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. * * * * * * * * * * * * * * * * * * * * */ /* * * * * * * 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。