com.cloudbees.syslog.util.CachingReference Maven / Gradle / Ivy
/*
* Copyright 2010-2013, CloudBees Inc.
*
* 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.cloudbees.syslog.util;
import javax.annotation.Nullable;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
* Maintains a cached version of the {@code Object} that it holds and handle the renewal of this object upon expiration.
*
* Greatly inspired by the {@code CachedData} sample provided in the javadoc
* of {@link java.util.concurrent.locks.ReentrantReadWriteLock}.
*
* {@code Object} is created implementing the {@link #newObject()} method.
*
*
* Sample to get an {@code InetAddress} refreshed against a DNS every 10 seconds:
*
* CachingReference myRemoteServerAddress = new CachingReference<InetAddress>(10, TimeUnit.SECONDS) {
* protected InetAddress newObject() {
* try {
* return InetAddress.getByName(myRemoteServerHostname);
* } catch () {
* throw new RuntimeException("Exception resolving '" + myRemoteServerHostname + "'", e);
* }
* }
* }
*
*
* @author Cyrille Le Clerc
*/
public abstract class CachingReference {
private final ReadWriteLock rwl = new ReentrantReadWriteLock();
private long lastCreationInNanos;
private long timeToLiveInNanos;
private E object;
public CachingReference(long timeToLiveInNanos) {
this.timeToLiveInNanos = timeToLiveInNanos;
}
public CachingReference(long timeToLive, TimeUnit timeToLiveUnit) {
this(TimeUnit.NANOSECONDS.convert(timeToLive, timeToLiveUnit));
}
/**
* @return the newly created object.
*/
@Nullable
protected abstract E newObject();
/**
* @return the up to date version of the {@code Object} hold by this reference.
*/
@Nullable
public E get() {
rwl.readLock().lock();
try {
if ((System.nanoTime() - lastCreationInNanos) > timeToLiveInNanos) {
// Must release read lock before acquiring write lock
rwl.readLock().unlock();
rwl.writeLock().lock();
try {
// Recheck state because another thread might have
// acquired write lock and changed state before we did.
if ((System.nanoTime() - lastCreationInNanos) > timeToLiveInNanos) {
object = newObject();
lastCreationInNanos = System.nanoTime();
}
} finally {
// Downgrade by acquiring read lock before releasing write lock
rwl.readLock().lock();
rwl.writeLock().unlock();
}
}
return object;
} finally {
rwl.readLock().unlock();
}
}
@Override
public String toString() {
return "CachingReference[" + this.object + "]";
}
}