1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
|
/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package java.util.concurrent.atomic;
/**
* An {@code AtomicStampedReference} maintains an object reference
* along with an integer "stamp", that can be updated atomically.
*
* <p>Implementation note: This implementation maintains stamped
* references by creating internal objects representing "boxed"
* [reference, integer] pairs.
*
* @since 1.5
* @author Doug Lea
* @param <V> The type of object referred to by this reference
*/
public class AtomicStampedReference<V> {
private static class Pair<T> {
final T reference;
final int stamp;
private Pair(T reference, int stamp) {
this.reference = reference;
this.stamp = stamp;
}
static <T> Pair<T> of(T reference, int stamp) {
return new Pair<T>(reference, stamp);
}
}
private volatile Pair<V> pair;
/**
* Creates a new {@code AtomicStampedReference} with the given
* initial values.
*
* @param initialRef the initial reference
* @param initialStamp the initial stamp
*/
public AtomicStampedReference(V initialRef, int initialStamp) {
pair = Pair.of(initialRef, initialStamp);
}
/**
* Returns the current value of the reference.
*
* @return the current value of the reference
*/
public V getReference() {
return pair.reference;
}
/**
* Returns the current value of the stamp.
*
* @return the current value of the stamp
*/
public int getStamp() {
return pair.stamp;
}
/**
* Returns the current values of both the reference and the stamp.
* Typical usage is {@code int[1] holder; ref = v.get(holder); }.
*
* @param stampHolder an array of size of at least one. On return,
* {@code stampholder[0]} will hold the value of the stamp.
* @return the current value of the reference
*/
public V get(int[] stampHolder) {
Pair<V> pair = this.pair;
stampHolder[0] = pair.stamp;
return pair.reference;
}
/**
* Atomically sets the value of both the reference and stamp
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current stamp is equal to the expected stamp.
*
* <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
* and does not provide ordering guarantees, so is only rarely an
* appropriate alternative to {@code compareAndSet}.
*
* @param expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedStamp the expected value of the stamp
* @param newStamp the new value for the stamp
* @return true if successful
*/
public boolean weakCompareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
return compareAndSet(expectedReference, newReference,
expectedStamp, newStamp);
}
/**
* Atomically sets the value of both the reference and stamp
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current stamp is equal to the expected stamp.
*
* @param expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedStamp the expected value of the stamp
* @param newStamp the new value for the stamp
* @return true if successful
*/
public boolean compareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
Pair<V> current = pair;
return
expectedReference == current.reference &&
expectedStamp == current.stamp &&
((newReference == current.reference &&
newStamp == current.stamp) ||
casPair(current, Pair.of(newReference, newStamp)));
}
/**
* Unconditionally sets the value of both the reference and stamp.
*
* @param newReference the new value for the reference
* @param newStamp the new value for the stamp
*/
public void set(V newReference, int newStamp) {
Pair<V> current = pair;
if (newReference != current.reference || newStamp != current.stamp)
this.pair = Pair.of(newReference, newStamp);
}
/**
* Atomically sets the value of the stamp to the given update value
* if the current reference is {@code ==} to the expected
* reference. Any given invocation of this operation may fail
* (return {@code false}) spuriously, but repeated invocation
* when the current value holds the expected value and no other
* thread is also attempting to set the value will eventually
* succeed.
*
* @param expectedReference the expected value of the reference
* @param newStamp the new value for the stamp
* @return true if successful
*/
public boolean attemptStamp(V expectedReference, int newStamp) {
Pair<V> current = pair;
return
expectedReference == current.reference &&
(newStamp == current.stamp ||
casPair(current, Pair.of(expectedReference, newStamp)));
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = UnsafeAccess.THE_ONE; // android-changed
private static final long pairOffset =
objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.class);
private boolean casPair(Pair<V> cmp, Pair<V> val) {
return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
}
static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
String field, Class<?> klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
}
|
