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package de.measite.minidns;
import java.util.LinkedHashMap;
import java.util.Map.Entry;
/**
* LRU based DNSCache backed by a LinkedHashMap.
*/
public class LRUCache implements DNSCache {
/**
* Internal miss count.
*/
protected long missCount = 0l;
/**
* Internal expire count (subset of misses that was caused by expire).
*/
protected long expireCount = 0l;
/**
* Internal hit count.
*/
protected long hitCount = 0l;
/**
* The internal capacity of the backend cache.
*/
protected int capacity;
/**
* The upper bound of the ttl. All longer TTLs will be capped by this ttl.
*/
protected long maxTTL;
/**
* The backend cache.
*/
protected LinkedHashMap<Question, DNSMessage> backend;
/**
* Create a new LRUCache with given capacity and upper bound ttl.
* @param capacity The internal capacity.
* @param maxTTL The upper bound for any ttl.
*/
@SuppressWarnings("serial")
public LRUCache(final int capacity, final long maxTTL) {
this.capacity = capacity;
this.maxTTL = maxTTL;
backend = new LinkedHashMap<Question,DNSMessage>(
Math.min(capacity + (capacity + 3) / 4 + 2, 11), 0.75f, true)
{
@Override
protected boolean removeEldestEntry(
Entry<Question, DNSMessage> eldest) {
return size() > capacity;
}
};
}
/**
* Create a new LRUCache with given capacity.
* @param capacity The capacity of this cache.
*/
public LRUCache(final int capacity) {
this(capacity, Long.MAX_VALUE);
}
@Override
public synchronized void put(Question q, DNSMessage message) {
if (message.getReceiveTimestamp() <= 0l) {
return;
}
backend.put(q, message);
}
@Override
public synchronized DNSMessage get(Question q) {
DNSMessage message = backend.get(q);
if (message == null) {
missCount++;
return null;
}
long ttl = maxTTL;
for (Record r : message.getAnswers()) {
ttl = Math.min(ttl, r.ttl);
}
for (Record r : message.getAdditionalResourceRecords()) {
ttl = Math.min(ttl, r.ttl);
}
if (message.getReceiveTimestamp() + ttl > System.currentTimeMillis()) {
missCount++;
expireCount++;
backend.remove(q);
return null;
} else {
hitCount++;
return message;
}
}
/**
* Clear all entries in this cache.
*/
public synchronized void clear() {
backend.clear();
missCount = 0l;
hitCount = 0l;
expireCount = 0l;
}
/**
* Get the miss count of this cache which is the number of fruitless
* get calls since this cache was last resetted.
* @return The number of cache misses.
*/
public long getMissCount() {
return missCount;
}
/**
* The number of expires (cache hits that have had a ttl to low to be
* retrieved).
* @return The expire count.
*/
public long getExpireCount() {
return expireCount;
}
/**
* The cache hit count (all sucessful calls to get).
* @return The hit count.
*/
public long getHitCount() {
return hitCount;
}
}
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