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Syslog Java Client
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/*
 * 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 + "]";
    }
}