Java多线程系列–“JUC原子类”02之 AtomicLong原子类
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概要
AtomicInteger, AtomicLong和AtomicBoolean这3个基本类型的原子类的原理和用法相似。本章以AtomicLong对基本类型的原子类进行介绍。内容包括：
AtomicLong介绍和函数列表
AtomicLong源码分析(基于JDK1.7.0_40)
AtomicLong示例
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AtomicLong介绍和函数列表
AtomicLong是作用是对长整形进行原子操作。
在32位操作系统中，64位的long 和 double 变量由于会被JVM当作两个分离的32位来进行操作，所以不具有原子性。而使用AtomicLong能让long的操作保持原子型。
AtomicLong函数列表
1 /*
2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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 * 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
36 package java.util.concurrent.atomic;
37 import sun.misc.Unsafe;
38
39 /**
40 * A {@code long} value that may be updated atomically. See the
41 * {@link java.util.concurrent.atomic} package specification for
42 * description of the properties of atomic variables. An
43 * {@code AtomicLong} is used in applications such as atomically
44 * incremented sequence numbers, and cannot be used as a replacement
45 * for a {@link java.lang.Long}. However, this class does extend
46 * {@code Number} to allow uniform access by tools and utilities that
47 * deal with numerically-based classes.
48 *
49 * @since 1.5
50 * @author Doug Lea
51 */
52 public class AtomicLong extends Number implements java.io.Serializable {
53 private static final long serialVersionUID = 1927816293512124184L;
54
55 // setup to use Unsafe.compareAndSwapLong for updates
56 private static final Unsafe unsafe = Unsafe.getUnsafe();
57 private static final long valueOffset;
58
59 /**
60 * Records whether the underlying JVM supports lockless
61 * compareAndSwap for longs. While the Unsafe.compareAndSwapLong
62 * method works in either case, some constructions should be
63 * handled at Java level to avoid locking user-visible locks.
64 */
65 static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();
66
67 /**
68 * Returns whether underlying JVM supports lockless CompareAndSet
69 * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
70 */
71 private static native boolean VMSupportsCS8();
72
73 static {
74 try {
75 valueOffset = unsafe.objectFieldOffset
76 (AtomicLong.class.getDeclaredField("value"));
77 } catch (Exception ex) { throw new Error(ex); }
78 }
79
80 private volatile long value;
81
82 /**
83 * Creates a new AtomicLong with the given initial value.
84 *
85 * @param initialValue the initial value
86 */
87 public AtomicLong(long initialValue) {
88 value = initialValue;
89 }
90
91 /**
92 * Creates a new AtomicLong with initial value {@code 0}.
93 */
94 public AtomicLong() {
95 }
96
97 /**
98 * Gets the current value.
99 *
100 * @return the current value
101 */
102 public final long get() {
103 return value;
104 }
105
106 /**
107 * Sets to the given value.
108 *
109 * @param newValue the new value
110 */
111 public final void set(long newValue) {
112 value = newValue;
113 }
114
115 /**
116 * Eventually sets to the given value.
117 *
118 * @param newValue the new value
119 * @since 1.6
120 */
121 public final void lazySet(long newValue) {
122 unsafe.putOrderedLong(this, valueOffset, newValue);
123 }
124
125 /**
126 * Atomically sets to the given value and returns the old value.
127 *
128 * @param newValue the new value
129 * @return the previous value
130 */
131 public final long getAndSet(long newValue) {
132 while (true) {
133 long current = get();
134 if (compareAndSet(current, newValue))
135 return current;
136 }
137 }
138
139 /**
140 * Atomically sets the value to the given updated value
141 * if the current value {@code ==} the expected value.
142 *
143 * @param expect the expected value
144 * @param update the new value
145 * @return true if successful. False return indicates that
146 * the actual value was not equal to the expected value.
147 */
148 public final boolean compareAndSet(long expect, long update) {
149 return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
150 }
151
152 /**
153 * Atomically sets the value to the given updated value
154 * if the current value {@code ==} the expected value.
155 *
156 * <p>May <a href="http://www.aichengxu.com/java/package-summary.html">fail spuriously</a>
157 * and does not provide ordering guarantees, so is only rarely an
158 * appropriate alternative to {@code compareAndSet}.
159 *
160 * @param expect the expected value
161 * @param update the new value
162 * @return true if successful.
163 */
164 public final boolean weakCompareAndSet(long expect, long update) {
165 return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
166 }
167
168 /**
169 * Atomically increments by one the current value.
170 *
171 * @return the previous value
172 */
173 public final long getAndIncrement() {
174 while (true) {
175 long current = get();
176 long next = current + 1;
177 if (compareAndSet(current, next))
178 return current;
179 }
180 }
181
182 /**
183 * Atomically decrements by one the current value.
184 *
185 * @return the previous value
186 */
187 public final long getAndDecrement() {
188 while (true) {
189 long current = get();
190 long next = current - 1;
191 if (compareAndSet(current, next))
192 return current;
193 }
194 }
195
196 /**
197 * Atomically adds the given value to the current value.
198 *
199 * @param delta the value to add
200 * @return the previous value
201 */
202 public final long getAndAdd(long delta) {
203 while (true) {
204 long current = get();
205 long next = current + delta;
206 if (compareAndSet(current, next))
207 return current;
208 }
209 }
210
211 /**
212 * Atomically increments by one the current value.
213 *
214 * @return the updated value
215 */
216 public final long incrementAndGet() {
217 for (;;) {
218 long current = get();
219 long next = current + 1;
220 if (compareAndSet(current, next))
221 return next;
222 }
223 }
224
225 /**
226 * Atomically decrements by one the current value.
227 *
228 * @return the updated value
229 */
230 public final long decrementAndGet() {
231 for (;;) {
232 long current = get();
233 long next = current - 1;
234 if (compareAndSet(current, next))
235 return next;
236 }
237 }
238
239 /**
240 * Atomically adds the given value to the current value.
241 *
242 * @param delta the value to add
243 * @return the updated value
244 */
245 public final long addAndGet(long delta) {
246 for (;;) {
247 long current = get();
248 long next = current + delta;
249 if (compareAndSet(current, next))
250 return next;
251 }
252 }
253
254 /**
255 * Returns the String representation of the current value.
256 * @return the String representation of the current value.
257 */
258 public String toString() {
259 return Long.toString(get());
260 }
261
262
263 public int intValue() {
264 return (int)get();
265 }
266
267 public long longValue() {
268 return get();
269 }
270
271 public float floatValue() {
272 return (float)get();
273 }
274
275 public double doubleValue() {
276 return (double)get();
277 }
278
279 }
AtomicLong的代码很简单，下面仅以incrementAndGet()为例，对AtomicLong的原理进行说明。
incrementAndGet()源码如下：
public final long incrementAndGet() {
for (;;) {
// 获取AtomicLong当前对应的long值
long current = get();
// 将current加1
long next = current + 1;
// 通过CAS函数，更新current的值
if (compareAndSet(current, next))
return next;
}
}
说明：
(01) incrementAndGet()首先会根据get()获取AtomicLong对应的long值。该值是volatile类型的变量，get()的源码如下：
// value是AtomicLong对应的long值
private volatile long value;
// 返回AtomicLong对应的long值
public final long get() {
return value;
}
(02) incrementAndGet()接着将current加1,然后通过CAS函数，将新的值赋值给value。
compareAndSet()的源码如下：
public final boolean compareAndSet(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}
compareAndSet()的作用是更新AtomicLong对应的long值。它会比较AtomicLong的原始值是否与expect相等，若相等的话，则设置AtomicLong的值为update。
AtomicLong示例
1 // LongTest.java的源码
2 import java.util.concurrent.atomic.AtomicLong;
3
4 public class LongTest {
5
6 public static void main(String[] args){
7
8 // 新建AtomicLong对象
9 AtomicLong mAtoLong = new AtomicLong();
10
11 mAtoLong.set(0x0123456789ABCDEFL);
12 System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
13 System.out.printf("%20s : 0x%016X\n", "intValue()", mAtoLong.intValue());
14 System.out.printf("%20s : 0x%016X\n", "longValue()", mAtoLong.longValue());
15 System.out.printf("%20s : %s\n", "doubleValue()", mAtoLong.doubleValue());
16 System.out.printf("%20s : %s\n", "floatValue()", mAtoLong.floatValue());
17
18 System.out.printf("%20s : 0x%016X\n", "getAndDecrement()", mAtoLong.getAndDecrement());
19 System.out.printf("%20s : 0x%016X\n", "decrementAndGet()", mAtoLong.decrementAndGet());
20 System.out.printf("%20s : 0x%016X\n", "getAndIncrement()", mAtoLong.getAndIncrement());
21 System.out.printf("%20s : 0x%016X\n", "incrementAndGet()", mAtoLong.incrementAndGet());
22
23 System.out.printf("%20s : 0x%016X\n", "addAndGet(0x10)", mAtoLong.addAndGet(0x10));
24 System.out.printf("%20s : 0x%016X\n", "getAndAdd(0x10)", mAtoLong.getAndAdd(0x10));
25
26 System.out.printf("\n%20s : 0x%016X\n", "get()", mAtoLong.get());
27
28 System.out.printf("%20s : %s\n", "compareAndSet()", mAtoLong.compareAndSet(0x12345679L, 0xFEDCBA9876543210L));
29 System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
30 }
31 }
运行结果：
get() : 0x0123456789ABCDEF
intValue() : 0x0000000089ABCDEF
longValue() : 0x0123456789ABCDEF
doubleValue() : 8.1985529216486896E16
floatValue() : 8.1985531E16
getAndDecrement() : 0x0123456789ABCDEF
decrementAndGet() : 0x0123456789ABCDED
getAndIncrement() : 0x0123456789ABCDED
incrementAndGet() : 0x0123456789ABCDEF
addAndGet(0x10) : 0x0123456789ABCDFF
getAndAdd(0x10) : 0x0123456789ABCDFF
get() : 0x0123456789ABCE0F
compareAndSet() : false
get() : 0x0123456789ABCE0F
因为水平有限，难免有疏忽或者不准确的地方，希望大家能够直接指出来，我会及时改正。一切为了知识的分享。
后续会有更多的精彩的内容分享给大家。
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