【JUC】Atomic原子类操作以及LongAdder底层详解
Atomic原子操作类
| Classes | Description | flag |
|---|---|---|
| AtomicBoolean | A boolean value that may be updated atomically. |
1 |
| AtomicInteger | An int value that may be updated atomically. |
1 |
| AtomicIntegerArray | An int array in which elements may be updated atomically. |
2 |
| AtmoicIntergerFieldUpdater | A reflection-based utility that enables atomic updates to designated volatile int fields of designated classes. |
4 |
| AtmoicLong | A long value that may be updated atomically. |
1 |
| AtmoicLongArray | A long array in which elements may be updated atomically. |
2 |
| AtmoicLongFieldUpdater | A reflection-based utility that enables atomic updates to designated volatile long fields of designated classes. |
4 |
| AtmoicReference | An object reference that may be updated atomically. | 3 |
| AtmoicReferenceArray | An array of object references in which elements may be updated atomically. | 2 |
| AtmoicReferenceFieldUpdater |
A reflection-based utility that enables atomic updates to designated volatile reference fields of designated classes. |
4 |
| AtmoicStampedReference | An AtomicStampedReference maintains an object reference along with an integer “stamp”, that can be updated atomically. |
3 |
| AtmoicMarkableReference | An AtomicMarkableReference maintains an object reference along with a mark bit, that can be updated atomically. |
3 |
| DoubleAccumulator | One or more variables that together maintain a running double value updated using a supplied function. |
5 |
| DoubleAdder | One or more variables that together maintain an initially zero double sum. |
5 |
| LongAccumulator | One or more variables that together maintain a running long value updated using a supplied function. |
5 |
| LongAdder | One or more variables that together maintain an initially zero long sum. |
5 |
1.基本类型原子类
AtmoicInteger
AtmoicBoolean
Atmoiclong
常用API
| api | 作用 |
|---|---|
| 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） |
class MyNumber
{
@Getter
private AtomicInteger atomicInteger = new AtomicInteger();
public void addPlusPlus()
{
atomicInteger.incrementAndGet();
}
}
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());
}
}
2.数组类型原子类
AtmoicIntegerArray
AtmoicLongArray
AtmoicReferenceArray
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));
}
}
3.引用类型原子类
AtmoicReference
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
* 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();
}
}
AtmoicStampedReference
携带版本号的引用类型原子类，可以解决ABA问题，修改几次
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();
}
}
AtmoicMarkableReference
原子更新带有标记位的引用类型对象,解决是否修改过,它的定义就是将状态戳简化为true|false
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();
}
4.对象的属性修改原子类
AtmoicIntegerFieldUpdater:原子更新对象中int类型字段的值
AtomicLongFieldUpdater:原子更新对象中Long类型字段的值
AtomicReferenceFieldUpdater:原子更新引用类型字段的值
目的是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);
}
}
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);
}
}
---------------------------------------------------------------------------------------------------
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);
}
}
5.原子操作增强类
DoubleAccumulator：
DoubleAdder
LongAccumulator
LongAdder
Accumulator提供了自定义的函数操作
Adder只能用来计算加法，且从零开始计算
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());
}
}
LongAdder具有高性能的特点
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());
}
}
LongAdder高性能原理
架构图
LongAdder是Striped64的子类
striped64重要的成员函数
/** Number of CPUS, to place bound on table size CPU数量，即cells数组的最大长度 */
static final int NCPU = Runtime.getRuntime().availableProcessors();
/**
* Table of cells. When non-null, size is a power of 2.
cells数组，为2的幂，2,4,8,16.....，方便以后位运算
*/
transient volatile Cell[] cells;
/**基础value值，当并发较低时，只累加该值主要用于没有竞争的情况，通过CAS更新。
* Base value, used mainly when there is no contention, but also as
* a fallback during table initialization races. Updated via CAS.
*/
transient volatile long base;
/**创建或者扩容Cells数组时使用的自旋锁变量调整单元格大小（扩容），创建单元格时使用的锁。0表示无锁，1有锁
* Spinlock (locked via CAS) used when resizing and/or creating Cells.
*/
transient volatile int cellsBusy;
advanceProbe():重置当前线程的hash值
getprobe():获取当前线程的hash值
casCellBusy():同过cas操作修改cellBusy的值，cas成功表示获取锁，返回ture
collide: 扩容意向，false表示一定不扩容，true表示可能
其中cell是其一个内部类
LongAdder的基本思路就是分散热点，将value值分散到一个Cell数组中，不同线程会命中到数组的不同槽中，各个线程只对自己槽中的那个值进行CAS操作，这样热点就被分散了，冲突的概率就小很多。如果要获取真正的long值，只要将各个槽中的变量值累加返回。
sum()会将所有Cell数组中的value和base累加作为返回值，核心的思想就是将之前AtomicLong一个value的更新压力分散到多个value中去，从而降级更新热点。
base变量：非竞态条件下，直接累加到该变量上
Cell[]数组：竞态条件下，累加个各个线程自己的槽Cell[i]中
Cell实现原理
1.最初无竞争时只更新base；2.如果更新base失败后，首次新建一个Cell[]数组3.当多个线程竞争同一个Cell比较激烈时，可能就要对Cell[]扩容
总览流程图
AtmoicLong和LongAdder比较
Java 8 性能改进：LongAdder vs AtomicLong |帕洛米诺实验室博客 (palominolabs.com)
| AtmoicLong | LongAdder | |
|---|---|---|
| 场景 | 线程安全，可允许一些性能损耗，要求高精度时可使用，低并发下的全局计算 | 当需要在高并发下有较好的性能表现，且对值的精确度要求不高时，可以使用，高并发下的全局计算 |
| 特点 | 保证精度，性能代价，高并发后性能急剧下降 | 保证性能，精度代价，sum求和后还有计算线程修改结果的话，最后结果不够准确 |
| 操作对象 | AtomicLong是多个线程针对单个热点值value进行原子操作 | LongAdder是每个线程拥有自己的槽，各个线程一般只对自己槽中的那个值进行CAS操作 |