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1 package cachingfilter;
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2
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3 import java.util.Map;
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4 import java.util.HashMap;
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5 import java.util.concurrent.locks.Lock;
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6 import java.util.concurrent.locks.ReentrantLock;
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7
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8
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9 public final class Locker<T> {
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10
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11 private static class Tracker {
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12 final ReentrantLock lock = new ReentrantLock();
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13 int waiters = 0;
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14 }
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15
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16 private final Map<T,Tracker> map = new HashMap<T,Tracker>();
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17
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18 public void lock(T obj) {
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19 Tracker t;
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20 synchronized(map) {
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21 t = map.get(obj);
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22 if( t == null ) {
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23 t = new Tracker();
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24 map.put(obj,t);
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25 }
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26 t.waiters++;
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27 }
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28 try {
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29 t.lock.lock();
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30 } finally {
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31 synchronized(map) {
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32 t.waiters--;
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33 }
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34 }
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35 }
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36
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37 public boolean unlock(T obj) {
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38 synchronized(map) {
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39 Tracker t = map.get(obj);
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40 if( t == null || !t.lock.isHeldByCurrentThread() )
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41 return false;
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42 t.lock.unlock();
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43 if( t.waiters==0 ) {
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44 map.remove(obj);
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45 }
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46 return true;
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47 }
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48 }
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49
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50 }
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