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This is the ehcache core module. Pair it with other modules for added functionality.

There is a newer version: 2.6.11

/**
 *  Copyright 2003-2010 Terracotta, 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 net.sf.ehcache.util;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.atomic.AtomicLong;

/**
 * Generates increasing identifiers (in a single VM only).
 * Not valid across multiple VMs. Yet, the identifier is based on time, so that the drifting
 * across a cluster should not ever be large...
 *
 * @author Alex Snaps
 */
public final class Timestamper {

    /**
     * Value for left shifting System.currentTimeMillis, freeing some space for the counter
     */
    public static final int BIN_DIGITS = Integer.getInteger("net.sf.ehcache.util.Timestamper.shift", 12);

    /**
     * What is one milliseconds, based on "counter value reserved space", for this Timestamper
     */
    public static final int ONE_MS = 1 << BIN_DIGITS;

    private static final Logger LOG = LoggerFactory.getLogger(Timestamper.class);
    private static final AtomicLong VALUE = new AtomicLong();

    private Timestamper() {
        //
    }

    /**
     * Returns an increasing unique value based on the System.currentTimeMillis()
     * with some additional reserved space for a counter.
     *
     * @see net.sf.ehcache.util.Timestamper#BIN_DIGITS
     * @return uniquely & increasing value
     */
    public static long next() {
        int runs = 0;
        while (true) {
            if (runs++ > 0 && LOG.isInfoEnabled()) {
                LOG.info("Thread spin-waits on time to pass. Looped "
                         + runs + " times, you might want to increase -Dnet.sf.ehcache.util.Timestamper.shift");
            }
            long base = System.currentTimeMillis() << BIN_DIGITS;
            long maxValue = base + ONE_MS - 1;

            for (long current = VALUE.get(), update = Math.max(base, current + 1); update < maxValue;
                 current = VALUE.get(), update = Math.max(base, current + 1)) {
                if (VALUE.compareAndSet(current, update)) {
                    return update;
                }
            }
        }
    }
}