Java多线程系列--“JUC原子类”03之 AtomicLongArray原子类
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。内容包括：
AtomicLongArray介绍和函数列表
AtomicLongArray源码分析(基于JDK1.7.0_40)
AtomicLongArray示例
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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 index 108 * @return the current value 109 */ 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 index 122 * @param newValue the new value 123 */ 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 index 132 * @param newValue the new value 133 * @since 1.6 134 */ 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 value 142 * and returns the old value. 143 * 144 * @param i the index 145 * @param newValue the new value 146 * @return the previous value 147 */ 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 given 159 * updated value if the current value {@code ==} the expected value. 160 * 161 * @param i the index 162 * @param expect the expected value 163 * @param update the new value 164 * @return true if successful. False return indicates that 165 * 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 given 177 * 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 an 181 * appropriate alternative to {@code compareAndSet}. 182 * 183 * @param i the index 184 * @param expect the expected value 185 * @param update the new value 186 * @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 index 196 * @return the previous value 197 */ 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 index 206 * @return the previous value 207 */ 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 index 216 * @param delta the value to add 217 * @return the previous value 218 */ 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 index 232 * @return the updated value 233 */ 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 index 242 * @return the updated value 243 */ 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 index 252 * @param delta the value to add 253 * @return the updated value 254 */ 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 array 268 */ 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) : 100 get(1) : 20 get(2) : 30 get(3) : 40 get(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
更多内容
2. Java多线程系列--“JUC原子类”02之 AtomicLong原子类