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- * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/./ */ 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 . * @author Doug Lea */ public class AtomicLong extends Number implements java.io.Serializable { private static final long serialVersionUID = L; // 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 = VMSupportsCS(); /** * Returns whether underlying JVM supports lockless CompareAndSet * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS. */ private static native boolean VMSupportsCS(); 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 }. */ 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" rel="external nofollow" >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(); } }

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

// LongTest.java的源码 import java.util.concurrent.atomic.AtomicLong; public class LongTest { public static void main(String[] args){ // 新建AtomicLong对象 AtomicLong mAtoLong = new AtomicLong(); mAtoLong.set(0x0123456789ABCDEFL); System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get()); System.out.printf("%20s : 0x%016X\n", "intValue()", mAtoLong.intValue()); System.out.printf("%20s : 0x%016X\n", "longValue()", mAtoLong.longValue()); System.out.printf("%20s : %s\n", "doubleValue()", mAtoLong.doubleValue()); System.out.printf("%20s : %s\n", "floatValue()", mAtoLong.floatValue()); System.out.printf("%20s : 0x%016X\n", "getAndDecrement()", mAtoLong.getAndDecrement()); System.out.printf("%20s : 0x%016X\n", "decrementAndGet()", mAtoLong.decrementAndGet()); System.out.printf("%20s : 0x%016X\n", "getAndIncrement()", mAtoLong.getAndIncrement()); System.out.printf("%20s : 0x%016X\n", "incrementAndGet()", mAtoLong.incrementAndGet()); System.out.printf("%20s : 0x%016X\n", "addAndGet(0x10)", mAtoLong.addAndGet(0x10)); System.out.printf("%20s : 0x%016X\n", "getAndAdd(0x10)", mAtoLong.getAndAdd(0x10)); System.out.printf("\n%20s : 0x%016X\n", "get()", mAtoLong.get()); System.out.printf("%20s : %s\n", "compareAndSet()", mAtoLong.compareAndSet(0x12345679L, 0xFEDCBA9876543210L)); System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get()); } }

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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