java中Atomic类之AtomicLongArray

概要

AtomicIntegerArray, AtomicLongArray, AtomicReferenceArray这3个数组类型的原子类的原理和用法相似。本章以AtomicLongArray对数组类型的原子类进行介绍.

 

AtomicLongArray介绍和函数列表

在"Java多线程系列--“JUC原子类”02之 AtomicLong原子类"中介绍过，AtomicLong是作用是对长整形进行原子操作。而AtomicLongArray的作用则是对"长整形数组"进行原子操作。

 

AtomicLongArray函数列表

// 创建给定长度的新 AtomicLongArray。AtomicLongArray(int length)// 创建与给定数组具有相同长度的新 AtomicLongArray，并从给定数组复制其所有元素。AtomicLongArray(long[] array)// 以原子方式将给定值添加到索引 i 的元素。long addAndGet(int i, long delta)// 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。boolean compareAndSet(int i, long expect, long update)// 以原子方式将索引 i 的元素减1。long decrementAndGet(int i)// 获取位置 i 的当前值。long get(int i)// 以原子方式将给定值与索引 i 的元素相加。long getAndAdd(int i, long delta)// 以原子方式将索引 i 的元素减 1。long getAndDecrement(int i)// 以原子方式将索引 i 的元素加 1。long getAndIncrement(int i)// 以原子方式将位置 i 的元素设置为给定值，并返回旧值。long getAndSet(int i, long newValue)// 以原子方式将索引 i 的元素加1。long incrementAndGet(int i)// 最终将位置 i 的元素设置为给定值。void lazySet(int i, long newValue)// 返回该数组的长度。int length()// 将位置 i 的元素设置为给定值。void set(int i, long newValue)// 返回数组当前值的字符串表示形式。String toString()// 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。boolean    weakCompareAndSet(int i, long expect, long update)

 

AtomicLongArray源码分析(基于JDK1.7.0_40)

AtomicLongArray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 import java.util.*; 39  40 /** 41  * A {@code long} array in which elements may be updated atomically. 42  * See the {@link java.util.concurrent.atomic} package specification 43  * for description of the properties of atomic variables. 44  * @since 1.5 45  * @author Doug Lea 46  */ 47 public class AtomicLongArray implements java.io.Serializable { 48     private static final long serialVersionUID = -2308431214976778248L; 49  50     private static final Unsafe unsafe = Unsafe.getUnsafe(); 51     private static final int base = unsafe.arrayBaseOffset(long[].class); 52     private static final int shift; 53     private final long[] array; 54  55     static { 56         int scale = unsafe.arrayIndexScale(long[].class); 57         if ((scale & (scale - 1)) != 0) 58             throw new Error("data type scale not a power of two"); 59         shift = 31 - Integer.numberOfLeadingZeros(scale); 60     } 61  62     private long checkedByteOffset(int i) { 63         if (i < 0 || i >= array.length) 64             throw new IndexOutOfBoundsException("index " + i); 65  66         return byteOffset(i); 67     } 68  69     private static long byteOffset(int i) { 70         return ((long) i << shift) + base; 71     } 72  73     /** 74      * Creates a new AtomicLongArray of the given length, with all 75      * elements initially zero. 76      * 77      * @param length the length of the array 78      */ 79     public AtomicLongArray(int length) { 80         array = new long[length]; 81     } 82  83     /** 84      * Creates a new AtomicLongArray with the same length as, and 85      * all elements copied from, the given array. 86      * 87      * @param array the array to copy elements from 88      * @throws NullPointerException if array is null 89      */ 90     public AtomicLongArray(long[] array) { 91         // Visibility guaranteed by final field guarantees 92         this.array = array.clone(); 93     } 94  95     /** 96      * Returns the length of the array. 97      * 98      * @return the length of the array 99      */100     public final int length() {101         return array.length;102     }103 104     /**105      * Gets the current value at position {@code i}.106      *107      * @param i the index108      * @return the current value109      */110     public final long get(int i) {111         return getRaw(checkedByteOffset(i));112     }113 114     private long getRaw(long offset) {115         return unsafe.getLongVolatile(array, offset);116     }117 118     /**119      * Sets the element at position {@code i} to the given value.120      *121      * @param i the index122      * @param newValue the new value123      */124     public final void set(int i, long newValue) {125         unsafe.putLongVolatile(array, checkedByteOffset(i), newValue);126     }127 128     /**129      * Eventually sets the element at position {@code i} to the given value.130      *131      * @param i the index132      * @param newValue the new value133      * @since 1.6134      */135     public final void lazySet(int i, long newValue) {136         unsafe.putOrderedLong(array, checkedByteOffset(i), newValue);137     }138 139 140     /**141      * Atomically sets the element at position {@code i} to the given value142      * and returns the old value.143      *144      * @param i the index145      * @param newValue the new value146      * @return the previous value147      */148     public final long getAndSet(int i, long newValue) {149         long offset = checkedByteOffset(i);150         while (true) {151             long current = getRaw(offset);152             if (compareAndSetRaw(offset, current, newValue))153                 return current;154         }155     }156 157     /**158      * Atomically sets the element at position {@code i} to the given159      * updated value if the current value {@code ==} the expected value.160      *161      * @param i the index162      * @param expect the expected value163      * @param update the new value164      * @return true if successful. False return indicates that165      * the actual value was not equal to the expected value.166      */167     public final boolean compareAndSet(int i, long expect, long update) {168         return compareAndSetRaw(checkedByteOffset(i), expect, update);169     }170 171     private boolean compareAndSetRaw(long offset, long expect, long update) {172         return unsafe.compareAndSwapLong(array, offset, expect, update);173     }174 175     /**176      * Atomically sets the element at position {@code i} to the given177      * updated value if the current value {@code ==} the expected value.178      *179      * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>180      * and does not provide ordering guarantees, so is only rarely an181      * appropriate alternative to {@code compareAndSet}.182      *183      * @param i the index184      * @param expect the expected value185      * @param update the new value186      * @return true if successful.187      */188     public final boolean weakCompareAndSet(int i, long expect, long update) {189         return compareAndSet(i, expect, update);190     }191 192     /**193      * Atomically increments by one the element at index {@code i}.194      *195      * @param i the index196      * @return the previous value197      */198     public final long getAndIncrement(int i) {199         return getAndAdd(i, 1);200     }201 202     /**203      * Atomically decrements by one the element at index {@code i}.204      *205      * @param i the index206      * @return the previous value207      */208     public final long getAndDecrement(int i) {209         return getAndAdd(i, -1);210     }211 212     /**213      * Atomically adds the given value to the element at index {@code i}.214      *215      * @param i the index216      * @param delta the value to add217      * @return the previous value218      */219     public final long getAndAdd(int i, long delta) {220         long offset = checkedByteOffset(i);221         while (true) {222             long current = getRaw(offset);223             if (compareAndSetRaw(offset, current, current + delta))224                 return current;225         }226     }227 228     /**229      * Atomically increments by one the element at index {@code i}.230      *231      * @param i the index232      * @return the updated value233      */234     public final long incrementAndGet(int i) {235         return addAndGet(i, 1);236     }237 238     /**239      * Atomically decrements by one the element at index {@code i}.240      *241      * @param i the index242      * @return the updated value243      */244     public final long decrementAndGet(int i) {245         return addAndGet(i, -1);246     }247 248     /**249      * Atomically adds the given value to the element at index {@code i}.250      *251      * @param i the index252      * @param delta the value to add253      * @return the updated value254      */255     public long addAndGet(int i, long delta) {256         long offset = checkedByteOffset(i);257         while (true) {258             long current = getRaw(offset);259             long next = current + delta;260             if (compareAndSetRaw(offset, current, next))261                 return next;262         }263     }264 265     /**266      * Returns the String representation of the current values of array.267      * @return the String representation of the current values of array268      */269     public String toString() {270         int iMax = array.length - 1;271         if (iMax == -1)272             return "[]";273 274         StringBuilder b = new StringBuilder();275         b.append('[');276         for (int i = 0; ; i++) {277             b.append(getRaw(byteOffset(i)));278             if (i == iMax)279                 return b.append(']').toString();280             b.append(',').append(' ');281         }282     }283 284 }

 

AtomicLongArray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(int i) {    return addAndGet(i, 1);}

说明：incrementAndGet()的作用是以原子方式将long数组的索引 i 的元素加1，并返回加1之后的值。

 

addAndGet()源码如下：

public long addAndGet(int i, long delta) {    // 检查数组是否越界    long offset = checkedByteOffset(i);    while (true) {        // 获取long型数组的索引 offset 的原始值        long current = getRaw(offset);        // 修改long型值        long next = current + delta;        // 通过CAS更新long型数组的索引 offset的值。        if (compareAndSetRaw(offset, current, next))            return next;    }}

说明：addAndGet()首先检查数组是否越界。如果没有越界的话，则先获取数组索引i的值；然后通过CAS函数更新i的值。

 

getRaw()源码如下：

private long getRaw(long offset) {    return unsafe.getLongVolatile(array, offset);}

说明：unsafe是通过Unsafe.getUnsafe()返回的一个Unsafe对象。通过Unsafe的CAS函数对long型数组的元素进行原子操作。如compareAndSetRaw()就是调用Unsafe的CAS函数，它的源码如下：

private boolean compareAndSetRaw(long offset, long expect, long update) {    return unsafe.compareAndSwapLong(array, offset, expect, update);}

 

AtomicLongArray示例

 1 // LongArrayTest.java的源码 2 import java.util.concurrent.atomic.AtomicLongArray; 3  4 public class LongArrayTest { 5      6     public static void main(String[] args){ 7  8         // 新建AtomicLongArray对象 9         long[] arrLong = new long[] {10, 20, 30, 40, 50};10         AtomicLongArray ala = new AtomicLongArray(arrLong);11 12         ala.set(0, 100);13         for (int i=0, len=ala.length(); i<len; i++) 14             System.out.printf("get(%d) : %s\n", i, ala.get(i));15 16         System.out.printf("%20s : %s\n", "getAndDecrement(0)", ala.getAndDecrement(0));17         System.out.printf("%20s : %s\n", "decrementAndGet(1)", ala.decrementAndGet(1));18         System.out.printf("%20s : %s\n", "getAndIncrement(2)", ala.getAndIncrement(2));19         System.out.printf("%20s : %s\n", "incrementAndGet(3)", ala.incrementAndGet(3));20 21         System.out.printf("%20s : %s\n", "addAndGet(100)", ala.addAndGet(0, 100));22         System.out.printf("%20s : %s\n", "getAndAdd(100)", ala.getAndAdd(1, 100));23 24         System.out.printf("%20s : %s\n", "compareAndSet()", ala.compareAndSet(2, 31, 1000));25         System.out.printf("%20s : %s\n", "get(2)", ala.get(2));26     }27 }

运行结果：

get(0) : 100get(1) : 20get(2) : 30get(3) : 40get(4) : 50  getAndDecrement(0) : 100  decrementAndGet(1) : 19  getAndIncrement(2) : 30  incrementAndGet(3) : 41      addAndGet(100) : 199      getAndAdd(100) : 19     compareAndSet() : true              get(2) : 1000