Java Code Examples for java.util.concurrent.ConcurrentMap#replace()

public synchronized void increment(String name, String hostname, Map<String, Set<String>> tags) {
    ConcurrentMap<CounterMetric, Integer> counters = ConcurrentMetricCounters.get();
    CounterMetric counterMetric = new CounterMetric(tags, name, hostname);
    Integer previousValue = counters.putIfAbsent(counterMetric, 1);
    if (previousValue != null){
        boolean ok = counters.replace(counterMetric, previousValue, previousValue + 1);
        // NOTE:
        // This while loop below should never be called since we are using a lock when flushing and
        // incrementing counters.
        while(!ok) {
            logger.warning("Couldn't increment counter " + name + " with value " + (previousValue + 1) +
                    " previousValue = " + previousValue);
            previousValue = counters.get(counterMetric);
            ok = counters.replace(counterMetric, previousValue, previousValue + 1);
        }
    }
    previousValue = previousValue == null ? 0 : previousValue;
    logger.fine("Counter " + name + " updated from previousValue " + previousValue + " to "
            + (previousValue + 1));
}
 

static Semaphore getLock(ConcurrentMap<String, Semaphore> locks2, String key) {
    Semaphore lock = locks2.get(key);
    if (lock == null) {
        Semaphore newLock = new Semaphore(1, true);
        locks2.get(null);
        locks2.put(null, null);
        locks2.remove(null);
        locks2.containsKey(null);
        locks2.containsValue(null);
        locks2.putIfAbsent(null, null);
        locks2.remove(null, null);
        locks2.replace(null, null);
        locks2.replace(null, null, null);
        lock = locks2.putIfAbsent(key, lock);
        // value, being null, will *always* throw NullPointerException
        if (lock == null)
            lock = newLock;
    }
    return lock;
}
 

/**
 * Update the store sequence id, e.g., upon executing in-memory compaction
 */
void updateStore(byte[] encodedRegionName, byte[] familyName, Long sequenceId,
    boolean onlyIfGreater) {
  if (sequenceId == null) {
    return;
  }
  Long highest = this.highestSequenceIds.get(encodedRegionName);
  if (highest == null || sequenceId > highest) {
    this.highestSequenceIds.put(encodedRegionName, sequenceId);
  }
  ImmutableByteArray familyNameWrapper = ImmutableByteArray.wrap(familyName);
  synchronized (this.tieLock) {
    ConcurrentMap<ImmutableByteArray, Long> m = getOrCreateLowestSequenceIds(encodedRegionName);
    boolean replaced = false;
    while (!replaced) {
      Long oldSeqId = m.get(familyNameWrapper);
      if (oldSeqId == null) {
        m.put(familyNameWrapper, sequenceId);
        replaced = true;
      } else if (onlyIfGreater) {
        if (sequenceId > oldSeqId) {
          replaced = m.replace(familyNameWrapper, oldSeqId, sequenceId);
        } else {
          return;
        }
      } else { // replace even if sequence id is not greater than oldSeqId
        m.put(familyNameWrapper, sequenceId);
        return;
      }
    }
  }
}
 

/**
 * replace(x, null, y) throws NPE
 */
public void testReplaceValue2_NullPointerException() {
    ConcurrentMap c = map();
    try {
        c.replace("whatever", null, "A");
        shouldThrow();
    } catch (NullPointerException success) {}
}
 

/**
 * @param item Item.
 */
public void onMappingAccepted(final MarshallerMappingItem item) {
    ConcurrentMap<Integer, MappedName> cache = getCacheFor(item.platformId());

    cache.replace(item.typeId(), new MappedName(item.className(), true));

    closProc.runLocalSafe(new MappingStoreTask(fileStore, item.platformId(), item.typeId(), item.className()));
}
 

public void testMapMethods() {
  Cache<Integer, Integer> cache = CaffeinatedGuava.build(Caffeine.newBuilder());

  ConcurrentMap<Integer, Integer> asMap = cache.asMap();

  cache.put(10, 100);
  cache.put(2, 52);

  asMap.replace(2, 79);
  asMap.replace(3, 60);

  assertEquals(null, cache.getIfPresent(3));
  assertEquals(null, asMap.get(3));

  assertEquals(Integer.valueOf(79), cache.getIfPresent(2));
  assertEquals(Integer.valueOf(79), asMap.get(2));

  asMap.replace(10, 100, 50);
  asMap.replace(2, 52, 99);

  assertEquals(Integer.valueOf(50), cache.getIfPresent(10));
  assertEquals(Integer.valueOf(50), asMap.get(10));
  assertEquals(Integer.valueOf(79), cache.getIfPresent(2));
  assertEquals(Integer.valueOf(79), asMap.get(2));

  asMap.remove(10, 100);
  asMap.remove(2, 79);

  assertEquals(Integer.valueOf(50), cache.getIfPresent(10));
  assertEquals(Integer.valueOf(50), asMap.get(10));
  assertEquals(null, cache.getIfPresent(2));
  assertEquals(null, asMap.get(2));

  asMap.putIfAbsent(2, 20);
  asMap.putIfAbsent(10, 20);

  assertEquals(Integer.valueOf(20), cache.getIfPresent(2));
  assertEquals(Integer.valueOf(20), asMap.get(2));
  assertEquals(Integer.valueOf(50), cache.getIfPresent(10));
  assertEquals(Integer.valueOf(50), asMap.get(10));
}
 

@Test(dataProvider = "guardedMap", expectedExceptions = NullPointerException.class)
public void replace_withNullValue(ConcurrentMap<Integer, Integer> map) {
  map.replace(1, null);
}
 

/**
 * Look-up the value through the cache. This always evaluates the
 * {@code subKeyFactory} function and optionally evaluates
 * {@code valueFactory} function if there is no entry in the cache for given
 * pair of (key, subKey) or the entry has already been cleared.
 *
 * @param key       possibly null key
 * @param parameter parameter used together with key to create sub-key and
 *                  value (should not be null)
 * @return the cached value (never null)
 * @throws NullPointerException if {@code parameter} passed in or
 *                              {@code sub-key} calculated by
 *                              {@code subKeyFactory} or {@code value}
 *                              calculated by {@code valueFactory} is null.
 */
public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);

    expungeStaleEntries();

    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }
        // else no supplier in cache
        // or a supplier that returned null (could be a cleared CacheValue
        // or a Factory that wasn't successful in installing the CacheValue)

        // lazily construct a Factory
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}
 

@Test(dataProvider = "guardedMap", expectedExceptions = NullPointerException.class)
public void replaceConditionally_withNullOldValue(ConcurrentMap<Integer, Integer> map) {
  map.replace(1, null, 3);
}
 

/**
 * Look-up the value through the cache. This always evaluates the
 * {@code subKeyFactory} function and optionally evaluates
 * {@code valueFactory} function if there is no entry in the cache for given
 * pair of (key, subKey) or the entry has already been cleared.
 *
 * @param key       possibly null key
 * @param parameter parameter used together with key to create sub-key and
 *                  value (should not be null)
 * @return the cached value (never null)
 * @throws NullPointerException if {@code parameter} passed in or
 *                              {@code sub-key} calculated by
 *                              {@code subKeyFactory} or {@code value}
 *                              calculated by {@code valueFactory} is null.
 */
public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);

    expungeStaleEntries();

    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }
        // else no supplier in cache
        // or a supplier that returned null (could be a cleared CacheValue
        // or a Factory that wasn't successful in installing the CacheValue)

        // lazily construct a Factory
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}
 

/**
 * Look-up the value through the cache. This always evaluates the
 * {@code subKeyFactory} function and optionally evaluates
 * {@code valueFactory} function if there is no entry in the cache for given
 * pair of (key, subKey) or the entry has already been cleared.
 *
 * @param key       possibly null key
 * @param parameter parameter used together with key to create sub-key and
 *                  value (should not be null)
 * @return the cached value (never null)
 * @throws NullPointerException if {@code parameter} passed in or
 *                              {@code sub-key} calculated by
 *                              {@code subKeyFactory} or {@code value}
 *                              calculated by {@code valueFactory} is null.
 */
public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);

    expungeStaleEntries();

    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }
        // else no supplier in cache
        // or a supplier that returned null (could be a cleared CacheValue
        // or a Factory that wasn't successful in installing the CacheValue)

        // lazily construct a Factory
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}
 

/**
 * Look-up the value through the cache. This always evaluates the
 * {@code subKeyFactory} function and optionally evaluates
 * {@code valueFactory} function if there is no entry in the cache for given
 * pair of (key, subKey) or the entry has already been cleared.
 *
 * @param key       possibly null key
 * @param parameter parameter used together with key to create sub-key and
 *                  value (should not be null)
 * @return the cached value (never null)
 * @throws NullPointerException if {@code parameter} passed in or
 *                              {@code sub-key} calculated by
 *                              {@code subKeyFactory} or {@code value}
 *                              calculated by {@code valueFactory} is null.
 */
public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);

    expungeStaleEntries();

    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }
        // else no supplier in cache
        // or a supplier that returned null (could be a cleared CacheValue
        // or a Factory that wasn't successful in installing the CacheValue)

        // lazily construct a Factory
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}
 

/**
 * Look-up the value through the cache. This always evaluates the
 * {@code subKeyFactory} function and optionally evaluates
 * {@code valueFactory} function if there is no entry in the cache for given
 * pair of (key, subKey) or the entry has already been cleared.
 *
 * @param key       possibly null key
 * @param parameter parameter used together with key to create sub-key and
 *                  value (should not be null)
 * @return the cached value (never null)
 * @throws NullPointerException if {@code parameter} passed in or
 *                              {@code sub-key} calculated by
 *                              {@code subKeyFactory} or {@code value}
 *                              calculated by {@code valueFactory} is null.
 */
public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);

    expungeStaleEntries();

    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }
        // else no supplier in cache
        // or a supplier that returned null (could be a cleared CacheValue
        // or a Factory that wasn't successful in installing the CacheValue)

        // lazily construct a Factory
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}
 

@Test(dataProvider = "guardedMap", expectedExceptions = NullPointerException.class)
public void replaceConditionally_withNullKeyAndValues(ConcurrentMap<Integer, Integer> map) {
  map.replace(null, null, null);
}
 

/**
 * Look-up the value through the cache. This always evaluates the
 * {@code subKeyFactory} function and optionally evaluates
 * {@code valueFactory} function if there is no entry in the cache for given
 * pair of (key, subKey) or the entry has already been cleared.
 *
 * @param key       possibly null key
 * @param parameter parameter used together with key to create sub-key and
 *                  value (should not be null)
 * @return the cached value (never null)
 * @throws NullPointerException if {@code parameter} passed in or
 *                              {@code sub-key} calculated by
 *                              {@code subKeyFactory} or {@code value}
 *                              calculated by {@code valueFactory} is null.
 */
public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);

    expungeStaleEntries();

    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }
        // else no supplier in cache
        // or a supplier that returned null (could be a cleared CacheValue
        // or a Factory that wasn't successful in installing the CacheValue)

        // lazily construct a Factory
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}
 

/**
 * Look-up the value through the cache. This always evaluates the
 * {@code subKeyFactory} function and optionally evaluates
 * {@code valueFactory} function if there is no entry in the cache for given
 * pair of (key, subKey) or the entry has already been cleared.
 *
 * @param key       possibly null key
 * @param parameter parameter used together with key to create sub-key and
 *                  value (should not be null)
 * @return the cached value (never null)
 * @throws NullPointerException if {@code parameter} passed in or
 *                              {@code sub-key} calculated by
 *                              {@code subKeyFactory} or {@code value}
 *                              calculated by {@code valueFactory} is null.
 */
public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);

    expungeStaleEntries();

    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }
        // else no supplier in cache
        // or a supplier that returned null (could be a cleared CacheValue
        // or a Factory that wasn't successful in installing the CacheValue)

        // lazily construct a Factory
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}
 

@Test(dataProvider = "guardedMap", expectedExceptions = NullPointerException.class)
public void replaceConditionally_withNullNewValue(ConcurrentMap<Integer, Integer> map) {
  map.replace(1, 2, null);
}
 

@Test(dataProvider = "guardedMap", expectedExceptions = NullPointerException.class)
public void replaceConditionally_withNullKeyAndNewValue(ConcurrentMap<Integer, Integer> map) {
  map.replace(null, 2, null);
}
 

/**
 * Attempts to compute a mapping for the specified key and its current
 * mapped value (or {@code null} if there is no current mapping). For
 * example, to either create or append a {@code String} msg to a value
 * mapping:
 *
 * <pre> {@code
 * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
 * (Method {@link #merge merge()} is often simpler to use for such purposes.)
 *
 * <p>If the remapping function returns {@code null}, the mapping is removed
 * (or remains absent if initially absent).  If the remapping function itself
 * throws an (unchecked) exception, the exception is rethrown, and the current
 * mapping is left unchanged.
 *
 * <p>The remapping function itself should not modify the passed map during
 * computation.
 *
 * <p><b>Implementation Requirements:</b><br>
 * The default implementation is equivalent to performing the following
 * steps for the {@code map}:
 *
 * <pre> {@code
 * for (;;) {
 *   V oldValue = map.get(key);
 *   V newValue = remappingFunction.apply(key, oldValue);
 *   if (newValue != null) {
 *     if ((oldValue != null)
 *       ? map.replace(key, oldValue, newValue)
 *       : map.putIfAbsent(key, newValue) == null)
 *       return newValue;
 *   } else if (oldValue == null || map.remove(key, oldValue)) {
 *     return null;
 *   }
 * }}</pre>
 * When multiple threads attempt updates, map operations and the
 * remapping function may be called multiple times.
 *
 * <p>This implementation assumes that the ConcurrentMap cannot contain null
 * values and {@code get()} returning null unambiguously means the key is
 * absent. Implementations which support null values <strong>must</strong>
 * override this default implementation.
 *
 * @param <K> the type of keys maintained by the passed map
 * @param <V> the type of mapped values in the passed map
 * @param map the {@code ConcurrentMap} on which to execute the {@code compute}
 * operation.
 * @param key key with which the specified value is to be associated
 * @param remappingFunction the remapping function to compute a value
 * @return the new value associated with the specified key, or null if none
 * @throws NullPointerException if the specified key is null and
 *         the map does not support null keys, or the
 *         remappingFunction is null
 * @throws UnsupportedOperationException if the {@code put} operation
 *         is not supported by the map (optional)
 * @throws ClassCastException if the class of the specified key or value
 *         prevents it from being stored in the map (optional)
 * @throws IllegalArgumentException if some property of the specified key
 *         or value prevents it from being stored in this map (optional)
 * @since 1.8
 */
public static <K, V> V compute(ConcurrentMap<K, V> map, K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
    Objects.requireNonNull(map);
    retry: for (;;) {
        V oldValue = map.get(key);
        // if putIfAbsent fails, opportunistically use its return value
        haveOldValue: for (;;) {
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue != null) {
                if (oldValue != null) {
                    if (map.replace(key, oldValue, newValue))
                        return newValue;
                }
                else if ((oldValue = map.putIfAbsent(key, newValue)) == null)
                    return newValue;
                else continue haveOldValue;
            } else if (oldValue == null || map.remove(key, oldValue)) {
                return null;
            }
            continue retry;
        }
    }
}
 

/**
 * Look-up the value through the cache. This always evaluates the
 * {@code subKeyFactory} function and optionally evaluates
 * {@code valueFactory} function if there is no entry in the cache for given
 * pair of (key, subKey) or the entry has already been cleared.
 *
 * @param key       possibly null key
 * @param parameter parameter used together with key to create sub-key and
 *                  value (should not be null)
 * @return the cached value (never null)
 * @throws NullPointerException if {@code parameter} passed in or
 *                              {@code sub-key} calculated by
 *                              {@code subKeyFactory} or {@code value}
 *                              calculated by {@code valueFactory} is null.
 */
public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);

    expungeStaleEntries();

    Object cacheKey = CacheKey.valueOf(key, refQueue);

    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
        }
    }

    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;

    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            V value = supplier.get();
            if (value != null) {
                return value;
            }
        }
        // else no supplier in cache
        // or a supplier that returned null (could be a cleared CacheValue
        // or a Factory that wasn't successful in installing the CacheValue)

        // lazily construct a Factory
        if (factory == null) {
            factory = new Factory(key, parameter, subKey, valuesMap);
        }

        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
            }
            // else retry with winning supplier
        } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
            } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
            }
        }
    }
}