https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2011 The Guava Authors
 *
 * 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.google.common.cache;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import com.google.common.collect.ImmutableMap;
import com.google.common.util.concurrent.ExecutionError;
import com.google.common.util.concurrent.UncheckedExecutionException;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;


A semi-persistent mapping from keys to values. Values are automatically loaded by the cache, and are stored in the cache until either evicted or manually invalidated. The common way to build instances is using CacheBuilder. 
Implementations of this interface are expected to be thread-safe, and can be safely accessed
by multiple concurrent threads.

When evaluated as a Function, a cache yields the same result as invoking getUnchecked. 
Author:Charles Fry
Since:11.0
/**
 * A semi-persistent mapping from keys to values. Values are automatically loaded by the cache, and
 * are stored in the cache until either evicted or manually invalidated. The common way to build
 * instances is using {@link CacheBuilder}.
 *
 * <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed
 * by multiple concurrent threads.
 *
 * <p>When evaluated as a {@link Function}, a cache yields the same result as invoking {@link
 * #getUnchecked}.
 *
 * @author Charles Fry
 * @since 11.0
 */
@GwtCompatible
public interface LoadingCache<K, V> extends Cache<K, V>, Function<K, V> {

  
Returns the value associated with key in this cache, first loading that value if necessary. No observable state associated with this cache is modified until loading completes. 
If another call to get or getUnchecked is currently loading the value for key, simply waits for that thread to finish and returns its loaded value. Note that multiple threads can concurrently load values for distinct keys. 
Caches loaded by a CacheLoader will call CacheLoader.load to load new values into the cache. Newly loaded values are added to the cache using 
Cache.asMap().putIfAbsent after loading has completed; if another value was associated with key while the new value was loading then a removal notification will be sent for the new value. 
If the cache loader associated with this cache is known not to throw checked exceptions, then prefer getUnchecked over this method. 
Throws:
/**
   * Returns the value associated with {@code key} in this cache, first loading that value if
   * necessary. No observable state associated with this cache is modified until loading completes.
   *
   * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
   * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
   * multiple threads can concurrently load values for distinct keys.
   *
   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
   * into the cache. Newly loaded values are added to the cache using {@code
   * Cache.asMap().putIfAbsent} after loading has completed; if another value was associated with
   * {@code key} while the new value was loading then a removal notification will be sent for the
   * new value.
   *
   * <p>If the cache loader associated with this cache is known not to throw checked exceptions,
   * then prefer {@link #getUnchecked} over this method.
   *
   * @throws ExecutionException if a checked exception was thrown while loading the value. ({@code
   *     ExecutionException} is thrown <a
   *     href="https://github.com/google/guava/wiki/CachesExplained#interruption">even if
   *     computation was interrupted by an {@code InterruptedException}</a>.)
   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
   *     value
   * @throws ExecutionError if an error was thrown while loading the value
   */
  V get(K key) throws ExecutionException;

  
Returns the value associated with key in this cache, first loading that value if necessary. No observable state associated with this cache is modified until loading completes. Unlike get, this method does not throw a checked exception, and thus should only be used in situations where checked exceptions are not thrown by the cache loader. 
If another call to get or getUnchecked is currently loading the value for key, simply waits for that thread to finish and returns its loaded value. Note that multiple threads can concurrently load values for distinct keys. 
Caches loaded by a CacheLoader will call CacheLoader.load to load new values into the cache. Newly loaded values are added to the cache using 
Cache.asMap().putIfAbsent after loading has completed; if another value was associated with key while the new value was loading then a removal notification will be sent for the new value. 
Warning: this method silently converts checked exceptions to unchecked exceptions, and should not be used with cache loaders which throw checked exceptions. In such cases use get instead. 
Throws:
  • UncheckedExecutionException – if an exception was thrown while loading the value. (As
    explained in the last paragraph above, this should be an unchecked exception only.)
  • ExecutionError – if an error was thrown while loading the value
/**
   * Returns the value associated with {@code key} in this cache, first loading that value if
   * necessary. No observable state associated with this cache is modified until loading completes.
   * Unlike {@link #get}, this method does not throw a checked exception, and thus should only be
   * used in situations where checked exceptions are not thrown by the cache loader.
   *
   * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
   * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
   * multiple threads can concurrently load values for distinct keys.
   *
   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
   * into the cache. Newly loaded values are added to the cache using {@code
   * Cache.asMap().putIfAbsent} after loading has completed; if another value was associated with
   * {@code key} while the new value was loading then a removal notification will be sent for the
   * new value.
   *
   * <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions,
   * and should not be used with cache loaders which throw checked exceptions. In such cases use
   * {@link #get} instead.
   *
   * @throws UncheckedExecutionException if an exception was thrown while loading the value. (As
   *     explained in the last paragraph above, this should be an unchecked exception only.)
   * @throws ExecutionError if an error was thrown while loading the value
   */
  V getUnchecked(K key);

  
Returns a map of the values associated with keys, creating or retrieving those values if necessary. The returned map contains entries that were already cached, combined with newly loaded entries; it will never contain null keys or values. 
Caches loaded by a CacheLoader will issue a single request to CacheLoader.loadAll for all keys which are not already present in the cache. All entries returned by CacheLoader.loadAll will be stored in the cache, over-writing any previously cached values. This method will throw an exception if CacheLoader.loadAll returns null, returns a map containing null keys or values, or fails to return an entry for each requested key. 
Note that duplicate elements in keys, as determined by Object.equals, will be ignored. 
Throws:
Since:11.0
/**
   * Returns a map of the values associated with {@code keys}, creating or retrieving those values
   * if necessary. The returned map contains entries that were already cached, combined with newly
   * loaded entries; it will never contain null keys or values.
   *
   * <p>Caches loaded by a {@link CacheLoader} will issue a single request to {@link
   * CacheLoader#loadAll} for all keys which are not already present in the cache. All entries
   * returned by {@link CacheLoader#loadAll} will be stored in the cache, over-writing any
   * previously cached values. This method will throw an exception if {@link CacheLoader#loadAll}
   * returns {@code null}, returns a map containing null keys or values, or fails to return an entry
   * for each requested key.
   *
   * <p>Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will
   * be ignored.
   *
   * @throws ExecutionException if a checked exception was thrown while loading the value. ({@code
   *     ExecutionException} is thrown <a
   *     href="https://github.com/google/guava/wiki/CachesExplained#interruption">even if
   *     computation was interrupted by an {@code InterruptedException}</a>.)
   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
   *     values
   * @throws ExecutionError if an error was thrown while loading the values
   * @since 11.0
   */
  ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException;

  
Throws:
  • UncheckedExecutionException – if an exception was thrown while loading the value. (As described in the documentation for getUnchecked, LoadingCache should be used as a Function only with cache loaders that throw only unchecked exceptions.)
Deprecated:Provided to satisfy the Function interface; use get or getUnchecked instead.
/**
   * @deprecated Provided to satisfy the {@code Function} interface; use {@link #get} or {@link
   *     #getUnchecked} instead.
   * @throws UncheckedExecutionException if an exception was thrown while loading the value. (As
   *     described in the documentation for {@link #getUnchecked}, {@code LoadingCache} should be
   *     used as a {@code Function} only with cache loaders that throw only unchecked exceptions.)
   */
  @Deprecated
  @Override
  V apply(K key);

  
Loads a new value for key key, possibly asynchronously. While the new value is loading the previous value (if any) will continue to be returned by get(key) unless it is evicted. If the new value is loaded successfully it will replace the previous value in the cache; if an exception is thrown while refreshing the previous value will remain, and the exception will be logged (using Logger) and swallowed.

Caches loaded by a CacheLoader will call CacheLoader.reload if the cache currently contains a value for key, and CacheLoader.load otherwise. Loading is asynchronous only if CacheLoader.reload was overridden with an asynchronous implementation. 
Returns without doing anything if another thread is currently loading the value for 
key. If the cache loader associated with this cache performs refresh asynchronously then this method may return before refresh completes. 
Since:11.0
/**
   * Loads a new value for key {@code key}, possibly asynchronously. While the new value is loading
   * the previous value (if any) will continue to be returned by {@code get(key)} unless it is
   * evicted. If the new value is loaded successfully it will replace the previous value in the
   * cache; if an exception is thrown while refreshing the previous value will remain, <i>and the
   * exception will be logged (using {@link java.util.logging.Logger}) and swallowed</i>.
   *
   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#reload} if the cache
   * currently contains a value for {@code key}, and {@link CacheLoader#load} otherwise. Loading is
   * asynchronous only if {@link CacheLoader#reload} was overridden with an asynchronous
   * implementation.
   *
   * <p>Returns without doing anything if another thread is currently loading the value for {@code
   * key}. If the cache loader associated with this cache performs refresh asynchronously then this
   * method may return before refresh completes.
   *
   * @since 11.0
   */
  void refresh(K key);

  
{@inheritDoc}

Note that although the view is modifiable, no method on the returned map will ever
cause entries to be automatically loaded.

/**
   * {@inheritDoc}
   *
   * <p><b>Note that although the view <i>is</i> modifiable, no method on the returned map will ever
   * cause entries to be automatically loaded.</b>
   */
  @Override
  ConcurrentMap<K, V> asMap();
}