE/CODE> < < /P> 但是,如果您想自己编写
基本上,对于缓存,我可以使用提供put和get方法的Map(其中key可以是字符串),并使用“timestamp”+“object”对的有序列表来管理过期时间。因此,清理线程可以检查第一个列表条目,并在过期时删除该对象。(删除第一个元素应该在O(1)时间内)我认为你的决定是正确的。
我将使用HasMead是精确的。 < P>我会考虑使用一个现有的库,比如<代码> EHCACHE/CODE> < < /P> 但是,如果您想自己编写,我不会使用后台线程,除非您需要它,因为它会增加复杂性。相反,我会让前台线程删除过期的条目
如果您只需要LRU缓存,我会使用
LinkedHashMap
。但是,如果您想要定时过期,我将使用带有优先级队列的HashMap
(这样您可以检查下一个过期条目是否已过期)缓存框架现在已经相当成熟:
- EhCache:
- Memcached:
但是,如果您坚持要重新发明轮子,请记住要考虑内存利用率。我经常看到一个实现糟糕的缓存(HashMap
)实际上变成了内存泄漏
请看科恩的回答:您所描述的建筑基本上是。还有其他类似的实现,如Guava(请参阅),但我认为它不支持ExpiringMap那样的每项过期。Guava Cachebuilder:
LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
.maximumSize(10000)
.expireAfterWrite(10, TimeUnit.MINUTES)
.removalListener(MY_LISTENER)
.build(
new CacheLoader<Key, Graph>() {
public Graph load(Key key) throws AnyException {
return createExpensiveGraph(key);
}
});
LoadingCache-graphs=CacheBuilder.newBuilder()
.最大尺寸(10000)
.expireAfterWrite(10,时间单位:分钟)
.removalListener(我的侦听器)
.建造(
新缓存加载程序(){
公共图加载(键)引发任何异常{
返回createExpensiveGraph(键);
}
});
因为WeekHashmap不适合缓存,但是您可以始终使用Map
,其值将符合GC for week引用的条件
最重要的是,我们总是将EhCache、Memcached和coherence作为流行的选择。如前面的答案所述,最好使用一种流行的内存缓存,如EhCache、Memcached等
但是,正如您希望通过自己的缓存实现它一样,它具有对象过期特性和较少的时间复杂性,我尝试这样实现它—(非常感谢任何测试评论/建议)
公共类ObjectCache{
私有易失性布尔关闭;
私有最终长对象;
私人最终长期生活;
私人最终长时间删除threadrundelay;
私有最终长objectsToRemovePerRemovalThreadRun;
私人最终原子目标;
私人最终地图库;
私有最终阻塞队列;
私有最终对象锁=新对象();
专用ScheduledExecutorService executorService;
公共对象缓存(长maxObjects、长timeToLive、长removalThreadRunDelay、长objectsToRemovePerRemovalThreadRun){
this.maxObjects=maxObjects;
this.timeToLive=timeToLive;
this.removalThreadRunDelay=removalThreadRunDelay;
this.objectsToRemovePerRemovalThreadRun=objectsToRemovePerRemovalThreadRun;
this.objectscont=new AtomicLong(0);
this.cachedDataStore=new HashMap();
this.queue=新建LinkedBlockingQueue();
}
公开作废认沽权(K键,V值){
if(key==null | | value==null){
抛出新的IllegalArgumentException(“键和值都不应为null”);
}
if(objectscont.get()+1>maxObjects){
抛出新的RuntimeException(“已达到最大对象限制。无法在缓存中存储更多对象。”);
}
//创建一个值包装器并将其添加到数据存储映射
CacheEntryWrapper entryWrapper=新的CacheEntryWrapper(键,值);
已同步(锁定){
cachedDataStore.put(key,entryWrapper);
}
//将缓存项引用添加到队列,该队列将由删除线程使用
add(entryWrapper.getCacheEntryReference());
objectsCount.incrementAndGet();
//如果尚未启动,请启动删除线程
if(executorService==null){
已同步(锁定){
if(executorService==null){
executorService=Executors.newSingleThreadScheduledExecutor();
executorService.scheduleWithFixedDelay(新的CacheEntryRemover(),0,removalThreadRunDelay,时间单位为毫秒);
}
}
}
}
公共V get(K键){
if(key==null){
抛出新的IllegalArgumentException(“键不能为空”);
}
CacheEntryWrapper入口包装器;
已同步(锁定){
entryWrapper=cachedDataStore.get(key);
if(entryWrapper!=null){
//重置上次访问时间
entryWrapper.resetLastAccessedTime();
//重置参考(以便清除弱参考)
entryWrapper.resetCacheEntryReference();
//将新引用添加到队列
add(entryWrapper.getCacheEntryReference());
}
}
return entryWrapper==null?null:entryWrapper.getValue();
}
公共无效删除(K键){
if(key==null){
抛出新的IllegalArgumentException(“键不能为空”);
}
CacheEntryWrapper入口包装器;
已同步(锁定){
entryWrapper=cachedDataStore.remove(键);
if(entryWrapper!=null){
//重置参考(以便清除弱参考)
public class ObjectCache<K, V> {
private volatile boolean shutdown;
private final long maxObjects;
private final long timeToLive;
private final long removalThreadRunDelay;
private final long objectsToRemovePerRemovalThreadRun;
private final AtomicLong objectsCount;
private final Map<K, CacheEntryWrapper> cachedDataStore;
private final BlockingQueue<CacheEntryReference> queue;
private final Object lock = new Object();
private ScheduledExecutorService executorService;
public ObjectCache(long maxObjects, long timeToLive, long removalThreadRunDelay, long objectsToRemovePerRemovalThreadRun) {
this.maxObjects = maxObjects;
this.timeToLive = timeToLive;
this.removalThreadRunDelay = removalThreadRunDelay;
this.objectsToRemovePerRemovalThreadRun = objectsToRemovePerRemovalThreadRun;
this.objectsCount = new AtomicLong(0);
this.cachedDataStore = new HashMap<K, CacheEntryWrapper>();
this.queue = new LinkedBlockingQueue<CacheEntryReference>();
}
public void put(K key, V value) {
if (key == null || value == null) {
throw new IllegalArgumentException("Key and Value both should be not null");
}
if (objectsCount.get() + 1 > maxObjects) {
throw new RuntimeException("Max objects limit reached. Can not store more objects in cache.");
}
// create a value wrapper and add it to data store map
CacheEntryWrapper entryWrapper = new CacheEntryWrapper(key, value);
synchronized (lock) {
cachedDataStore.put(key, entryWrapper);
}
// add the cache entry reference to queue which will be used by removal thread
queue.add(entryWrapper.getCacheEntryReference());
objectsCount.incrementAndGet();
// start the removal thread if not started already
if (executorService == null) {
synchronized (lock) {
if (executorService == null) {
executorService = Executors.newSingleThreadScheduledExecutor();
executorService.scheduleWithFixedDelay(new CacheEntryRemover(), 0, removalThreadRunDelay, TimeUnit.MILLISECONDS);
}
}
}
}
public V get(K key) {
if (key == null) {
throw new IllegalArgumentException("Key can not be null");
}
CacheEntryWrapper entryWrapper;
synchronized (lock) {
entryWrapper = cachedDataStore.get(key);
if (entryWrapper != null) {
// reset the last access time
entryWrapper.resetLastAccessedTime();
// reset the reference (so the weak reference is cleared)
entryWrapper.resetCacheEntryReference();
// add the new reference to queue
queue.add(entryWrapper.getCacheEntryReference());
}
}
return entryWrapper == null ? null : entryWrapper.getValue();
}
public void remove(K key) {
if (key == null) {
throw new IllegalArgumentException("Key can not be null");
}
CacheEntryWrapper entryWrapper;
synchronized (lock) {
entryWrapper = cachedDataStore.remove(key);
if (entryWrapper != null) {
// reset the reference (so the weak reference is cleared)
entryWrapper.resetCacheEntryReference();
}
}
objectsCount.decrementAndGet();
}
public void shutdown() {
shutdown = true;
executorService.shutdown();
queue.clear();
cachedDataStore.clear();
}
public static void main(String[] args) throws Exception {
ObjectCache<Long, Long> cache = new ObjectCache<>(1000000, 60000, 1000, 1000);
long i = 0;
while (i++ < 10000) {
cache.put(i, i);
}
i = 0;
while(i++ < 100) {
Thread.sleep(1000);
System.out.println("Data store size: " + cache.cachedDataStore.size() + ", queue size: " + cache.queue.size());
}
cache.shutdown();
}
private class CacheEntryRemover implements Runnable {
public void run() {
if (!shutdown) {
try {
int count = 0;
CacheEntryReference entryReference;
while ((entryReference = queue.peek()) != null && count++ < objectsToRemovePerRemovalThreadRun) {
long currentTime = System.currentTimeMillis();
CacheEntryWrapper cacheEntryWrapper = entryReference.getWeakReference().get();
if (cacheEntryWrapper == null || !cachedDataStore.containsKey(cacheEntryWrapper.getKey())) {
queue.poll(100, TimeUnit.MILLISECONDS); // remove the reference object from queue as value is removed from cache
} else if (currentTime - cacheEntryWrapper.getLastAccessedTime().get() > timeToLive) {
synchronized (lock) {
// get the cacheEntryWrapper again just to find if put() has overridden the same key or remove() has removed it already
CacheEntryWrapper newCacheEntryWrapper = cachedDataStore.get(cacheEntryWrapper.getKey());
// poll the queue if -
// case 1 - value is removed from cache
// case 2 - value is overridden by new value
// case 3 - value is still in cache but it is old now
if (newCacheEntryWrapper == null || newCacheEntryWrapper != cacheEntryWrapper || currentTime - cacheEntryWrapper.getLastAccessedTime().get() > timeToLive) {
queue.poll(100, TimeUnit.MILLISECONDS);
newCacheEntryWrapper = newCacheEntryWrapper == null ? cacheEntryWrapper : newCacheEntryWrapper;
if (currentTime - newCacheEntryWrapper.getLastAccessedTime().get() > timeToLive) {
remove(newCacheEntryWrapper.getKey());
}
} else {
break; // try next time
}
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
private class CacheEntryWrapper {
private K key;
private V value;
private AtomicLong lastAccessedTime;
private CacheEntryReference cacheEntryReference;
public CacheEntryWrapper(K key, V value) {
this.key = key;
this.value = value;
this.lastAccessedTime = new AtomicLong(System.currentTimeMillis());
this.cacheEntryReference = new CacheEntryReference(this);
}
public K getKey() {
return key;
}
public V getValue() {
return value;
}
public AtomicLong getLastAccessedTime() {
return lastAccessedTime;
}
public CacheEntryReference getCacheEntryReference() {
return cacheEntryReference;
}
public void resetLastAccessedTime() {
lastAccessedTime.set(System.currentTimeMillis());
}
public void resetCacheEntryReference() {
cacheEntryReference.clear();
cacheEntryReference = new CacheEntryReference(this);
}
}
private class CacheEntryReference {
private WeakReference<CacheEntryWrapper> weakReference;
public CacheEntryReference(CacheEntryWrapper entryWrapper) {
this.weakReference = new WeakReference<CacheEntryWrapper>(entryWrapper);
}
public WeakReference<CacheEntryWrapper> getWeakReference() {
return weakReference;
}
public void clear() {
weakReference.clear();
}
}
}