1. AtomicLong是基本原子类中的一种
AtomicLong是对长整形进行原子操作。
1.1 AtomicLong类的函数列表
// 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)
1.2 AtomicLong类的完整源码(基于JDK1.7.0_40)
/* * 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(); } }
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()首先会调用get()方法读取AtomicLong中value的值，该变量是volatile类型的变量，get()的源码如下：
// 返回AtomicLong中敏感变量value的值
public final long get() {
return value;
}
(02) 然后通过unsafe对象的CAS函数，对变量value进行更新
compareAndSet()的源码如下：
public final boolean compareAndSet(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}
compareAndSet()方法中，它会调用unsafe对象的CAS函数（compareAndSwapLong）。比较AtomicLong的当前value值与expect是否相等，若相等则直接更新；若不相等则循环执行步骤(01)和(02)，直到更新成功并返回新值为止。