com.google.common.cache

Class CacheLoader<K,V>

java.lang.Object

com.google.common.cache.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, if you're able to use lambda expressions it can be specified even more easily:

 CacheLoader<Key, Graph> loader = CacheLoader.from(key -> createExpensiveGraph(key));
 

Since:

10.0

Author:

Charles Fry

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	CacheLoader.InvalidCacheLoadException Thrown to indicate that an invalid response was returned from a call to CacheLoader.
static class	CacheLoader.UnsupportedLoadingOperationException Exception thrown by loadAll() to indicate that it is not supported.

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	CacheLoader() Constructor for use by subclasses.

Method Summary

Modifier and Type	Method and Description
static <K,V> CacheLoader<K,V>	asyncReloading(CacheLoader<K,V> loader, Executor executor) Returns a CacheLoader which wraps loader, executing calls to reload(K, V) using executor.
static <K,V> CacheLoader<K,V>	from(Function<K,V> function) Returns a cache loader that uses function to load keys, and without supporting either reloading or bulk loading.
static <V> CacheLoader<Object,V>	from(Supplier<V> supplier) Returns a cache loader based on an existing supplier instance.
abstract V	load(K key) Computes or retrieves the value corresponding to key.
Map<K,V>	loadAll(Iterable<? extends K> keys) Computes or retrieves the values corresponding to keys.
ListenableFuture<V>	reload(K key, V oldValue) Computes or retrieves a replacement value corresponding to an already-cached key.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

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(long, java.util.concurrent.TimeUnit), 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(long, java.util.concurrent.TimeUnit).

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, and without supporting either reloading or bulk loading. This is most useful when you can pass a lambda expression. Otherwise it is useful mostly when you already have an existing function instance.

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.

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

asyncReloading

@GwtIncompatible
public static <K,V> CacheLoader<K,V> asyncReloading(CacheLoader<K,V> loader,
                                                                     Executor executor)

Returns a CacheLoader which wraps loader, executing calls to reload(K, V) using executor.

This method is useful only when loader.reload has a synchronous implementation, such as the default implementation.

Since:

17.0