AtomicInteger等对象出现的目的主要是为了解决在多线程环境下变量计数的问题，例如常用的i++,i--操作，它们不是线程安全的，AtomicInteger引入后，就不必在进行i++和i--操作时，进行加锁操作，在我们日常工作中，有很多业务场景需要在多线程环境下进行变量的计数：订单数统计、访问量统计、累计相应时长统计等。
demo 源码：https://github.com/mantuliu/javaAdvance
下面我们先分析一下AtomicInteger的源代码。通过源码分析我们知道，AtomicInteger的核心就是一个CAS算法(CompareAndSwap)，比较并交换算法，此算法是由unsafe的底层代码实现，它是一个原子的操作，原理就是：如果内存中的实际值与update值相同，则将实际值更新为expect值，反之则返回失败，由上层系统循环获取实际值后，再次调用此CAS算法：
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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*/ /*
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* 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; /**
* An {@code int} 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 AtomicInteger} is used in applications such as atomically
* incremented counters, and cannot be used as a replacement for an
* {@link java.lang.Integer}. 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 AtomicInteger extends Number implements java.io.Serializable {
private static final long serialVersionUID = 6214790243416807050L; // setup to use Unsafe.compareAndSwapInt for updates
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long valueOffset;//value值的偏移地址 static {
try {
valueOffset = unsafe.objectFieldOffset
(AtomicInteger.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
} private volatile int value;//实际的值 /**
* Creates a new AtomicInteger with the given initial value.
*
* @param initialValue the initial value
*/
public AtomicInteger(int initialValue) {
value = initialValue;//初始化
} /**
* Creates a new AtomicInteger with initial value {@code 0}.
*/
public AtomicInteger() {
} /**
* Gets the current value.
*
* @return the current value
*/
public final int get() {
return value;//返回value值
} /**
* Sets to the given value.
*
* @param newValue the new value
*/
public final void set(int newValue) {
value = newValue;//设置新值,因为没有判断oldvalue，所以此操作是非线程安全的
} /**
* Eventually sets to the given value.
*
* @param newValue the new value
* @since 1.6
*/
public final void lazySet(int newValue) {
unsafe.putOrderedInt(this, valueOffset, newValue);//与set操作效果一样，只是采用的是unsafe对象中通过偏移地址来设置值的方式
} /**
* Atomically sets to the given value and returns the old value.
*
* @param newValue the new value
* @return the previous value
*/
public final int getAndSet(int newValue) {//原子操作，设置新值，返回老值
for (;;) {
int current = get();
if (compareAndSet(current, newValue))//通过CAS算法，比较current的值和实际值是否一致，如果一致则设置为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(int expect, int update) {
return unsafe.compareAndSwapInt(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(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
} /**
* Atomically increments by one the current value.
*
* @return the previous value
*/
public final int getAndIncrement() {//i++操作
for (;;) {
int current = get();//获取当前值
int next = current + 1;//当前值+1
if (compareAndSet(current, next))//比较current值和实际的值是否一致，如不一致，则继续循环
return current;
}
} /**
* Atomically decrements by one the current value.
*
* @return the previous value
*/
public final int getAndDecrement() {
for (;;) {
int current = get();
int 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 int getAndAdd(int delta) {//例如：当我们统计接口的响应时间时，可以利用此方法
for (;;) {
int current = get();
int next = current + delta;
if (compareAndSet(current, next))
return current;
}
} /**
* Atomically increments by one the current value.
*
* @return the updated value
*/
public final int incrementAndGet() {
for (;;) {
int current = get();
int next = current + 1;
if (compareAndSet(current, next))
return next;
}
} /**
* Atomically decrements by one the current value.
*
* @return the updated value
*/
public final int decrementAndGet() {
for (;;) {
int current = get();
int 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 int addAndGet(int delta) {
for (;;) {
int current = get();
int 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 Integer.toString(get());
} public int intValue() {
return get();
} public long longValue() {
return (long)get();
} public float floatValue() {
return (float)get();
} public double doubleValue() {
return (double)get();
} }
下面，我们为四种情况(同步关键字、ReentrantLock公平锁和非公平锁、AtomicInteger)做一下性能对比分析，当我们看到上面的代码分析后，我们判断AtomicInteger应该比加锁的方式快，但是实验的结果表明，AtomicInteger只比ReentrantLock加公平锁的情况快几十倍，比其它两种方式略慢一些。
四个demo都用100个线程来循环模拟下单60秒钟：
demo Lesson8SyncIntPerform:在使用同步关键字加锁的情况下100个线程循环下单数为：677337556
demo Lesson8SyncIntPerform:在使用同步关键字加锁的情况下100个线程循环下单数为：755994691
demo Lesson8AtomicIntPerform:在使用AtomicInteger的情况下100个线程循环下单数为：562359607
demo Lesson8AtomicIntPerform:在使用AtomicInteger的情况下100个线程循环下单数为：575367967
demo Lesson8LockIntPerform:在使用ReentrantLock加非公平锁的情况下100个线程循环下单数为：857239882
demo Lesson8LockIntPerform:在使用ReentrantLock加非公平锁的情况下100个线程循环下单数为：860364303
demo Lesson8LockFairIntPerform:在使用ReentrantLock加公平锁的情况下100个线程循环下单数为：19153640
demo Lesson8LockFairIntPerform:在使用ReentrantLock加公平锁的情况下100个线程循环下单数为：19076567
上面的实验结果表明，在jdk1.6及后续的版本中(本实验的jdk版本是1.7，操作系统为windows操作系统)，已经对于synchronized关键字的性能优化了很多，已经和ReentrantLock的性能差不多，加锁的效果比不加锁时使用AtomicInteger性能效果还要略好一些，但是公平锁的性能明显降低，其它三种情况下的性能是公平锁性能的几十倍以上，这和公平锁每次试图占有锁时，都必须先要进等待队列，按照FIFO的顺序去获取锁，因此在我们的实验情景下，使用公平锁的线程进行了频繁切换，而频繁切换线程，性能必然会下降的厉害，这也告诫了我们在实际的开发过程中，在需要使用公平锁的情景下，务必要考虑线程的切换频率。