Re: multithreaded cache?

Robert Klemme <>
Thu, 17 May 2012 12:06:34 +0200
On 05/17/2012 11:54 AM, Robert Klemme wrote:

On 05/15/2012 11:14 AM, bugbear wrote:

However, if the underlying function is slow
and/or resource hungry (consider cacheing
a ray traced image!) many threads can
end up calling the real function (second
and subsequent threads to the first get a miss
during the first threads call to the underlying function).

"obviously..." what I want is for only
the thread that FIRST has a cache miss
calls the underlying function, whilst other
threads (for the same key) wait.

I provide a variant of Silvio's, Eric's and Daniel's solution which
should yield higher throughput because it works without read write
locking. You can find it as gist in case the code is garbled in the
newsgroup posting:

There was one important detail missing. This is the corrected code
(gist is updated as well):

package clj;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

  * The cache works with as few locking as possible. Lookup is done in
two steps
  * on cache miss:
  * <ol>
  * <li>On a cache miss a retriever is inserted into the cache which
will obtain
  * the value synchronized from a {@link Calculator}.</li>
  * <li>Once calculation has finished a simple lock free reference to
the value
  * replaces the retriever in the cache and the value is returned.</li>
  * </ol>
  * @author robert klemme
  * @param <K>
  * key type
  * @param <V>
  * value type
public final class LazyCache<K, V> {
      * Calculate values from given keys.
      * @param <K>
      * key type
      * @param <V>
      * value type
     public interface Calculator<K, V> {
         V get(K key);

      * Obtain a value.
      * @param <V>
      * value type.
     private interface Reference<V> {
         V get();

      * Stupid simple reference which only hands out a fixed value all
the time
      * without synchronization.
      * @param <V>
      * value type.
     private static final class Ref<V> implements Reference<V> {
         private final V val;

         public Ref(V val) {
             this.val = val;

         public V get() {
             return val;

     /** Mapping from keys to objects which yield values. */
     private final ConcurrentMap<K, Reference<V>> map = new
ConcurrentHashMap<K, Reference<V>>();

     /** User provided. */
     private final Calculator<K, V> calc;

      * Create a cache.
      * @param calc
      * user must provide a reasonable implementation, not
      * <code>null</code>.
     public LazyCache(final Calculator<K, V> calc) {
         if (calc == null)
             throw new NullPointerException();
         this.calc = calc;

      * Get a value from the cache. The value might have to be calculated.
      * @param key
      * lookup key.
      * @return value, might even be <code>null</code> depending on
     public V get(final K key) {
         Reference<V> ref = map.get(key);

         if (ref == null) {
             // miss
             ref = new Reference<V>() {
                 private V val;
                 private boolean here = false; // superfluous but explicit

                 public synchronized V get() {
                     if (!here) {
                         val = calc.get(key);
                         here = true;
                         // next time lock free access:
                         Reference<V> x = map.put(key, new Ref<V>(val));
                         assert x == this;

                     return val;

             final Reference<V> old = map.putIfAbsent(key, ref);

             if (old != null)
                 ref = old; // someone else was faster

         return ref.get();

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