--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/cachingfilter/Locker.java	Thu Mar 21 19:15:52 2019 -0600
@@ -0,0 +1,50 @@
+package cachingfilter;
+
+import java.util.Map;
+import java.util.HashMap;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
+
+public final class Locker<T> {
+
+	private static class Tracker {
+		final ReentrantLock lock = new ReentrantLock();
+		int waiters = 0;
+	}
+
+	private final Map<T,Tracker> map = new HashMap<T,Tracker>();
+
+	public void lock(T obj) {
+		Tracker t;
+		synchronized(map) {
+			t = map.get(obj);
+			if( t == null ) {
+				t = new Tracker();
+				map.put(obj,t);
+			}
+			t.waiters++;
+		}
+		try {
+			t.lock.lock();
+		} finally {
+			synchronized(map) {
+				t.waiters--;
+			}
+		}
+	}
+
+	public boolean unlock(T obj) {
+		synchronized(map) {
+			Tracker t = map.get(obj);
+			if( t == null || !t.lock.isHeldByCurrentThread() )
+				return false;
+			t.lock.unlock();
+			if( t.waiters==0 ) {
+				map.remove(obj);
+			}
+			return true;
+		}
+	}
+
+}