lang/java/avro/src/main/java/org/apache/avro/util/WeakIdentityHashMap.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at
 *
 * https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.avro.util;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

/**
 * Implements a combination of WeakHashMap and IdentityHashMap. Useful for
 * caches that need to key off of a == comparison instead of a .equals.
 *
 * <b> This class is not a general-purpose Map implementation! While this class
 * implements the Map interface, it intentionally violates Map's general
 * contract, which mandates the use of the equals method when comparing objects.
 * This class is designed for use only in the rare cases wherein
 * reference-equality semantics are required.
 *
 * Note that this implementation is not synchronized. </b>
 */
public class WeakIdentityHashMap<K, V> implements Map<K, V> {
  private final ReferenceQueue<K> queue = new ReferenceQueue<>();
  private Map<IdentityWeakReference, V> backingStore = new HashMap<>();

  public WeakIdentityHashMap() {
  }

  @Override
  public void clear() {
    backingStore.clear();
    reap();
  }

  @Override
  public boolean containsKey(Object key) {
    reap();
    return backingStore.containsKey(new IdentityWeakReference(key));
  }

  @Override
  public boolean containsValue(Object value) {
    reap();
    return backingStore.containsValue(value);
  }

  @Override
  public Set<Map.Entry<K, V>> entrySet() {
    reap();
    Set<Map.Entry<K, V>> ret = new HashSet<>();
    for (Map.Entry<IdentityWeakReference, V> ref : backingStore.entrySet()) {
      final K key = ref.getKey().get();
      final V value = ref.getValue();
      Map.Entry<K, V> entry = new Map.Entry<K, V>() {
        @Override
        public K getKey() {
          return key;
        }

        @Override
        public V getValue() {
          return value;
        }

        @Override
        public V setValue(V value) {
          throw new UnsupportedOperationException();
        }
      };
      ret.add(entry);
    }
    return Collections.unmodifiableSet(ret);
  }

  @Override
  public Set<K> keySet() {
    reap();
    Set<K> ret = new HashSet<>();
    for (IdentityWeakReference ref : backingStore.keySet()) {
      ret.add(ref.get());
    }
    return Collections.unmodifiableSet(ret);
  }

  @Override
  public boolean equals(Object o) {
    if (!(o instanceof WeakIdentityHashMap)) {
      return false;
    }
    return backingStore.equals(((WeakIdentityHashMap) o).backingStore);
  }

  @Override
  public V get(Object key) {
    reap();
    return backingStore.get(new IdentityWeakReference(key));
  }

  @Override
  public V put(K key, V value) {
    reap();
    return backingStore.put(new IdentityWeakReference(key), value);
  }

  @Override
  public int hashCode() {
    reap();
    return backingStore.hashCode();
  }

  @Override
  public boolean isEmpty() {
    reap();
    return backingStore.isEmpty();
  }

  @Override
  public void putAll(Map t) {
    throw new UnsupportedOperationException();
  }

  @Override
  public V remove(Object key) {
    reap();
    return backingStore.remove(new IdentityWeakReference(key));
  }

  @Override
  public int size() {
    reap();
    return backingStore.size();
  }

  @Override
  public Collection<V> values() {
    reap();
    return backingStore.values();
  }

  private synchronized void reap() {
    Object zombie = queue.poll();

    while (zombie != null) {
      IdentityWeakReference victim = (IdentityWeakReference) zombie;
      backingStore.remove(victim);
      zombie = queue.poll();
    }
  }

  class IdentityWeakReference extends WeakReference<K> {
    int hash;

    @SuppressWarnings("unchecked")
    IdentityWeakReference(Object obj) {
      super((K) obj, queue);
      hash = System.identityHashCode(obj);
    }

    @Override
    public int hashCode() {
      return hash;
    }

    @Override
    public boolean equals(Object o) {
      if (this == o) {
        return true;
      }
      if (!(o instanceof WeakIdentityHashMap.IdentityWeakReference)) {
        return false;
      }
      IdentityWeakReference ref = (IdentityWeakReference) o;
      return this.get() == ref.get();
    }
  }
}