Package com.google.common.cache

Class CacheLoader<K,​V>

  • @GwtCompatible(emulated=true)
public abstract class CacheLoader<K,​V>
extends Object
    Computes or retrieves values, based on a key, for use in populating a LoadingCache.

    Most implementations will only need to implement load(K). Other methods may be overridden as desired.

    Usage example: 

    
 CacheLoader<Key, Graph> loader = new CacheLoader<Key, Graph>() {
   public Graph load(Key key) throws AnyException {
     return createExpensiveGraph(key);
   }
 };
 LoadingCache<Key, Graph> cache = CacheBuilder.newBuilder().build(loader);

    Since this example doesn't support reloading or bulk loading, it can also be specified much more simply: 

    
 CacheLoader<Key, Graph> loader = CacheLoader.from(key -> createExpensiveGraph(key));
    Since:
    10.0
    Author:
    Charles Fry

    • Constructor Detail

      • CacheLoader

        protected CacheLoader()
        Constructor for use by subclasses.

    • Method Detail

      • load

        public abstract V load​(K key)
                throws Exception
        Computes or retrieves the value corresponding to key.
        Parameters:
        key - the non-null key whose value should be loaded
        Returns:
        the value associated with key; must not be null
        Throws:
        Exception - if unable to load the result
        InterruptedException - if this method is interrupted. InterruptedException is treated like any other Exception in all respects except that, when it is caught, the thread's interrupt status is set

      • reload

        @GwtIncompatible
public ListenableFuture<V> reload​(K key,
                                  V oldValue)
                           throws Exception
        Computes or retrieves a replacement value corresponding to an already-cached key. This method is called when an existing cache entry is refreshed by CacheBuilder.refreshAfterWrite(java.time.Duration), or through a call to LoadingCache.refresh(K).

        This implementation synchronously delegates to load(K). It is recommended that it be overridden with an asynchronous implementation when using CacheBuilder.refreshAfterWrite(java.time.Duration).

        Note: all exceptions thrown by this method will be logged and then swallowed.

        Parameters:
        key - the non-null key whose value should be loaded
        oldValue - the non-null old value corresponding to key
        Returns:
        the future new value associated with key; must not be null, must not return null
        Throws:
        Exception - if unable to reload the result
        InterruptedException - if this method is interrupted. InterruptedException is treated like any other Exception in all respects except that, when it is caught, the thread's interrupt status is set
        Since:
        11.0

      • loadAll

        public Map<K,​V> loadAll​(Iterable<? extends K> keys)
                       throws Exception
        Computes or retrieves the values corresponding to keys. This method is called by LoadingCache.getAll(java.lang.Iterable<? extends K>).

        If the returned map doesn't contain all requested keys then the entries it does contain will be cached, but getAll will throw an exception. If the returned map contains extra keys not present in keys then all returned entries will be cached, but only the entries for keys will be returned from getAll.

        This method should be overridden when bulk retrieval is significantly more efficient than many individual lookups. Note that LoadingCache.getAll(java.lang.Iterable<? extends K>) will defer to individual calls to LoadingCache.get(K) if this method is not overridden.

        Parameters:
        keys - the unique, non-null keys whose values should be loaded
        Returns:
        a map from each key in keys to the value associated with that key; may not contain null values
        Throws:
        Exception - if unable to load the result
        InterruptedException - if this method is interrupted. InterruptedException is treated like any other Exception in all respects except that, when it is caught, the thread's interrupt status is set
        Since:
        11.0

      • from

        public static <K,​V> CacheLoader<K,​V> from​(Function<K,​V> function)
        Returns a cache loader that uses function to load keys, without supporting either reloading or bulk loading. This allows creating a cache loader using a lambda expression.

        The returned object is serializable if function is serializable.

        Parameters:
        function - the function to be used for loading values; must never return null
        Returns:
        a cache loader that loads values by passing each key to function

      • from

        public static <V> CacheLoader<Object,​V> from​(Supplier<V> supplier)
        Returns a cache loader based on an existing supplier instance. Note that there's no need to create a new supplier just to pass it in here; just subclass CacheLoader and implement load instead.

        The returned object is serializable if supplier is serializable.

        Parameters:
        supplier - the supplier to be used for loading values; must never return null
        Returns:
        a cache loader that loads values by calling Supplier.get(), irrespective of the key