Java中有那么一些类,是以Atomic开头的。这一系列的类我们称之为原子操作类。以最简单的类AtomicInteger为例。它相当于一个int变量,我们执行Int的 i++ 的时候并不是一个原子操作。而使用AtomicInteger的incrementAndGet却能保证原子操作。 更新变量这种场景下效果和 synchronized 相同,却要简单高效的多。
AtomicBoolean
AtomicInteger
AtomicIntegerArray
AtomicIntegerFieldUpdater
AtomicLong
AtomicLongArray
AtomicLongFieldUpdater
AtomicMarkableReference
AtomicReference
AtomicReferenceArray
AtomicReferenceFieldUpdater
AtomicStampedReference
DoubleAccumulator
DoubleAdder
LongAccumulator
LongAdder
public final int get() //获取当前的值
public final int getAndSet(int newValue)//获取当前的值,并设置新的值
public final int getAndIncrement()//获取当前的值,并自增
public final int getAndDecrement() //获取当前的值,并自减
public final int getAndAdd(int delta) //获取当前的值,并加上预期的值
boolean compareAndSet(int expect, int update) //如果输入的数值等于预期值,则以原子方式将该值设置为输入值(update)
tosleep→countDownLatch
主线程需要等待多个组件加载完毕,之后再继续执行。因为无法法准确判断,使用toSleep无法准确设置线程睡眠时间导致很大问题。countDownLatch则可以使用此问题
class MyNumber
{
@Getter
private AtomicInteger atomicInteger = new AtomicInteger();
public void addPlusPlus()
{
atomicInteger.incrementAndGet();
}
}
/**
* @auther zzyy
* @create 2020-07-03 17:16
*/
public class AtomicIntegerDemo
{
public static void main(String[] args) throws InterruptedException
{
MyNumber myNumber = new MyNumber();
CountDownLatch countDownLatch = new CountDownLatch(100);
for (int i = 1; i <=100; i++) {
new Thread(() -> {
try
{
for (int j = 1; j <=5000; j++)
{
myNumber.addPlusPlus();
}
}finally {
countDownLatch.countDown();
}
},String.valueOf(i)).start();
}
countDownLatch.await();
System.out.println(myNumber.getAtomicInteger().get());
}
}
CountDownLatch概念
CountDownLatch是一个同步工具类,用来协调多个线程之间的同步,或者说起到线程之间的通信(而不是用作互斥的作用)。
CountDownLatch能够使一个线程在等待另外一些线程完成各自工作之后,再继续执行。使用一个计数器进行实现。计数器初始值为线程的数量。当每一个线程完成自己任务后,计数器的值就会减一。当计数器的值为0时,表示所有的线程都已经完成一些任务,然后在CountDownLatch上等待的线程就可以恢复执行接下来的任务。
CountDownLatch的用法
1、某一线程在开始运行前等待n个线程执行完毕。将CountDownLatch的计数器初始化为new CountDownLatch(n),每当一个任务线程执行完毕,就将计数器减1 countdownLatch.countDown(),当计数器的值变为0时,在CountDownLatch上await()的线程就会被唤醒。一个典型应用场景就是启动一个服务时,主线程需要等待多个组件加载完毕,之后再继续执行。
2、实现多个线程开始执行任务的最大并行性。注意是并行性,不是并发,强调的是多个线程在某一时刻同时开始执行。类似于赛跑,将多个线程放到起点,等待发令枪响,然后同时开跑。做法是初始化一个共享的CountDownLatch(1),将其计算器初始化为1,多个线程在开始执行任务前首先countdownlatch.await(),当主线程调用countDown()时,计数器变为0,多个线程同时被唤醒。
public class AtomicIntegerArrayDemo
{
public static void main(String[] args)
{
AtomicIntegerArray atomicIntegerArray = new AtomicIntegerArray(new int[5]);
//AtomicIntegerArray atomicIntegerArray = new AtomicIntegerArray(5);
//AtomicIntegerArray atomicIntegerArray = new AtomicIntegerArray(new int[]{1,2,3,4,5});
for (int i = 0; i <atomicIntegerArray.length(); i++) {
System.out.println(atomicIntegerArray.get(i));
}
System.out.println();
System.out.println();
System.out.println();
int tmpInt = 0;
tmpInt = atomicIntegerArray.getAndSet(0,1122);
System.out.println(tmpInt+"\t"+atomicIntegerArray.get(0));
atomicIntegerArray.getAndIncrement(1);
atomicIntegerArray.getAndIncrement(1);
tmpInt = atomicIntegerArray.getAndIncrement(1);
System.out.println(tmpInt+"\t"+atomicIntegerArray.get(1));
}
}
AtomicReference
@Getter
@ToString
@AllArgsConstructor
class User
{
String userName;
int age;
}
public class AtomicReferenceDemo
{
public static void main(String[] args)
{
User z3 = new User("z3",24);
User li4 = new User("li4",26);
AtomicReference<User> atomicReferenceUser = new AtomicReference<>();
atomicReferenceUser.set(z3);
System.out.println(atomicReferenceUser.compareAndSet(z3,li4)+"\t"+atomicReferenceUser.get().toString());
System.out.println(atomicReferenceUser.compareAndSet(z3,li4)+"\t"+atomicReferenceUser.get().toString());
}
}
自旋锁SpinLockDemo
/**
* 题目:实现一个自旋锁
* 自旋锁好处:循环比较获取没有类似wait的阻塞。
*
* 通过CAS操作完成自旋锁,A线程先进来调用myLock方法自己持有锁5秒钟,B随后进来后发现
* 当前有线程持有锁,不是null,所以只能通过自旋等待,直到A释放锁后B随后抢到。
*/
public class SpinLockDemo
{
AtomicReference<Thread> atomicReference = new AtomicReference<>();
public void myLock()
{
Thread thread = Thread.currentThread();
System.out.println(Thread.currentThread().getName()+"\t come in");
while(!atomicReference.compareAndSet(null,thread))
{
}
}
public void myUnLock()
{
Thread thread = Thread.currentThread();
atomicReference.compareAndSet(thread,null);
System.out.println(Thread.currentThread().getName()+"\t myUnLock over");
}
public static void main(String[] args)
{
SpinLockDemo spinLockDemo = new SpinLockDemo();
new Thread(() -> {
spinLockDemo.myLock();
//暂停一会儿线程
try { TimeUnit.SECONDS.sleep( 5 ); } catch (InterruptedException e) { e.printStackTrace(); }
spinLockDemo.myUnLock();
},"A").start();
//暂停一会儿线程,保证A线程先于B线程启动并完成
try { TimeUnit.SECONDS.sleep( 1 ); } catch (InterruptedException e) { e.printStackTrace(); }
new Thread(() -> {
spinLockDemo.myLock();
spinLockDemo.myUnLock();
},"B").start();
}
}
AtomicStampedReference
public class ABADemo
{
static AtomicInteger atomicInteger = new AtomicInteger(100);
static AtomicStampedReference atomicStampedReference = new AtomicStampedReference(100,1);
public static void main(String[] args)
{
abaProblem();
abaResolve();
}
public static void abaResolve()
{
new Thread(() -> {
int stamp = atomicStampedReference.getStamp();
System.out.println("t3 ----第1次stamp "+stamp);
try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }
atomicStampedReference.compareAndSet(100,101,stamp,stamp+1);
System.out.println("t3 ----第2次stamp "+atomicStampedReference.getStamp());
atomicStampedReference.compareAndSet(101,100,atomicStampedReference.getStamp(),atomicStampedReference.getStamp()+1);
System.out.println("t3 ----第3次stamp "+atomicStampedReference.getStamp());
},"t3").start();
new Thread(() -> {
int stamp = atomicStampedReference.getStamp();
System.out.println("t4 ----第1次stamp "+stamp);
//暂停几秒钟线程
try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
boolean result = atomicStampedReference.compareAndSet(100, 20210308, stamp, stamp + 1);
System.out.println(Thread.currentThread().getName()+"\t"+result+"\t"+atomicStampedReference.getReference());
},"t4").start();
}
public static void abaProblem()
{
new Thread(() -> {
atomicInteger.compareAndSet(100,101);
atomicInteger.compareAndSet(101,100);
},"t1").start();
try { TimeUnit.MILLISECONDS.sleep(200); } catch (InterruptedException e) { e.printStackTrace(); }
new Thread(() -> {
atomicInteger.compareAndSet(100,20210308);
System.out.println(atomicInteger.get());
},"t2").start();
}
}
AtomicMarkableReference
public class ABADemo
{
static AtomicInteger atomicInteger = new AtomicInteger(100);
static AtomicStampedReference<Integer> stampedReference = new AtomicStampedReference<>(100,1);
static AtomicMarkableReference<Integer> markableReference = new AtomicMarkableReference<>(100,false);
public static void main(String[] args)
{
new Thread(() -> {
atomicInteger.compareAndSet(100,101);
atomicInteger.compareAndSet(101,100);
System.out.println(Thread.currentThread().getName()+"\t"+"update ok");
},"t1").start();
new Thread(() -> {
//暂停几秒钟线程
try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }
atomicInteger.compareAndSet(100,2020);
},"t2").start();
//暂停几秒钟线程
try { TimeUnit.SECONDS.sleep(2); } catch (InterruptedException e) { e.printStackTrace(); }
System.out.println(atomicInteger.get());
System.out.println();
System.out.println();
System.out.println();
System.out.println("============以下是ABA问题的解决,让我们知道引用变量中途被更改了几次=========================");
new Thread(() -> {
System.out.println(Thread.currentThread().getName()+"\t 1次版本号"+stampedReference.getStamp());
//故意暂停200毫秒,让后面的t4线程拿到和t3一样的版本号
try { TimeUnit.MILLISECONDS.sleep(200); } catch (InterruptedException e) { e.printStackTrace(); }
stampedReference.compareAndSet(100,101,stampedReference.getStamp(),stampedReference.getStamp()+1);
System.out.println(Thread.currentThread().getName()+"\t 2次版本号"+stampedReference.getStamp());
stampedReference.compareAndSet(101,100,stampedReference.getStamp(),stampedReference.getStamp()+1);
System.out.println(Thread.currentThread().getName()+"\t 3次版本号"+stampedReference.getStamp());
},"t3").start();
new Thread(() -> {
int stamp = stampedReference.getStamp();
System.out.println(Thread.currentThread().getName()+"\t =======1次版本号"+stamp);
//暂停2秒钟,让t3先完成ABA操作了,看看自己还能否修改
try { TimeUnit.SECONDS.sleep(2); } catch (InterruptedException e) { e.printStackTrace(); }
boolean b = stampedReference.compareAndSet(100, 2020, stamp, stamp + 1);
System.out.println(Thread.currentThread().getName()+"\t=======2次版本号"+stampedReference.getStamp()+"\t"+stampedReference.getReference());
},"t4").start();
System.out.println();
System.out.println();
System.out.println();
System.out.println("============AtomicMarkableReference不关心引用变量更改过几次,只关心是否更改过======================");
new Thread(() -> {
boolean marked = markableReference.isMarked();
System.out.println(Thread.currentThread().getName()+"\t 1次版本号"+marked);
try { TimeUnit.MILLISECONDS.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); }
markableReference.compareAndSet(100,101,marked,!marked);
System.out.println(Thread.currentThread().getName()+"\t 2次版本号"+markableReference.isMarked());
markableReference.compareAndSet(101,100,markableReference.isMarked(),!markableReference.isMarked());
System.out.println(Thread.currentThread().getName()+"\t 3次版本号"+markableReference.isMarked());
},"t5").start();
new Thread(() -> {
boolean marked = markableReference.isMarked();
System.out.println(Thread.currentThread().getName()+"\t 1次版本号"+marked);
//暂停几秒钟线程
try { TimeUnit.MILLISECONDS.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); }
markableReference.compareAndSet(100,2020,marked,!marked);
System.out.println(Thread.currentThread().getName()+"\t"+markableReference.getReference()+"\t"+markableReference.isMarked());
},"t6").start();
}
}
以一种线程安全的方式操作非线程安全对象内的某些字段
更新的对象属性必须使用 public volatile 修饰符。
因为对象的属性修改类型原子类都是抽象类,所以每次使用都必须使用静态方法newUpdater()创建一个更新器,并且需要设置想要更新的类和属性。
class BankAccount
{
private String bankName = "CCB";//银行
public volatile int money = 0;//钱数
AtomicIntegerFieldUpdater<BankAccount> accountAtomicIntegerFieldUpdater = AtomicIntegerFieldUpdater.newUpdater(BankAccount.class,"money");
//不加锁+性能高,局部微创
public void transferMoney(BankAccount bankAccount)
{
accountAtomicIntegerFieldUpdater.incrementAndGet(bankAccount);
}
}
/**
* @auther zzyy
* @create 2020-07-14 18:06
* 以一种线程安全的方式操作非线程安全对象的某些字段。
* 需求:
* 1000个人同时向一个账号转账一元钱,那么累计应该增加1000元,
* 除了synchronized和CAS,还可以使用AtomicIntegerFieldUpdater来实现。
*/
public class AtomicIntegerFieldUpdaterDemo
{
public static void main(String[] args)
{
BankAccount bankAccount = new BankAccount();
for (int i = 1; i <=1000; i++) {
int finalI = i;
new Thread(() -> {
bankAccount.transferMoney(bankAccount);
},String.valueOf(i)).start();
}
//暂停毫秒
try { TimeUnit.MILLISECONDS.sleep(500); } catch (InterruptedException e) { e.printStackTrace(); }
System.out.println(bankAccount.money);
}
}
class MyVar
{
public volatile Boolean isInit = Boolean.FALSE;
AtomicReferenceFieldUpdater<MyVar,Boolean> atomicReferenceFieldUpdater = AtomicReferenceFieldUpdater.newUpdater(MyVar.class,Boolean.class,"isInit");
public void init(MyVar myVar)
{
if(atomicReferenceFieldUpdater.compareAndSet(myVar,Boolean.FALSE,Boolean.TRUE))
{
System.out.println(Thread.currentThread().getName()+"\t"+"---init.....");
//暂停几秒钟线程
try { TimeUnit.SECONDS.sleep(2); } catch (InterruptedException e) { e.printStackTrace(); }
System.out.println(Thread.currentThread().getName()+"\t"+"---init.....over");
}else{
System.out.println(Thread.currentThread().getName()+"\t"+"------其它线程正在初始化");
}
}
}
/**
* 多线程并发调用一个类的初始化方法,如果未被初始化过,将执行初始化工作,要求只能初始化一次
*/
public class AtomicIntegerFieldUpdaterDemo
{
public static void main(String[] args) throws InterruptedException
{
MyVar myVar = new MyVar();
for (int i = 1; i <=5; i++) {
new Thread(() -> {
myVar.init(myVar);
},String.valueOf(i)).start();
}
}
}
【参考】volatile解决多线程内存不可见问题。对于一写多读,是可以解决变量同步问题,但是如果多写,同样无法解决线程安全问题。说明:如果是count-++操作,使用如下类实现:AtomicInteger count=new AtomicInteger(;count.addAndGet(1);如果是JDK8,推荐使用LongAdder对像,比AtomicLong性能更好(减少乐观锁的重试次数)。
方法名 | 说明 |
---|---|
void add(long x) | 将当前的value加x。 |
void increment() | 将当前的value加1。 |
void decremen() | 将当前的value减1 |
long sun() | 返回当前值。特别注意,在没有并发更新vlue的情况下,sum会返回一个精确值,在存在并long sumo发的情况下,sum不保证返回精确值。 |
void reset() | 将value重置为O,可用于替代重新new一个LongAdder,.但此方法只可以在没有并发更新的void reseto情况下使用。 |
long sumThenReset() | 获取当前value,并将value重置为0。 |
LongAdder只能用来计算加法,且从零开始计算
LongAccumulator提供了自定义的函数操作
//long类型的聚合器,需要传入一个long类型的二元操作,可以用来计算各种聚合操作,包括加乘等
import java.util.concurrent.atomic.LongAccumulator;
import java.util.concurrent.atomic.LongAdder;
import java.util.function.LongBinaryOperator;
public class LongAccumulatorDemo
{
LongAdder longAdder = new LongAdder();
public void add_LongAdder()
{
longAdder.increment();
}
//LongAccumulator longAccumulator = new LongAccumulator((x, y) -> x + y,0);
LongAccumulator longAccumulator = new LongAccumulator(new LongBinaryOperator()
{
@Override
public long applyAsLong(long left, long right)
{
return left - right;
}
},777);
public void add_LongAccumulator()
{
longAccumulator.accumulate(1);
}
public static void main(String[] args)
{
LongAccumulatorDemo demo = new LongAccumulatorDemo();
demo.add_LongAccumulator();
demo.add_LongAccumulator();
System.out.println(demo.longAccumulator.longValue());
}
}
public class LongAdderAPIDemo
{
public static void main(String[] args)
{
LongAdder longAdder = new LongAdder();
longAdder.increment();
longAdder.increment();
longAdder.increment();
System.out.println(longAdder.longValue());
LongAccumulator longAccumulator = new LongAccumulator((x,y) -> x * y,2);
longAccumulator.accumulate(1);
longAccumulator.accumulate(2);
longAccumulator.accumulate(3);
System.out.println(longAccumulator.longValue());
}
}
class ClickNumberNet
{
int number = 0;
public synchronized void clickBySync()
{
number++;
}
AtomicLong atomicLong = new AtomicLong(0);
public void clickByAtomicLong()
{
atomicLong.incrementAndGet();
}
LongAdder longAdder = new LongAdder();
public void clickByLongAdder()
{
longAdder.increment();
}
LongAccumulator longAccumulator = new LongAccumulator((x,y) -> x + y,0);
public void clickByLongAccumulator()
{
longAccumulator.accumulate(1);
}
}
/**
* @auther zzyy
* @create 2020-05-21 22:23
* 50个线程,每个线程100W次,总点赞数出来
*/
public class LongAdderDemo2
{
public static void main(String[] args) throws InterruptedException
{
ClickNumberNet clickNumberNet = new ClickNumberNet();
long startTime;
long endTime;
CountDownLatch countDownLatch = new CountDownLatch(50);
CountDownLatch countDownLatch2 = new CountDownLatch(50);
CountDownLatch countDownLatch3 = new CountDownLatch(50);
CountDownLatch countDownLatch4 = new CountDownLatch(50);
startTime = System.currentTimeMillis();
for (int i = 1; i <=50; i++) {
new Thread(() -> {
try
{
for (int j = 1; j <=100 * 10000; j++) {
clickNumberNet.clickBySync();
}
}finally {
countDownLatch.countDown();
}
},String.valueOf(i)).start();
}
countDownLatch.await();
endTime = System.currentTimeMillis();
System.out.println("----costTime: "+(endTime - startTime) +" 毫秒"+"\t clickBySync result: "+clickNumberNet.number);
startTime = System.currentTimeMillis();
for (int i = 1; i <=50; i++) {
new Thread(() -> {
try
{
for (int j = 1; j <=100 * 10000; j++) {
clickNumberNet.clickByAtomicLong();
}
}finally {
countDownLatch2.countDown();
}
},String.valueOf(i)).start();
}
countDownLatch2.await();
endTime = System.currentTimeMillis();
System.out.println("----costTime: "+(endTime - startTime) +" 毫秒"+"\t clickByAtomicLong result: "+clickNumberNet.atomicLong);
startTime = System.currentTimeMillis();
for (int i = 1; i <=50; i++) {
new Thread(() -> {
try
{
for (int j = 1; j <=100 * 10000; j++) {
clickNumberNet.clickByLongAdder();
}
}finally {
countDownLatch3.countDown();
}
},String.valueOf(i)).start();
}
countDownLatch3.await();
endTime = System.currentTimeMillis();
System.out.println("----costTime: "+(endTime - startTime) +" 毫秒"+"\t clickByLongAdder result: "+clickNumberNet.longAdder.sum());
startTime = System.currentTimeMillis();
for (int i = 1; i <=50; i++) {
new Thread(() -> {
try
{
for (int j = 1; j <=100 * 10000; j++) {
clickNumberNet.clickByLongAccumulator();
}
}finally {
countDownLatch4.countDown();
}
},String.valueOf(i)).start();
}
countDownLatch4.await();
endTime = System.currentTimeMillis();
System.out.println("----costTime: "+(endTime - startTime) +" 毫秒"+"\t clickByLongAccumulator result: "+clickNumberNet.longAccumulator.longValue());
}
}
AtomicLong
LongAdder
原理
场景
缺陷
原理
场景
缺陷