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本文是学习尚硅谷周阳老师《JUC并发编程》的总结（文末有链接）。

基本类型原子类

AtomicInteger
AtomicLong
AtomicBoolean

AtomicInteger 的方法 getAndIncrement 和 incrementAndGet 的区别：

两个方法都能实现对当前值加 1 ，但返回值不同，getAndIncrement 方法返回加 1 前的旧值，incrementAndGet 方法返回加 1 后的新值。

package juc.cas;import java.util.concurrent.atomic.AtomicInteger;public class ShareNumCas {private AtomicInteger num = new AtomicInteger(0);public int add() {return num.getAndIncrement();}public int add2() {return num.incrementAndGet();}public int getNum() {return num.get();}
}public class NumPlusDemo {public static void main(String[] args) {ShareNumCas share = new ShareNumCas();System.out.println("before add num is " + share.getNum());System.out.println("add return " + share.add());System.out.println("after add num is " + share.getNum());}
}public class NumPlusDemo2 {public static void main(String[] args) {ShareNumCas share = new ShareNumCas();System.out.println("before add num is " + share.getNum());System.out.println("add2 return " + share.add2());System.out.println("after add num is " + share.getNum());}
}

输出：

before add num is 0
add return 0
after add num is 1before add num is 0
add2 return 1
after add num is 1

数组类型原子类

AtomicIntegerArray
AtomicLongArray
AtomicReferenceArray

示例代码：

package juc.atomic;import java.util.concurrent.atomic.AtomicIntegerArray;public class AtomicIntegerArrayDemo {public static void main(String[] args) {AtomicIntegerArray array = new AtomicIntegerArray(5);// AtomicIntegerArray array = new AtomicIntegerArray(new int[]{1, 2, 3, 4, 5});for (int i = 0; i < array.length(); i++) {System.out.println(array.get(i));}System.out.println();int tmp = array.getAndSet(0, 100);System.out.println(tmp + " " + array.get(0));tmp = array.getAndIncrement(0);System.out.println(tmp + " " + array.get(0));}
}

输出：

引用类型原子类

AtomicReference
AtomicStampedReference
AtomicMarkableReference

AtomicReference 实现自旋锁

package juc.cas;import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;public class SpinLockDemo {AtomicReference<Thread> atomicReference = new AtomicReference<>();public void lock() {Thread thread = Thread.currentThread();System.out.println(thread.getName() + " come in");while (!atomicReference.compareAndSet(null, thread)) {}}public void unlock() {Thread thread = Thread.currentThread();atomicReference.compareAndSet(thread, null);System.out.println(thread.getName() + " task over, unlock");}public static void main(String[] args) {SpinLockDemo spinLockDemo = new SpinLockDemo();new Thread(() -> {spinLockDemo.lock();try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {throw new RuntimeException(e);}spinLockDemo.unlock();}, "A").start();try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {throw new RuntimeException(e);}new Thread(() -> {spinLockDemo.lock();spinLockDemo.unlock();}, "B").start();}
}

输出：

A come in
B come in
A task over, unlock
B task over, unlock

AtomicStampedReference VS AtomicMarkableReference

AtomicStampedReference 可以记录对象被修改过几次

AtomicMarkableReference 只能记录对象是否被修改过

AtomicMarkableReference 示例代码：

package juc.atomic;import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicMarkableReference;public class AtomicMarkableReferenceDemo {static AtomicMarkableReference<Integer> atomicMarkableReference = new AtomicMarkableReference<>(100, false);public static void main(String[] args) {new Thread(() -> {boolean marked = atomicMarkableReference.isMarked();System.out.println(Thread.currentThread().getName() + " init marked = " + marked);try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {throw new RuntimeException(e);}boolean result = atomicMarkableReference.compareAndSet(100, 101, marked, !marked);System.out.println(Thread.currentThread().getName() + " result = " + result +", marked = " + atomicMarkableReference.isMarked() + ", value = " + atomicMarkableReference.getReference());}, "t1").start();new Thread(() -> {boolean marked = atomicMarkableReference.isMarked();System.out.println(Thread.currentThread().getName() + " init marked = " + marked);try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {throw new RuntimeException(e);}boolean result = atomicMarkableReference.compareAndSet(100, 102, marked, !marked);System.out.println(Thread.currentThread().getName() + " result = " + result+ ", marked = " + atomicMarkableReference.isMarked() + ", value = " + atomicMarkableReference.getReference());}, "t2").start();}
}

输出：

t2 init marked = false
t1 init marked = false
t1 result = true, marked = true, value = 101
t2 result = false, marked = true, value = 101

对象的属性修改原子类

AtomicIntegerFieldUpdater
AtomicLongFieldUpdater
AtomicReferenceFieldUpdater<T,V>

下例中可以将 money 定义为 AtomicInteger，已经有了 AtomicInteger ，AtomicIntegerFieldUpdater 有什么使用场景？

AtomicIntegerFieldUpdater 示例代码:

package juc.atomic;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;class BankAccount {String bankName = "CCB";public volatile int money = 0;AtomicIntegerFieldUpdater<BankAccount> updater =AtomicIntegerFieldUpdater.newUpdater(BankAccount.class, "money");public void add(BankAccount bankAccount) {updater.getAndIncrement(bankAccount);}
}
public class AtomicIntegerFieldUpdaterDemo {public static void main(String[] args) throws InterruptedException {BankAccount bankAccount = new BankAccount();CountDownLatch countDownLatch = new CountDownLatch(10);for (int i = 0; i < 10; i++) {new Thread(() -> {for (int j = 0; j < 1000; j++) {bankAccount.add(bankAccount);}countDownLatch.countDown();}).start();}countDownLatch.await();System.out.println("after add money is " + bankAccount.money);}
}

输出：

after add money is 10000

AtomicReferenceFieldUpdater<T,V> 示例代码：

package juc.atomic;import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;class MyVar {public volatile Boolean isInit = Boolean.FALSE;AtomicReferenceFieldUpdater<MyVar, Boolean> updater =AtomicReferenceFieldUpdater.newUpdater(MyVar.class, Boolean.class, "isInit");public void init(MyVar myVar) {if(updater.compareAndSet(myVar, Boolean.FALSE, Boolean.TRUE)) {System.out.println(Thread.currentThread().getName() + " start init");try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.println(Thread.currentThread().getName() + " init success");} else {System.out.println(Thread.currentThread().getName() + " other thread is initing...");}}
}public class AtomicReferenceFieldUpdaterDemo {public static void main(String[] args) {MyVar myVar = new MyVar();for (int i = 0; i < 5; i++) {new Thread(() -> myVar.init(myVar), "t" + i).start();}}
}

输出：

t3 other thread is initing...
t1 other thread is initing...
t4 other thread is initing...
t2 other thread is initing...
t0 start init
t0 init success

原子操作增强类

LongAdder
LongAccumulator
DoubleAdder
DoubleAccumulator

LongAdder VS LongAccumulator

LongAdder 的初始值只能为 0
LongAdder 只能进行加减操作
LongAccumulator 没有上面两个限制，更灵活

LongAdder#sum() 不会加锁，不会影响对数的操作。

LongAdder 简单示例：

package juc.atomic;import java.util.concurrent.atomic.LongAdder;public class LongAdderDemo {public static void main(String[] args) {LongAdder longAdder = new LongAdder();longAdder.increment();longAdder.increment();longAdder.increment();longAdder.increment();longAdder.increment();System.out.println(longAdder.sum());}
}

输出：

LongAccumulator 简单示例：

package juc.atomic;import java.util.concurrent.atomic.LongAccumulator;public class LongAccumulatorDemo {public static void main(String[] args) {LongAccumulator longAccumulator = new LongAccumulator((x, y) -> x + y, 0);// 可以使用方法引用简写// LongAccumulator longAccumulator = new LongAccumulator(Long::sum, 0);longAccumulator.accumulate(1);System.out.println(longAccumulator.get());longAccumulator.accumulate(2);System.out.println(longAccumulator.get());}
}

输出：

1
3

各种计数性能比较

package juc.atomic;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.LongAccumulator;
import java.util.concurrent.atomic.LongAdder;class ClickNum {int number = 0;public synchronized void add() {number++;}AtomicLong atomicLong = new AtomicLong(0);public void addAtomic() {atomicLong.getAndIncrement();}LongAdder longAdder = new LongAdder();public void addLongAdder() {longAdder.increment();}LongAccumulator longAccumulator = new LongAccumulator((x, y) -> x + y, 0);public void addLongAccumulator() {longAccumulator.accumulate(1);}
}public class CountPerfTester {private static final int THREADS_COUNT = 50;private static final int TEN_THOUSANDS = 10000;public static void main(String[] args) throws InterruptedException {ClickNum clickNum = new ClickNum();long start = 0L;long end = 0L;CountDownLatch countDownLatch1 = new CountDownLatch(THREADS_COUNT);CountDownLatch countDownLatch2 = new CountDownLatch(THREADS_COUNT);CountDownLatch countDownLatch3 = new CountDownLatch(THREADS_COUNT);CountDownLatch countDownLatch4 = new CountDownLatch(THREADS_COUNT);start = System.currentTimeMillis();for (int i = 0; i < THREADS_COUNT; i++) {new Thread(() -> {try {for (int j = 1; j <= 100 * TEN_THOUSANDS; j++) {clickNum.add();}} finally {countDownLatch1.countDown();}}).start();}countDownLatch1.await();end = System.currentTimeMillis();System.out.println("synchronized add cost time : " + (end - start) + " ms, " + "result is " + clickNum.number);start = System.currentTimeMillis();for (int i = 0; i < THREADS_COUNT; i++) {new Thread(() -> {try {for (int j = 1; j <= 100 * TEN_THOUSANDS; j++) {clickNum.addAtomic();}} finally {countDownLatch2.countDown();}}).start();}countDownLatch2.await();end = System.currentTimeMillis();System.out.println("addAtomic cost time : " + (end - start) + " ms, " + "result is " + clickNum.atomicLong.get());start = System.currentTimeMillis();for (int i = 0; i < THREADS_COUNT; i++) {new Thread(() -> {try {for (int j = 1; j <= 100 * TEN_THOUSANDS; j++) {clickNum.addLongAdder();}} finally {countDownLatch3.countDown();}}).start();}countDownLatch3.await();end = System.currentTimeMillis();System.out.println("addLongAdder cost time : " + (end - start) + " ms, " + "result is " + clickNum.longAdder.sum());start = System.currentTimeMillis();for (int i = 0; i < THREADS_COUNT; i++) {new Thread(() -> {try {for (int j = 1; j <= 100 * TEN_THOUSANDS; j++) {clickNum.addLongAccumulator();}} finally {countDownLatch4.countDown();}}).start();}countDownLatch4.await();end = System.currentTimeMillis();System.out.println("addLongAccumulator cost time : " + (end - start) + " ms, " + "result is " + clickNum.longAccumulator.get());}
}

输出：

synchronized add cost time : 1352 ms, result is 50000000
addAtomic cost time : 844 ms, result is 50000000
addLongAdder cost time : 97 ms, result is 50000000
addLongAccumulator cost time : 74 ms, result is 50000000

可以看出：

LongAdder 和 LongAccumulator 的时间是 synchronized 和 AtomicLong 的十分之一左右。

LongAdder 高性能原理说明

LongAdder 的基本思路是分散热点，将 value 值分散到一个 Cell 数组中，不同的线程会命中到不同的数组元素中，各个线程只对自己数组元素的 value 进行 CAS 操作，冲突概率会小很多，sum() 方法会对 base 和所有 Cell 数组的值累加作为返回值。

某个线程分配到哪个 Cell 的方法是：对线程 id 进行 hash 求值，再根据 hash 值映射到 Cell 数组的某个下标。

Cell 数组元素是 2 的 n 次方，最大是 CPU 的核数。

LongAdder 源码分析

add(1L)

最初无竞争时只更新 base
如果更新 base 失败时，首次新增一个 Cell 数组
当多个线程对同一个 Cell 竞争激烈时，可能就会对 Cell 数组扩容

    public void add(long x) {Cell[] cs; long b, v; int m; Cell c;if ((cs = cells) != null || !casBase(b = base, b + x)) {int index = getProbe();boolean uncontended = true;if (cs == null || (m = cs.length - 1) < 0 ||(c = cs[index & m]) == null ||!(uncontended = c.cas(v = c.value, v + x)))longAccumulate(x, null, uncontended, index);}}

判断代码中有赋值，也有方法调用，简化了代码。

longAccumulator

分为 3 种情况：

Cell 数组已经初始化

 这种情况下主要进行对某个 Cell 元素的初始化赋值，在某个 Cell 元素上重试 CAS，对 Cell 数组扩容

Cell 数组未初始化

 这种情况下首次创建 Cell 数组，包含 2 个元素

Cell 数组正在初始化，则尝试在 base 上重试 CAS 操作

sum

    /*** Returns the current sum.  The returned value is <em>NOT</em> an* atomic snapshot; invocation in the absence of concurrent* updates returns an accurate result, but concurrent updates that* occur while the sum is being calculated might not be* incorporated.** @return the sum*/public long sum() {Cell[] cs = cells;long sum = base;if (cs != null) {for (Cell c : cs)if (c != null)sum += c.value;}return sum;}

sum 会将所有 Cell 数组中的 value 和 base 累加作为返回值，sum 执行时并没有限制对 base 和 Cells 的值更新，所以返回值可能会损失一些精度。LongAdder 不是强一致性的，是最终一致性的。

参考

尚硅谷JUC并发编程 (https://www.bilibili.com/video/BV1ar4y1x727?spm_id_from=333.788.videopod.episodes&vd_source=9266b9af652d5902d068c94b9d60116f&p=80)