并发读源码——AtomicInteger/AtomicLong/AtomicStampedReference

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1. AtomicInteger描述

AtomicInteger从名字上看是操作Integer整数的，但Integer是线程不安全的，AtomicInteger是线程安全的。AtomicInteger的作用可以把两个Integer对象的加减乘除等操作变成一个原子操作，如果对Integer的操作不用AtomicInteger，也可以选择用synchronized锁住两个Integer对象的操作，synchronized是重量级锁，悲观锁，资源开销大，而AtomicInteger中的操作方法采用的轻量级锁，乐观锁，下面会进行分析。

2. 源码解析

AtomicInteger的原理就是运用了乐观锁
悲观锁与乐观锁区别：

• 悲观锁：在并发操作之前认为冲突概率比较大，读操作之前就上锁。比如synchronized关键字
• 乐观锁：在并发操作之前认为冲突的概率比较小，读操作不上锁，等到写操作时，再判断数据在此期间是否被修改了，如果数据被修改了，则就把数据重新读出来，重新执行该过程；如果原始数据没被修改，就直接把新值写回去。

AtomicInteger的乐观锁是通过Unsafe下的compareAndSwapInt方法执行的，AtomicInteger是通过乐观锁中的自旋锁方式获取锁的（自旋锁：不放弃CPU，空转，不停的重试，典型的就是通过for进行循环）。

package java.util.concurrent.atomic;
import java.util.function.IntUnaryOperator;
import java.util.function.IntBinaryOperator;
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 updatesprivate static final Unsafe unsafe = Unsafe.getUnsafe();/*valueOffset是下面定义的value在AtomicInteger类内存的偏移量，通过这个偏移量可以定位value的值，下面读写AtomicInteger 对象中的值都是valueOffset进行操作的*/private static final long valueOffset;/*因为value是private类型的，是不能直接获取的，因此通过反射形式获取value变量，然后通过java的native方法unsafe.objectFieldOffset获取value相对AtomicInteger类的内存偏移量*/static {try {valueOffset = unsafe.objectFieldOffset(AtomicInteger.class.getDeclaredField("value"));} catch (Exception ex) { throw new Error(ex); }}private volatile int value;/*初始化时指定value的值*/public AtomicInteger(int initialValue) {value = initialValue;}/*默认初始化方式，value值默认为0，因为value是int类型的*/public AtomicInteger() {}/*获取value值*/public final int get() {return value;}/*修改value值，因为value是volatile类型的，会立即把value中的值刷入主内存，其它线程可以看到修改*/public final void set(int newValue) {value = newValue;}/*lazySet方法现实中很少用，不会立即把value刷入主内存，但最终会刷值，保证最终一致性*/public final void lazySet(int newValue) {unsafe.putOrderedInt(this, valueOffset, newValue);}/*先获取value的值，然后修改value值为新值newValue，整个动作是原子性的，通过unsafe.getAndSetInt方式实现*/public final int getAndSet(int newValue) {return unsafe.getAndSetInt(this, valueOffset, newValue);}/*封装了Unsafe中compareAndSwapInt的native函数，compareAndSet有两个参数，expect指变量的旧值，是读出来的值，写回去的时候希望没有没其他线程修改，所以是expect；第二个参数是update，值变量的新值，修改过的，希望写入的value值。当expect等于变量当前值时，说明在修改期间没有其他线程对此变量进行修改，所以可以成功写入，变量更新为update，返回true，否则返回false*//*compareAndSwapInt有4个参数，第一个参数是AtomictInteger对象，第二个是成员变量value相对AtomictInteger类的内存偏移量，expect和update保持不变*//*** 如果内存中的value成员变量值等于expect的值，就把update值更新到value中，compareAndSwapInt方法是原子的，先比较，如果相等就直接更新（根据value在AtomicInteger中内存偏移量valueOffset来更新）*/public final boolean compareAndSet(int expect, int update) {return unsafe.compareAndSwapInt(this, valueOffset, expect, update);}/*** weakCompareAndSet方法与compareAndSet类似，但weakCompareAndSet不会插入内存屏障，不能保障volatile的原子性*/public final boolean weakCompareAndSet(int expect, int update) {return unsafe.compareAndSwapInt(this, valueOffset, expect, update);}/*** 先获取value成员变量的值，然后对value增加1*/public final int getAndIncrement() {return unsafe.getAndAddInt(this, valueOffset, 1);}/*** 先获取value成员变量的值，然后对value减1public final int getAndDecrement() {return unsafe.getAndAddInt(this, valueOffset, -1);}/*** 先获取value成员变量的值，然后把delta加到value上*/public final int getAndAdd(int delta) {return unsafe.getAndAddInt(this, valueOffset, delta);}/*** 先对成员变量value增加1，然后把修改后的值更新到value中*/public final int incrementAndGet() {return unsafe.getAndAddInt(this, valueOffset, 1) + 1;}/*** 先对成员变量value减1，然后把修改后的值更新到value中*/public final int decrementAndGet() {return unsafe.getAndAddInt(this, valueOffset, -1) - 1;}/*** 先对成员变量value增加delta，然后把修改后的值更新到value中*/public final int addAndGet(int delta) {return unsafe.getAndAddInt(this, valueOffset, delta) + delta;}/*** 先获取value的值赋值给prev，然后对prev执行功能函数updateFunction，把结果next更新到value中*//*该方法用到了自旋锁，当prev的值等于内存中的value值时，才会把next的值更新到value中，否则该方法会进行自旋，即一遍遍的获取锁重试，当prev的值等于内存中的value值时进行更新*/public final int getAndUpdate(IntUnaryOperator updateFunction) {int prev, next;do {prev = get();next = updateFunction.applyAsInt(prev);} while (!compareAndSet(prev, next));return prev;}/*** 该方法与上面方法原理相同，只是先更新后，再获取value值*/public final int updateAndGet(IntUnaryOperator updateFunction) {int prev, next;do {prev = get();next = updateFunction.applyAsInt(prev);} while (!compareAndSet(prev, next));return next;}/*** 该方法与上面方法原理相同，只是功能函数变为了IntBinaryOperator，执行两个整数返回一个整数；* 先获取value值，然后功能函数执行后的结果更新到value中*/public final int getAndAccumulate(int x,IntBinaryOperator accumulatorFunction) {int prev, next;do {prev = get();next = accumulatorFunction.applyAsInt(prev, x);} while (!compareAndSet(prev, next));return prev;}/*** 该方法与上面函数类似，只是先更新value值，然后获取返回更新后的value值*/public final int accumulateAndGet(int x,IntBinaryOperator accumulatorFunction) {int prev, next;do {prev = get();next = accumulatorFunction.applyAsInt(prev, x);} while (!compareAndSet(prev, next));return next;}/*** 把value值转化为String 类型*/public String toString() {return Integer.toString(get());}/***获取value成员变量值*/public int intValue() {return get();}/*** 把value值转化为long类型*/public long longValue() {return (long)get();}/*** 把value值转化为float类型*/public float floatValue() {return (float)get();}/*** 把value值转化为double类型*/public double doubleValue() {return (double)get();}}

3. AtomicInteger演示示例

package com.lzj.atomic;import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.IntBinaryOperator;
import java.util.function.IntUnaryOperator;public class AtomicIntegerDemo2 {public static void main(String[] args) {get();set();getAndSet();getAndIncrement();getAndDecrement();getAndAdd();incrementAndGet();decrementAndGet();addAndGet();getAndUpdate();updateAndGet();getAndAccumulate();accumulateAndGet();}/*测试AtomicInterger.get方法*/public static void get() {AtomicInteger at = new AtomicInteger(10);int value = at.get();System.out.println("get() : " + value);}/*测试AtomicInteger.get方法*/public static void set() {AtomicInteger at = new AtomicInteger(10);at.set(20);System.out.println("set()" + at.get());}/*测试AtomicInteger.getAndSet*/public static void getAndSet() {AtomicInteger at = new AtomicInteger(10);int oldValue = at.getAndSet(20);System.out.println("getAndSet : " + at.get());System.out.println("getAndSet : " + oldValue);}/*测试AtomicInteger.getAndIncrement*/public static void getAndIncrement() {AtomicInteger at = new AtomicInteger(10);at.getAndIncrement();System.out.println("getAndIncrement : " + at.get());}public static void getAndDecrement() {AtomicInteger at = new AtomicInteger(10);at.getAndDecrement();System.out.println("getAndDecrement : " + at.get());}/*测试AtomicInteger.getAndAdd*/public static void getAndAdd() {AtomicInteger at = new AtomicInteger(10);at.getAndAdd(5);System.out.println("getAndAdd : " + at.get());}/*测试AtomicInteger.incrementAndGet*/public static void incrementAndGet() {AtomicInteger at = new AtomicInteger(10);int current = at.incrementAndGet();System.out.println("incrementAndGet() : " + at.get());System.out.println("incrementAndGet() : " + current);}/*AtomicInteger.decrementAndGet*/public static void decrementAndGet() {AtomicInteger at = new AtomicInteger(10);int current = at.decrementAndGet();System.out.println("decrementAndGet() : " + at.get());System.out.println("decrementAndGet() : " + current);}/*测试AtomicInteger.addAndGet*/public static void addAndGet() {AtomicInteger at = new AtomicInteger(10);int current = at.addAndGet(10);System.out.println("addAndGet() : " + at.get());System.out.println("current : " + current);}/*测试AtomicInteger.getAndUpdate*/public static void getAndUpdate() {AtomicInteger at = new AtomicInteger(11);IntUnaryOperator updateFunction = x -> x/2;int old = at.getAndUpdate(updateFunction);System.out.println("getAndUpdate() : " + at.get());System.out.println("getAndUpdate() : " + old);}/*测试AtomicInteger.updateAndGet*/public static void updateAndGet() {AtomicInteger at = new AtomicInteger(11);IntUnaryOperator updateFunction = x -> x/2;int current = at.updateAndGet(updateFunction);System.out.println("updateAndGet() : " + at.get());System.out.println("updateAndGet() : " + current);}/*测试AtomicInteger。getAndAccumulate*/public static void getAndAccumulate() {AtomicInteger at = new AtomicInteger(11);IntBinaryOperator accumulatorFunction = (x,y) -> x / y;int old = at.getAndAccumulate(2, accumulatorFunction);System.out.println("getAndAccumulate() : " + at.get());System.out.println("getAndAccumulate() : " + old);}/*测试AtomicInteger.accumulateAndGet*/public static void accumulateAndGet() {AtomicInteger at = new AtomicInteger(11);IntBinaryOperator accumulatorFunction = (x,y) -> x / y;int current = at.accumulateAndGet(2, accumulatorFunction);System.out.println("accumulateAndGet() : " + at.get());System.out.println("accumulateAndGet() : " + current);}
}

执行结果如下

get() : 10
set()20
getAndSet : 20
getAndSet : 10
getAndIncrement : 11
getAndDecrement : 9
getAndAdd : 15
incrementAndGet() : 11
incrementAndGet() : 11
decrementAndGet() : 9
decrementAndGet() : 9
addAndGet() : 20
current : 20
getAndUpdate() : 5
getAndUpdate() : 11
updateAndGet() : 5
updateAndGet() : 5
getAndAccumulate() : 5
getAndAccumulate() : 11
accumulateAndGet() : 5
accumulateAndGet() : 5

4. 多线程用法

package com.lzj.atomic;import java.util.concurrent.atomic.AtomicInteger;public class AtomicDemo1 {public static AtomicInteger count = new AtomicInteger(10);public void add() {count.getAndIncrement();System.out.println("thread1 : " + count);}public void dec() {System.out.println("thread2 : " + count);count.getAndDecrement();}public static void main(String[] args) {AtomicDemo1  atomicDemo1 = new AtomicDemo1();Runnable run1 = new Runnable() {public void run() {atomicDemo1.add();System.out.println("thread1 : " + atomicDemo1.count);}};Runnable run2 = new Runnable() {public void run() {atomicDemo1.dec();System.out.println("thread2 : " + atomicDemo1.count);}};Thread thread1 = new Thread(run1);Thread thread2 = new Thread(run2);thread1.start();thread2.start();}
}

另外，AtomicLong与AtomicInteger除了value的类型不一样，其它几乎都类似。
AtomicStampedReference与AtomicInteger源码基本类似，只是AtomicInteger用来处理整数类型，通过自旋锁的方式利用compareAndSwapInt来解决CAS问题；而AtomicStampedReference用来处理对象，通过自旋锁的方式利用compareAndSwapObject来解决CAS问题，关于AtomicStampedReference的详解参考AtomicStampedReference

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