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" 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。
