Guava---Cache

Cache

概述

A semi-persistent mapping from keys to values.
Cache entries are manually added using {@link #get(Object, Callable)} or {@link #put(Object, Object)}, and are stored in the cache until either evicted or manually invalidated.
The common way to build instances is using {@link CacheBuilder}.

k-v半持久化映射；

Cache的entry 被手动使用get(Object, Callable), put(Object, Object);

创建Cache实例通常使用CacheBuilder；

Implementations of this interface are expected to be thread-safe, and can be safely accessed by multiple concurrent threads.

Cache的实现应该是线程安全的；

/**
     * @param <K> the type of the cache's keys, which are not permitted to be null
     * @param <V> the type of the cache's values, which are not permitted to be null
     */
    public interface Cache<K, V> {
        /**
         * Returns the value associated with {@code key} in this cache, or {@code null} if there is no cached value for {@code key}.
         */
        V getIfPresent(@CompatibleWith("K") Object key);
 
        /**
         * Returns the value associated with {@code key} in this cache, obtaining that value from {@code loader} if necessary.  返回key关联的value，如果不存在，从Loader中获取
         * The method improves upon the conventional "if cached, return; otherwise create, cache and return" pattern.  改进了 if cached, return; otherwise create, cache and return 的范式
         * For further improvements, use {@link LoadingCache} and its {@link LoadingCache#get(Object) get(K)} method instead of this one.
         */
        V get(K key, Callable<? extends V> loader) throws ExecutionException;
 
        /**
         * Associates {@code value} with {@code key} in this cache.
         * If the cache previously contained a value associated with {@code key}, the old value is replaced by {@code value}. 如果key存在，new替换old
         * Prefer {@link #get(Object, Callable)} when using the conventional "if cached, return; otherwise create, cache and return" pattern.
         * @param key
         * @param value
         */
        void put(K key, V value);
 
        /**
         * Discards any cached value for key {@code key}.
         */
        void invalidate(@CompatibleWith("K") Object key);
 
        /**
         * For discussion of <? extends Object>, see getAllPresent.
         */
        void invalidateAll(Iterable<? extends Object> keys);
 
        /**
         * Discards all entries in the cache.
         */
        void invalidateAll();
    }

链路

CacheBuilder.newBuilder().build()

// com.google.common.cache.CacheBuilder.newBuilder
    public final class CacheBuilder<K, V> {
 
        private CacheBuilder() {
 
        }
 
        public static CacheBuilder<Object, Object> newBuilder() {
            return new CacheBuilder<>();
        }
 
        public <K1 extends K, V1 extends V> Cache<K1, V1> build() {
            checkWeightWithWeigher();
            checkNonLoadingCache();
            return new LocalCache.LocalManualCache<>(this);
        }
    }
 
    // com.google.common.cache.LocalCache
    class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
        static class LocalManualCache<K, V> implements Cache<K, V>, Serializable {
            final LocalCache<K, V> localCache;
 
            private LocalManualCache(LocalCache<K, V> localCache) {
                this.localCache = localCache;
            }
        }
    }

Put

// com.google.common.cache.LocalCache
    class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
 
        static class LocalManualCache<K, V> implements Cache<K, V>, Serializable {
            final LocalCache<K, V> localCache;
 
            private LocalManualCache(LocalCache<K, V> localCache) {
                this.localCache = localCache;
            }
 
            public void put(K key, V value) {
                localCache.put(key, value);
            }
        }
 
        static class Segment<K, V> extends ReentrantLock {
 
            V put(K key, int hash, V value, boolean onlyIfAbsent) {
 
            }
        }
 
        /** The segments, each of which is a specialized hash table. */
        final Segment<K, V>[] segments;
 
        /**
         * Shift value for indexing within segments.
         * Helps prevent entries that end up in the same segment from also ending up in the same bucket.
         */
        final int segmentShift;
 
        /**
         * Mask value for indexing into segments.
         * The upper bits of a key's hash code are used to choose the segment.
         */
        final int segmentMask;
 
        public V put(K key, V value) {
            checkNotNull(key);
            checkNotNull(value);
            int hash = hash(key);
            return segmentFor(hash).put(key, hash, value, false);
        }
 
        Segment<K, V> segmentFor(int hash) {
            // TODO(fry): Lazily create segments?
            return segments[(hash >>> segmentShift) & segmentMask];
        }
 
 
    }