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/*
* Licensed to Elasticsearch under one or more contributor
* license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright
* ownership. Elasticsearch licenses this file to you 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 org.elasticsearch.common.util;
import org.elasticsearch.common.unit.TimeValue;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* A very simple single object cache that allows non-blocking refresh calls
* triggered by expiry time.
*/
public abstract class SingleObjectCache<T>{
private volatile T cached;
private Lock refreshLock = new ReentrantLock();
private final TimeValue refreshInterval;
protected long lastRefreshTimestamp = 0;
protected SingleObjectCache(TimeValue refreshInterval, T initialValue) {
if (initialValue == null) {
throw new IllegalArgumentException("initialValue must not be null");
}
this.refreshInterval = refreshInterval;
cached = initialValue;
}
/**
* Returns the currently cached object and potentially refreshes the cache before returning.
*/
public T getOrRefresh() {
if (needsRefresh()) {
if(refreshLock.tryLock()) {
try {
if (needsRefresh()) { // check again!
cached = refresh();
assert cached != null;
lastRefreshTimestamp = System.currentTimeMillis();
}
} finally {
refreshLock.unlock();
}
}
}
assert cached != null;
return cached;
}
/**
* Returns a new instance to cache
*/
protected abstract T refresh();
/**
* Returns <code>true</code> iff the cache needs to be refreshed.
*/
protected boolean needsRefresh() {
if (refreshInterval.millis() == 0) {
return true;
}
final long currentTime = System.currentTimeMillis();
return (currentTime - lastRefreshTimestamp) > refreshInterval.millis();
}
}
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