1 /*
2 * Written by Doug Lea with assistance from members of JCP JSR-166
3 * Expert Group and released to the public domain, as explained at
4 * http://creativecommons.org/publicdomain/zero/1.0/
5 */
6
7 /*
8 * Source:
9 * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/LongAdder.java?revision=1.17
10 */
11
12 package com.google.common.cache;
13
14 import com.google.common.annotations.GwtCompatible;
15 import java.io.IOException;
16 import java.io.ObjectInputStream;
17 import java.io.ObjectOutputStream;
18 import java.io.Serializable;
19 import java.util.concurrent.atomic.AtomicLong;
20
21 /**
22 * One or more variables that together maintain an initially zero {@code long} sum. When updates
23 * (method {@link #add}) are contended across threads, the set of variables may grow dynamically to
24 * reduce contention. Method {@link #sum} (or, equivalently, {@link #longValue}) returns the current
25 * total combined across the variables maintaining the sum.
26 *
27 * <p>This class is usually preferable to {@link AtomicLong} when multiple threads update a common
28 * sum that is used for purposes such as collecting statistics, not for fine-grained synchronization
29 * control. Under low update contention, the two classes have similar characteristics. But under
30 * high contention, expected throughput of this class is significantly higher, at the expense of
31 * higher space consumption.
32 *
33 * <p>This class extends {@link Number}, but does <em>not</em> define methods such as {@code
34 * equals}, {@code hashCode} and {@code compareTo} because instances are expected to be mutated, and
35 * so are not useful as collection keys.
36 *
37 * <p><em>jsr166e note: This class is targeted to be placed in java.util.concurrent.atomic.</em>
38 *
39 * @since 1.8
40 * @author Doug Lea
41 */
42 @GwtCompatible(emulated = true)
43 final class LongAdder extends Striped64 implements Serializable, LongAddable {
44 private static final long serialVersionUID = 7249069246863182397L;
45
46 /** Version of plus for use in retryUpdate */
47 @Override
48 final long fn(long v, long x) {
49 return v + x;
50 }
51
52 /** Creates a new adder with initial sum of zero. */
53 public LongAdder() {}
54
55 /**
56 * Adds the given value.
57 *
58 * @param x the value to add
59 */
60 @Override
61 public void add(long x) {
62 Cell[] as;
63 long b, v;
64 int[] hc;
65 Cell a;
66 int n;
67 if ((as = cells) != null || !casBase(b = base, b + x)) {
68 boolean uncontended = true;
69 if ((hc = threadHashCode.get()) == null
70 || as == null
71 || (n = as.length) < 1
72 || (a = as[(n - 1) & hc[0]]) == null
73 || !(uncontended = a.cas(v = a.value, v + x))) retryUpdate(x, hc, uncontended);
74 }
75 }
76
77 /** Equivalent to {@code add(1)}. */
78 @Override
79 public void increment() {
80 add(1L);
81 }
82
83 /** Equivalent to {@code add(-1)}. */
84 public void decrement() {
85 add(-1L);
86 }
87
88 /**
89 * Returns the current sum. The returned value is <em>NOT</em> an atomic snapshot; invocation in
90 * the absence of concurrent updates returns an accurate result, but concurrent updates that occur
91 * while the sum is being calculated might not be incorporated.
92 *
93 * @return the sum
94 */
95 @Override
96 public long sum() {
97 long sum = base;
98 Cell[] as = cells;
99 if (as != null) {
100 int n = as.length;
101 for (int i = 0; i < n; ++i) {
102 Cell a = as[i];
103 if (a != null) sum += a.value;
104 }
105 }
106 return sum;
107 }
108
109 /**
110 * Resets variables maintaining the sum to zero. This method may be a useful alternative to
111 * creating a new adder, but is only effective if there are no concurrent updates. Because this
112 * method is intrinsically racy, it should only be used when it is known that no threads are
113 * concurrently updating.
114 */
115 public void reset() {
116 internalReset(0L);
117 }
118
119 /**
120 * Equivalent in effect to {@link #sum} followed by {@link #reset}. This method may apply for
121 * example during quiescent points between multithreaded computations. If there are updates
122 * concurrent with this method, the returned value is <em>not</em> guaranteed to be the final
123 * value occurring before the reset.
124 *
125 * @return the sum
126 */
127 public long sumThenReset() {
128 long sum = base;
129 Cell[] as = cells;
130 base = 0L;
131 if (as != null) {
132 int n = as.length;
133 for (int i = 0; i < n; ++i) {
134 Cell a = as[i];
135 if (a != null) {
136 sum += a.value;
137 a.value = 0L;
138 }
139 }
140 }
141 return sum;
142 }
143
144 /**
145 * Returns the String representation of the {@link #sum}.
146 *
147 * @return the String representation of the {@link #sum}
148 */
149 @Override
150 public String toString() {
151 return Long.toString(sum());
152 }
153
154 /**
155 * Equivalent to {@link #sum}.
156 *
157 * @return the sum
158 */
159 @Override
160 public long longValue() {
161 return sum();
162 }
163
164 /** Returns the {@link #sum} as an {@code int} after a narrowing primitive conversion. */
165 @Override
166 public int intValue() {
167 return (int) sum();
168 }
169
170 /** Returns the {@link #sum} as a {@code float} after a widening primitive conversion. */
171 @Override
172 public float floatValue() {
173 return (float) sum();
174 }
175
176 /** Returns the {@link #sum} as a {@code double} after a widening primitive conversion. */
177 @Override
178 public double doubleValue() {
179 return (double) sum();
180 }
181
182 private void writeObject(ObjectOutputStream s) throws IOException {
183 s.defaultWriteObject();
184 s.writeLong(sum());
185 }
186
187 private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
188 s.defaultReadObject();
189 busy = 0;
190 cells = null;
191 base = s.readLong();
192 }
193 }