001/**
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.camel.util;
018
019import java.util.concurrent.atomic.AtomicLong;
020
021public final class ReferenceCount {
022    private final AtomicLong count;
023    private final Runnable onFirst;
024    private final Runnable onRelease;
025
026    private ReferenceCount(Runnable onFirst, Runnable onRelease) {
027        this.count = new AtomicLong(0);
028        this.onFirst = ObjectHelper.notNull(onFirst, "onFirst");
029        this.onRelease = ObjectHelper.notNull(onRelease, "onRelease");
030    }
031
032    /**
033     * Returns the reference count.
034     */
035    public long get() {
036        return count.get();
037    }
038
039    /**
040     * Increases the reference count invoke onFirst on the first increment;
041     */
042    public void retain() throws IllegalStateException {
043        while (true) {
044            long v = count.get();
045            if (v < 0) {
046                throw new IllegalStateException("Released");
047            }
048
049            if (count.compareAndSet(v, v + 1)) {
050                if (v == 0) {
051                    this.onFirst.run();
052                }
053
054                break;
055            }
056        }
057    }
058
059    /**
060     * Decreases the reference count and invoke onRelease if the reference count reaches {@code 0}.
061     */
062    public void release() throws IllegalStateException {
063        while (true) {
064            long v = count.get();
065            if (v <= 0) {
066                throw new IllegalStateException("ReferenceCount already released");
067            }
068
069            if (count.compareAndSet(v, v - 1)) {
070                if (v == 1) {
071                    onRelease.run();
072                }
073
074                break;
075            }
076        }
077    }
078
079    // *******************************
080    // Helpers
081    // *******************************
082
083    public static ReferenceCount on(Runnable onFirst, Runnable onRelease) {
084        return new ReferenceCount(onFirst, onRelease);
085    }
086
087    public static ReferenceCount onRelease(Runnable onRelease) {
088        return new ReferenceCount(() -> { }, onRelease);
089    }
090}