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## 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:
  10.0
Author:
  Charles Fry

Nested Class Summary
`static class` | `CacheLoader.InvalidCacheLoadException` Thrown to indicate that an invalid response was returned from a call to `CacheLoader`.

Constructor Summary
`protected` | `CacheLoader()` Constructor for use by subclasses.

Method Summary
`static
`<K,V> CacheLoader<K,V>`
` | `from(Function<K,V> function)` Returns a cache loader based on an existing function instance.
`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:
    `CacheLoader.InvalidCacheLoadException` - 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
    `Exception`

### reload

    @GwtIncompatible(value="Futures")
    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:
    `CacheLoader.InvalidCacheLoadException` - 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
    `Exception`
  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)`.

  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 overriden when bulk retrieval is significantly more efficient than many individual lookups. Note that `LoadingCache.getAll(java.lang.Iterable)` will defer to individual calls to `LoadingCache.get(K)` if this method is not overriden.

  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:
    `CacheLoader.InvalidCacheLoadException` - 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
    `Exception`
  Since:
    11.0

### from

    @Beta
    public static <K,V> CacheLoader<K,V> from(Function<K,V> function)

  Returns a cache loader based on an existing function instance. Note that there's no need to create a new function just to pass it in here; just subclass `CacheLoader` and implement `load` instead.

  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

    @Beta
    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

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