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
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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
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
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