juc之atomic的学习

atomic基本类型

常见方法

public final int get() public final int getAndSet(int newValue)public final int incrementAndGet()public final int decrementAndGet()public final int addAndGet(int delta)//获取当前的值并加上预期的值//如果当前的值和期待值expect相同&＃xff0c;则把当前值设置为update
public final boolean compareAndSet(int expect, int update)

 atomic*FieldUpdater

代码示例

package atomic;import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;/*** 描述&＃xff1a; 演示AtomicIntegerFieldUpdater的用法不支持对static变量的升级*/
public class AtomicIntegerFieldUpdaterDemo implements Runnable{static Candidate tom;static Candidate peter;public static AtomicIntegerFieldUpdater scoreUpdater &＃61; AtomicIntegerFieldUpdater.newUpdater(Candidate.class, "score");&＃64;Overridepublic void run() {for (int i &＃61; 0; i <10000; i&＃43;&＃43;) {peter.score&＃43;&＃43;;scoreUpdater.getAndIncrement(tom);}}public static class Candidate {volatile int score;}public static void main(String[] args) throws InterruptedException {tom&＃61;new Candidate();peter&＃61;new Candidate();AtomicIntegerFieldUpdaterDemo r &＃61; new AtomicIntegerFieldUpdaterDemo();Thread t1 &＃61; new Thread(r);Thread t2 &＃61; new Thread(r);t1.start();t2.start();t1.join();t2.join();System.out.println("普通变量&＃xff1a;"&＃43;peter.score);System.out.println("升级后的结果"&＃43; tom.score);}
}


adder累加器

package atomic;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.LongAdder;/*** 描述&＃xff1a; 演示高并发场景下&＃xff0c;LongAdder比AtomicLong性能好*/
public class LongAdderDemo {public static void main(String[] args) throws InterruptedException {LongAdder counter &＃61; new LongAdder();ExecutorService service &＃61; Executors.newFixedThreadPool(20);long start &＃61; System.currentTimeMillis();for (int i &＃61; 0; i <10000; i&＃43;&＃43;) {service.submit(new Task(counter));}service.shutdown();while (!service.isTerminated()) {}long end &＃61; System.currentTimeMillis();System.out.println(counter.sum());System.out.println("LongAdder耗时&＃xff1a;" &＃43; (end - start));}private static class Task implements Runnable {private LongAdder counter;public Task(LongAdder counter) {this.counter &＃61; counter;}&＃64;Overridepublic void run() {for (int i &＃61; 0; i <10000; i&＃43;&＃43;) {counter.increment();}}}
}


为什么LongAdder比较快&＃xff1f;

因为AtomicLong需要flush和refresh.让不同的线程中看见主内存的最新值。

而Adder每个线程只在自己线程中操作。