001/*
002 * (C) Copyright 2006-2011 Nuxeo SA (http://nuxeo.com/) and others.
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 *     http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 *
016 * Contributors:
017 *     Nuxeo - initial API and implementation
018 *
019 * $Id$
020 */
021
022package org.nuxeo.ecm.core.utils;
023
024/**
025 * Generate session IDs.
026 * <p>
027 * Session IDs are long values that must be unique on the same JVM. Each call of the {@link SIDGenerator#next()} method
028 * returns an unique ID (unique relative to the current running JVM).
029 *
030 * @author <a href="mailto:[email protected]">Bogdan Stefanescu</a>
031 */
032public final class SIDGenerator {
033
034    private static int count = 0;
035
036    private static final int COUNT_OFFSET = 32;
037
038    private SIDGenerator() {
039    }
040
041    /**
042     * The long unique id is generated as follow:
043     * <p>
044     * On the first 32 bits we put an integer value incremented at each call and that is reset to 0 when the it reaches
045     * the max integer range.
046     * <p>
047     * On the last 32 bits the most significant part of the current timestamp in milliseconds.
048     *
049     * @return the next unique id in this JVM
050     */
051    public static synchronized long next() {
052        if (count == Integer.MAX_VALUE) {
053            count = 0;
054        }
055        long ms = System.currentTimeMillis();
056        long id = (int) ms;
057        id = Long.rotateLeft(id, COUNT_OFFSET);
058        return id + count++;
059    }
060
061}