Java多线程系列--“JUC原子类”02之 AtomicLong原子类

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

转载请注明出处：/article/4708144.html

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="package-summary.html#Spurious">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