skywang12345

导航

统计
 

 

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:http://www.cnblogs.com/skywang12345/p/3514593.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

// 构造函数
AtomicLong()
// 创建值为initialValue的AtomicLong对象
AtomicLong(long initialValue)
// 以原子方式设置当前值为newValue。
final void set(long newValue) 
// 获取当前值
final long get() 
// 以原子方式将当前值减 1,并返回减1后的值。等价于“--num”
final long decrementAndGet() 
// 以原子方式将当前值减 1,并返回减1前的值。等价于“num--”
final long getAndDecrement() 
// 以原子方式将当前值加 1,并返回加1后的值。等价于“++num”
final long incrementAndGet() 
// 以原子方式将当前值加 1,并返回加1前的值。等价于“num++”
final long getAndIncrement()    
// 以原子方式将delta与当前值相加,并返回相加后的值。
final long addAndGet(long delta) 
// 以原子方式将delta添加到当前值,并返回相加前的值。
final long getAndAdd(long delta) 
// 如果当前值 == expect,则以原子方式将该值设置为update。成功返回true,否则返回false,并且不修改原值。
final boolean compareAndSet(long expect, long update)
// 以原子方式设置当前值为newValue,并返回旧值。
final long getAndSet(long newValue)
// 返回当前值对应的int值
int intValue() 
// 获取当前值对应的long值
long longValue()    
// 以 float 形式返回当前值
float floatValue()    
// 以 double 形式返回当前值
double doubleValue()    
// 最后设置为给定值。延时设置变量值,这个等价于set()方法,但是由于字段是volatile类型的,因此次字段的修改会比普通字段(非volatile字段)有稍微的性能延时(尽管可以忽略),所以如果不是想立即读取设置的新值,允许在“后台”修改值,那么此方法就很有用。如果还是难以理解,这里就类似于启动一个后台线程如执行修改新值的任务,原线程就不等待修改结果立即返回(这种解释其实是不正确的,但是可以这么理解)。
final void lazySet(long newValue)
// 如果当前值 == 预期值,则以原子方式将该设置为给定的更新值。JSR规范中说:以原子方式读取和有条件地写入变量但不 创建任何 happen-before 排序,因此不提供与除 weakCompareAndSet 目标外任何变量以前或后续读取或写入操作有关的任何保证。大意就是说调用weakCompareAndSet时并不能保证不存在happen-before的发生(也就是可能存在指令重排序导致此操作失败)。但是从Java源码来看,其实此方法并没有实现JSR规范的要求,最后效果和compareAndSet是等效的,都调用了unsafe.compareAndSwapInt()完成操作。
final boolean weakCompareAndSet(long expect, long update)

 

AtomicLong源码分析(基于JDK1.7.0_40)

AtomicLong的完整源码

  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 }
View Code

 

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

 


更多内容

1. Java多线程系列--“JUC原子类”01之 框架

2. Java多线程系列目录(共xx篇)

 

posted on 2014-01-17 23:18 如果天空不死 阅读(...) 评论(...) 编辑 收藏