org.eclipse.edc.util.collection.Cache Maven / Gradle / Ivy
/*
* Copyright (c) 2024 Bayerische Motoren Werke Aktiengesellschaft (BMW AG)
*
* This program and the accompanying materials are made available under the
* terms of the Apache License, Version 2.0 which is available at
* https://www.apache.org/licenses/LICENSE-2.0
*
* SPDX-License-Identifier: Apache-2.0
*
* Contributors:
* Bayerische Motoren Werke Aktiengesellschaft (BMW AG) - initial API and implementation
*
*/
package org.eclipse.edc.util.collection;
import java.time.Clock;
import java.time.Instant;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.function.Function;
/**
* Cache, that maintains a map of key-value pairs, where values have an individual expiry. When values expire, they are re-fetched
* and put back in the cache.
*
* Values are not stored directly, but are wrapped in a {@link TimestampedValue}. When getting values from the cache, one has to provide
* a {@code cacheEntryUpdateFunction}, which encapsulates the re-fetching of the expired value.
*
* This cache is thread-safe.
*/
public class Cache {
private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
private final Map> cache = new HashMap<>();
private final Function cacheEntryUpdateFunction;
private final long validity;
private final Clock clock;
public Cache(Function cacheEntryUpdateFunction, long validity) {
this(cacheEntryUpdateFunction, validity, Clock.systemUTC());
}
public Cache(Function cacheEntryUpdateFunction, long validity, Clock clock) {
this.cacheEntryUpdateFunction = cacheEntryUpdateFunction;
this.validity = validity;
this.clock = clock;
}
/**
* Gets an entry from the cache, or - if the entry is expired - calls the refresh function, and then gets the value.
*
* @param key The key of the value to get.
* @return the value
*/
public V get(K key) {
V value;
lock.readLock().lock();
try {
if (isEntryExpired(key)) {
lock.readLock().unlock(); // unlock read, acquire write -> "upgrade" lock
lock.writeLock().lock();
try {
if (isEntryExpired(key)) {
var newEntry = cacheEntryUpdateFunction.apply(key);
cache.put(key, new TimestampedValue<>(newEntry, Instant.now(), validity));
}
} finally {
lock.readLock().lock(); // downgrade lock
lock.writeLock().unlock();
}
}
value = cache.get(key).value();
} finally {
lock.readLock().unlock();
}
return value;
}
private boolean isEntryExpired(K key) {
var timestampedValue = cache.get(key);
if (timestampedValue == null) return true;
return timestampedValue.isExpired(clock);
}
}