org.cache2k.expiry.ExpiryPolicy Maven / Gradle / Ivy
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package org.cache2k.expiry;
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* Copyright (C) 2000 - 2021 headissue GmbH, Munich
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* 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,
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import org.cache2k.Cache2kBuilder;
import org.cache2k.CacheEntry;
import org.cache2k.DataAware;
import org.cache2k.annotation.Nullable;
import java.util.concurrent.TimeUnit;
/**
* A custom policy which allows to calculate a dynamic expiry time for an entry after an
* insert or update.
*
* For some expiry calculations it is useful to know the previous entry, e.g. to detect
* whether the stored data was updated. If a previous value is present in the cache,
* it is passed to this method. Expired old entries will be passed in also, if still present
* in the cache.
*
*
The expiry policy is also used for refresh ahead, determining the time when an entry should
* be automatically refreshed.
*
* @author Jens Wilke
* @see
* cache2k user guide - Expiry and Refresh
* @see Cache2kBuilder#expiryPolicy(ExpiryPolicy)
* @see Cache2kBuilder#refreshAhead(boolean)
* @see Cache2kBuilder#expireAfterWrite(long, TimeUnit)
*/
public interface ExpiryPolicy extends ExpiryTimeValues, DataAware {
/**
* Returns the time of expiry in milliseconds since epoch.
*
* By default expiry itself happens lenient, which means the expiry happens
* zero or some milliseconds after the obtained time. If sharp expiry is requested,
* the value will not be returned any more by the cache when the point in time is reached.
* The cache parameters {@link Cache2kBuilder#sharpExpiry(boolean)}
* and {@link Cache2kBuilder#refreshAhead(boolean)} influence the behaviour.
* It is also possible to request a sharp timeout for some entries. This is done
* by returning a negative time value, see the further comments for the return
* value below.
*
*
Inserts or updates: It is possible to return different expiry times for
* inserts or updates. An update can be detected by the presence of the old entry.
*
*
Calling cache operations: It is illegal to call any
* cache methods from this method. The outcome is undefined and it can
* cause a deadlock.
*
*
Calling time:
The method is called from the cache whenever a
* cache entry is updated. However, it is legal that the cache calls the
* method at arbitrary times during the entry lifecycle.
*
* {@code null} values: If the loader returns a {@code null} value, the expiry
* policy will be called, regardless of the {@link Cache2kBuilder#permitNullValues} setting.
* If the expiry policy returns a {@link #NOW} the entry will be removed. If the expiry
* policy returns a different time value, a {@code NullPointerException} will be propagated
* if {@code null} values are not permitted.
*
*
Loader exceptions
The expiry policy is only called for a successful load
* operation.
*
* API rationale: The recently loaded or inserted data is not passed in via a cache
* entry object. Using a cache entry is desirable for API design reasons to have less parameter.
* But the "real" entry can only be filled after the expiry policy has been run, passing
* in an entry object would mean to build a temporary object, increasing GC load. Second, the
* properties that are needed by the implementation are available directly. The downside, OTOH,
* 4-arity breaks Java 8 lambdas. Time values: For performance reasons the long type
* is used to represent the time and not an object. Also it allows a simple offset calculation.
*
* @param key the cache key used for inserting or loading
* @param value the value to be cached. May be {@code null} if the loader returns {@code null},
* regardless of the {@link Cache2kBuilder#permitNullValues} setting.
* @param loadTime The time the entry was inserted or loaded. If a loader was used,
* this is the time before the loader was called.
* @param currentEntry entry representing the current mapping, if there is a value present.
* {@code null} if there is no current mapping, or, if the previous load
* operation had thrown an exception. This can be used for adapting the
* expiry time to the amount of data changes.
* @return the time of expiry in millis since epoch. {@link #NOW} if it should not
* cached. If {@link Cache2kBuilder#refreshAhead} is enabled the return value
* {@link #NOW} will trigger an immediate refresh.
* The return value {@link #ETERNAL} means that there is no specific expiry time
* known or needed. The effective expiry duration will never be longer than the configured
* expiry value via {@link Cache2kBuilder#expireAfterWrite(long, TimeUnit)}.
* If a negative value is returned, the negated value will be the expiry time
* used, but sharp expiry is requested. Use {@link Expiry#toSharpTime(long)} to have a
* more expressive code. Switching on
* {@link Cache2kBuilder#sharpExpiry(boolean)} means always sharp expiry.
*
* @see ValueWithExpiryTime#getCacheExpiryTime()
*/
long calculateExpiryTime(K key, V value, long loadTime, @Nullable CacheEntry currentEntry);
}