java多线程系类：JUC原子类：02之AtomicLog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：http://www.cnblogs.com/skywang12345/p/3514593.html

为什么要使用AtomicLog原子类：

现在我们从java角度分析一下：为什么要使用atomiclong 而不是 long？

首先你得了解synchronized（同步锁），atomiclong 可以理解是加了synchronized的long。

现在我们思考一下你这个java场景，必然是跟多线程有关，而且可能用于编写计数器。

了解JVM指令的，都知道如果赋值一个long变量，编译后，会产生多个JVM语言指令，如果处于多线程情况下对于这个变量进行加减操作会导致数据不一致。

为避免此问题， Java 引入了原子变量 AtomicLong 。当一个线程正在操作一个原子变量时，即使其他线程也想要操作这个变量，类的实现中含有一个检查那步骤操作是否完成的机制。 基本上，操作获取变量的值，改变本地变量值，然后尝试以新值代替旧值。如果旧值还是一样，那么就改变它。如果不一样，方法再次开始操作。这个操作称为  Compare and Set （校对注：简称 CAS ，比较并交换的意思）。

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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

// 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

/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/

/*
*
*
*
*
*
* 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/publicdomain/zero/1.0/
*/

package java.util.concurrent.atomic;
import sun.misc.Unsafe;

/**
* A {@code long} value that may be updated atomically. See the
* {@link java.util.concurrent.atomic} package specification for
* description of the properties of atomic variables. An
* {@code AtomicLong} is used in applications such as atomically
* incremented sequence numbers, and cannot be used as a replacement
* for a {@link java.lang.Long}. However, this class does extend
* {@code Number} to allow uniform access by tools and utilities that
* deal with numerically-based classes.
*
* @since 1.5
* @author Doug Lea
*/
public class AtomicLong extends Number implements java.io.Serializable {
private static final long serialVersionUID = 1927816293512124184L;

// setup to use Unsafe.compareAndSwapLong for updates
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long valueOffset;

/**
* Records whether the underlying JVM supports lockless
* compareAndSwap for longs. While the Unsafe.compareAndSwapLong
* method works in either case, some constructions should be
* handled at Java level to avoid locking user-visible locks.
*/
static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();

/**
* Returns whether underlying JVM supports lockless CompareAndSet
* for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
*/
private static native boolean VMSupportsCS8();

static {
try {
valueOffset = unsafe.objectFieldOffset
(AtomicLong.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}

private volatile long value;

/**
* Creates a new AtomicLong with the given initial value.
*
* @param initialValue the initial value
*/
public AtomicLong(long initialValue) {
value = initialValue;
}

/**
* Creates a new AtomicLong with initial value {@code 0}.
*/
public AtomicLong() {
}

/**
* Gets the current value.
*
* @return the current value
*/
public final long get() {
return value;
}

/**
* Sets to the given value.
*
* @param newValue the new value
*/
public final void set(long newValue) {
value = newValue;
}

/**
* Eventually sets to the given value.
*
* @param newValue the new value
* @since 1.6
*/
public final void lazySet(long newValue) {
unsafe.putOrderedLong(this, valueOffset, newValue);
}

/**
* Atomically sets to the given value and returns the old value.
*
* @param newValue the new value
* @return the previous value
*/
public final long getAndSet(long newValue) {
while (true) {
long current = get();
if (compareAndSet(current, newValue))
return current;
}
}

/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
*
* @param expect the expected value
* @param update the new value
* @return true if successful. False return indicates that
* the actual value was not equal to the expected value.
*/
public final boolean compareAndSet(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}

/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
*
* <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 expect the expected value
* @param update the new value
* @return true if successful.
*/
public final boolean weakCompareAndSet(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}

/**
* Atomically increments by one the current value.
*
* @return the previous value
*/
public final long getAndIncrement() {
while (true) {
long current = get();
long next = current + 1;
if (compareAndSet(current, next))
return current;
}
}

/**
* Atomically decrements by one the current value.
*
* @return the previous value
*/
public final long getAndDecrement() {
while (true) {
long current = get();
long next = current - 1;
if (compareAndSet(current, next))
return current;
}
}

/**
* Atomically adds the given value to the current value.
*
* @param delta the value to add
* @return the previous value
*/
public final long getAndAdd(long delta) {
while (true) {
long current = get();
long next = current + delta;
if (compareAndSet(current, next))
return current;
}
}

/**
* Atomically increments by one the current value.
*
* @return the updated value
*/
public final long incrementAndGet() {
for (;;) {
long current = get();
long next = current + 1;
if (compareAndSet(current, next))
return next;
}
}

/**
* Atomically decrements by one the current value.
*
* @return the updated value
*/
public final long decrementAndGet() {
for (;;) {
long current = get();
long next = current - 1;
if (compareAndSet(current, next))
return next;
}
}

/**
* Atomically adds the given value to the current value.
*
* @param delta the value to add
* @return the updated value
*/
public final long addAndGet(long delta) {
for (;;) {
long current = get();
long next = current + delta;
if (compareAndSet(current, next))
return next;
}
}

/**
* Returns the String representation of the current value.
* @return the String representation of the current value.
*/
public String toString() {
return Long.toString(get());
}

public int intValue() {
return (int)get();
}

public long longValue() {
return get();
}

public float floatValue() {
return (float)get();
}

public double doubleValue() {
return (double)get();
}

}

  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