java/com/android/dialer/util/ExpirableCache.java

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed 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
 *
 *      http://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 com.android.dialer.util;

import android.util.LruCache;
import java.util.concurrent.atomic.AtomicInteger;
import javax.annotation.concurrent.Immutable;
import javax.annotation.concurrent.ThreadSafe;

/**
 * An LRU cache in which all items can be marked as expired at a given time and it is possible to
 * query whether a particular cached value is expired or not.
 *
 * <p>A typical use case for this is caching of values which are expensive to compute but which are
 * still useful when out of date.
 *
 * <p>Consider a cache for contact information:
 *
 * <pre>{@code
 * private ExpirableCache<String, Contact> mContactCache;
 * }</pre>
 *
 * which stores the contact information for a given phone number.
 *
 * <p>When we need to store contact information for a given phone number, we can look up the info in
 * the cache:
 *
 * <pre>{@code
 * CachedValue<Contact> cachedContact = mContactCache.getCachedValue(phoneNumber);
 * }</pre>
 *
 * We might also want to fetch the contact information again if the item is expired.
 *
 * <pre>
 *     if (cachedContact.isExpired()) {
 *         fetchContactForNumber(phoneNumber,
 *                 new FetchListener() {
 *                     &#64;Override
 *                     public void onFetched(Contact contact) {
 *                         mContactCache.put(phoneNumber, contact);
 *                     }
 *                 });
 *     }</pre>
 *
 * and insert it back into the cache when the fetch completes.
 *
 * <p>At a certain point we want to expire the content of the cache because we know the content may
 * no longer be up-to-date, for instance, when resuming the activity this is shown into:
 *
 * <pre>
 *     &#64;Override
 *     protected onResume() {
 *         // We were paused for some time, the cached value might no longer be up to date.
 *         mContactCache.expireAll();
 *         super.onResume();
 *     }
 * </pre>
 *
 * The values will be still available from the cache, but they will be expired.
 *
 * <p>If interested only in the value itself, not whether it is expired or not, one should use the
 * {@link #getPossiblyExpired(Object)} method. If interested only in non-expired values, one should
 * use the {@link #get(Object)} method instead.
 *
 * <p>This class wraps around an {@link LruCache} instance: it follows the {@link LruCache} behavior
 * for evicting items when the cache is full. It is possible to supply your own subclass of LruCache
 * by using the {@link #create(LruCache)} method, which can define a custom expiration policy. Since
 * the underlying cache maps keys to cached values it can determine which items are expired and
 * which are not, allowing for an implementation that evicts expired items before non expired ones.
 *
 * <p>This class is thread-safe.
 *
 * @param <K> the type of the keys
 * @param <V> the type of the values
 */
@ThreadSafe
public class ExpirableCache<K, V> {

  /**
   * The current generation of items added to the cache.
   *
   * <p>Items in the cache can belong to a previous generation, but in that case they would be
   * expired.
   *
   * @see ExpirableCache.CachedValue#isExpired()
   */
  private final AtomicInteger generation;
  /** The underlying cache used to stored the cached values. */
  private LruCache<K, CachedValue<V>> cache;

  private ExpirableCache(LruCache<K, CachedValue<V>> cache) {
    this.cache = cache;
    generation = new AtomicInteger(0);
  }

  /**
   * Creates a new {@link ExpirableCache} that wraps the given {@link LruCache}.
   *
   * <p>The created cache takes ownership of the cache passed in as an argument.
   *
   * @param <K> the type of the keys
   * @param <V> the type of the values
   * @param cache the cache to store the value in
   * @return the newly created expirable cache
   * @throws IllegalArgumentException if the cache is not empty
   */
  public static <K, V> ExpirableCache<K, V> create(LruCache<K, CachedValue<V>> cache) {
    return new ExpirableCache<K, V>(cache);
  }

  /**
   * Creates a new {@link ExpirableCache} with the given maximum size.
   *
   * @param <K> the type of the keys
   * @param <V> the type of the values
   * @return the newly created expirable cache
   */
  public static <K, V> ExpirableCache<K, V> create(int maxSize) {
    return create(new LruCache<K, CachedValue<V>>(maxSize));
  }

  /**
   * Returns the cached value for the given key, or null if no value exists.
   *
   * <p>The cached value gives access both to the value associated with the key and whether it is
   * expired or not.
   *
   * <p>If not interested in whether the value is expired, use {@link #getPossiblyExpired(Object)}
   * instead.
   *
   * <p>If only wants values that are not expired, use {@link #get(Object)} instead.
   *
   * @param key the key to look up
   */
  public CachedValue<V> getCachedValue(K key) {
    return cache.get(key);
  }

  /**
   * Returns the value for the given key, or null if no value exists.
   *
   * <p>When using this method, it is not possible to determine whether the value is expired or not.
   * Use {@link #getCachedValue(Object)} to achieve that instead. However, if using {@link
   * #getCachedValue(Object)} to determine if an item is expired, one should use the item within the
   * {@link CachedValue} and not call {@link #getPossiblyExpired(Object)} to get the value
   * afterwards, since that is not guaranteed to return the same value or that the newly returned
   * value is in the same state.
   *
   * @param key the key to look up
   */
  public V getPossiblyExpired(K key) {
    CachedValue<V> cachedValue = getCachedValue(key);
    return cachedValue == null ? null : cachedValue.getValue();
  }

  /**
   * Returns the value for the given key only if it is not expired, or null if no value exists or is
   * expired.
   *
   * <p>This method will return null if either there is no value associated with this key or if the
   * associated value is expired.
   *
   * @param key the key to look up
   */
  public V get(K key) {
    CachedValue<V> cachedValue = getCachedValue(key);
    return cachedValue == null || cachedValue.isExpired() ? null : cachedValue.getValue();
  }

  /**
   * Puts an item in the cache.
   *
   * <p>Newly added item will not be expired until {@link #expireAll()} is next called.
   *
   * @param key the key to look up
   * @param value the value to associate with the key
   */
  public void put(K key, V value) {
    cache.put(key, newCachedValue(value));
  }

  /**
   * Mark all items currently in the cache as expired.
   *
   * <p>Newly added items after this call will be marked as not expired.
   *
   * <p>Expiring the items in the cache does not imply they will be evicted.
   */
  public void expireAll() {
    generation.incrementAndGet();
  }

  /**
   * Creates a new {@link CachedValue} instance to be stored in this cache.
   *
   * <p>Implementation of {@link LruCache#create(K)} can use this method to create a new entry.
   */
  public CachedValue<V> newCachedValue(V value) {
    return new GenerationalCachedValue<V>(value, generation);
  }

  /**
   * A cached value stored inside the cache.
   *
   * <p>It provides access to the value stored in the cache but also allows to check whether the
   * value is expired.
   *
   * @param <V> the type of value stored in the cache
   */
  public interface CachedValue<V> {

    /** Returns the value stored in the cache for a given key. */
    V getValue();

    /**
     * Checks whether the value, while still being present in the cache, is expired.
     *
     * @return true if the value is expired
     */
    boolean isExpired();
  }

  /** Cached values storing the generation at which they were added. */
  @Immutable
  private static class GenerationalCachedValue<V> implements ExpirableCache.CachedValue<V> {

    /** The value stored in the cache. */
    public final V value;
    /** The generation at which the value was added to the cache. */
    private final int generation;
    /** The atomic integer storing the current generation of the cache it belongs to. */
    private final AtomicInteger cacheGeneration;

    /**
     * @param cacheGeneration the atomic integer storing the generation of the cache in which this
     *     value will be stored
     */
    public GenerationalCachedValue(V value, AtomicInteger cacheGeneration) {
      this.value = value;
      this.cacheGeneration = cacheGeneration;
      // Snapshot the current generation.
      generation = this.cacheGeneration.get();
    }

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

    @Override
    public boolean isExpired() {
      return generation != cacheGeneration.get();
    }
  }
}