1/*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation.  Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25/*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36package java.util.concurrent.atomic;
37
38import java.io.Serializable;
39
40/**
41 * One or more variables that together maintain an initially zero
42 * {@code long} sum.  When updates (method {@link #add}) are contended
43 * across threads, the set of variables may grow dynamically to reduce
44 * contention. Method {@link #sum} (or, equivalently, {@link
45 * #longValue}) returns the current total combined across the
46 * variables maintaining the sum.
47 *
48 * <p>This class is usually preferable to {@link AtomicLong} when
49 * multiple threads update a common sum that is used for purposes such
50 * as collecting statistics, not for fine-grained synchronization
51 * control.  Under low update contention, the two classes have similar
52 * characteristics. But under high contention, expected throughput of
53 * this class is significantly higher, at the expense of higher space
54 * consumption.
55 *
56 * <p>LongAdders can be used with a {@link
57 * java.util.concurrent.ConcurrentHashMap} to maintain a scalable
58 * frequency map (a form of histogram or multiset). For example, to
59 * add a count to a {@code ConcurrentHashMap<String,LongAdder> freqs},
60 * initializing if not already present, you can use {@code
61 * freqs.computeIfAbsent(key, k -> new LongAdder()).increment();}
62 *
63 * <p>This class extends {@link Number}, but does <em>not</em> define
64 * methods such as {@code equals}, {@code hashCode} and {@code
65 * compareTo} because instances are expected to be mutated, and so are
66 * not useful as collection keys.
67 *
68 * @since 1.8
69 * @author Doug Lea
70 */
71public class LongAdder extends Striped64 implements Serializable {
72    private static final long serialVersionUID = 7249069246863182397L;
73
74    /**
75     * Creates a new adder with initial sum of zero.
76     */
77    public LongAdder() {
78    }
79
80    /**
81     * Adds the given value.
82     *
83     * @param x the value to add
84     */
85    public void add(long x) {
86        Cell[] as; long b, v; int m; Cell a;
87        if ((as = cells) != null || !casBase(b = base, b + x)) {
88            boolean uncontended = true;
89            if (as == null || (m = as.length - 1) < 0 ||
90                (a = as[getProbe() & m]) == null ||
91                !(uncontended = a.cas(v = a.value, v + x)))
92                longAccumulate(x, null, uncontended);
93        }
94    }
95
96    /**
97     * Equivalent to {@code add(1)}.
98     */
99    public void increment() {
100        add(1L);
101    }
102
103    /**
104     * Equivalent to {@code add(-1)}.
105     */
106    public void decrement() {
107        add(-1L);
108    }
109
110    /**
111     * Returns the current sum.  The returned value is <em>NOT</em> an
112     * atomic snapshot; invocation in the absence of concurrent
113     * updates returns an accurate result, but concurrent updates that
114     * occur while the sum is being calculated might not be
115     * incorporated.
116     *
117     * @return the sum
118     */
119    public long sum() {
120        Cell[] as = cells;
121        long sum = base;
122        if (as != null) {
123            for (Cell a : as)
124                if (a != null)
125                    sum += a.value;
126        }
127        return sum;
128    }
129
130    /**
131     * Resets variables maintaining the sum to zero.  This method may
132     * be a useful alternative to creating a new adder, but is only
133     * effective if there are no concurrent updates.  Because this
134     * method is intrinsically racy, it should only be used when it is
135     * known that no threads are concurrently updating.
136     */
137    public void reset() {
138        Cell[] as = cells;
139        base = 0L;
140        if (as != null) {
141            for (Cell a : as)
142                if (a != null)
143                    a.reset();
144        }
145    }
146
147    /**
148     * Equivalent in effect to {@link #sum} followed by {@link
149     * #reset}. This method may apply for example during quiescent
150     * points between multithreaded computations.  If there are
151     * updates concurrent with this method, the returned value is
152     * <em>not</em> guaranteed to be the final value occurring before
153     * the reset.
154     *
155     * @return the sum
156     */
157    public long sumThenReset() {
158        Cell[] as = cells;
159        long sum = base;
160        base = 0L;
161        if (as != null) {
162            for (Cell a : as) {
163                if (a != null) {
164                    sum += a.value;
165                    a.reset();
166                }
167            }
168        }
169        return sum;
170    }
171
172    /**
173     * Returns the String representation of the {@link #sum}.
174     * @return the String representation of the {@link #sum}
175     */
176    public String toString() {
177        return Long.toString(sum());
178    }
179
180    /**
181     * Equivalent to {@link #sum}.
182     *
183     * @return the sum
184     */
185    public long longValue() {
186        return sum();
187    }
188
189    /**
190     * Returns the {@link #sum} as an {@code int} after a narrowing
191     * primitive conversion.
192     */
193    public int intValue() {
194        return (int)sum();
195    }
196
197    /**
198     * Returns the {@link #sum} as a {@code float}
199     * after a widening primitive conversion.
200     */
201    public float floatValue() {
202        return (float)sum();
203    }
204
205    /**
206     * Returns the {@link #sum} as a {@code double} after a widening
207     * primitive conversion.
208     */
209    public double doubleValue() {
210        return (double)sum();
211    }
212
213    /**
214     * Serialization proxy, used to avoid reference to the non-public
215     * Striped64 superclass in serialized forms.
216     * @serial include
217     */
218    private static class SerializationProxy implements Serializable {
219        private static final long serialVersionUID = 7249069246863182397L;
220
221        /**
222         * The current value returned by sum().
223         * @serial
224         */
225        private final long value;
226
227        SerializationProxy(LongAdder a) {
228            value = a.sum();
229        }
230
231        /**
232         * Returns a {@code LongAdder} object with initial state
233         * held by this proxy.
234         *
235         * @return a {@code LongAdder} object with initial state
236         * held by this proxy
237         */
238        private Object readResolve() {
239            LongAdder a = new LongAdder();
240            a.base = value;
241            return a;
242        }
243    }
244
245    /**
246     * Returns a
247     * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAdder.SerializationProxy">
248     * SerializationProxy</a>
249     * representing the state of this instance.
250     *
251     * @return a {@link SerializationProxy}
252     * representing the state of this instance
253     */
254    private Object writeReplace() {
255        return new SerializationProxy(this);
256    }
257
258    /**
259     * @param s the stream
260     * @throws java.io.InvalidObjectException always
261     */
262    private void readObject(java.io.ObjectInputStream s)
263        throws java.io.InvalidObjectException {
264        throw new java.io.InvalidObjectException("Proxy required");
265    }
266
267}
268