juc之atomic的學習

atomic基本類型

常見方法

public final int get() 

public final int getAndSet(int newValue)

public final int incrementAndGet()

public final int decrementAndGet()

public final int addAndGet(int delta)//獲取當前的值並加上預期的值

//如果當前的值和期待值expect相同，則把當前值設置爲update
public final boolean compareAndSet(int expect, int update)

 atomic*FieldUpdater

代碼示例

package atomic;

import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;

/**
 * 描述：     演示AtomicIntegerFieldUpdater的用法
 */
public class AtomicIntegerFieldUpdaterDemo implements Runnable{

    static Candidate tom;
    static Candidate peter;

    public static AtomicIntegerFieldUpdater<Candidate> scoreUpdater = AtomicIntegerFieldUpdater
            .newUpdater(Candidate.class, "score");

    @Override
    public void run() {
        for (int i = 0; i < 10000; i++) {
            peter.score++;
            scoreUpdater.getAndIncrement(tom);
        }
    }

    public static class Candidate {

        volatile int score;
    }

    public static void main(String[] args) throws InterruptedException {
        tom=new Candidate();
        peter=new Candidate();
        AtomicIntegerFieldUpdaterDemo r = new AtomicIntegerFieldUpdaterDemo();
        Thread t1 = new Thread(r);
        Thread t2 = new Thread(r);
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println("普通變量："+peter.score);
        System.out.println("升級後的結果"+ 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：     演示高併發場景下，LongAdder比AtomicLong性能好
 */
public class LongAdderDemo {

    public static void main(String[] args) throws InterruptedException {
        LongAdder counter = new LongAdder();
        ExecutorService service = Executors.newFixedThreadPool(20);
        long start = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            service.submit(new Task(counter));
        }
        service.shutdown();
        while (!service.isTerminated()) {

        }
        long end = System.currentTimeMillis();
        System.out.println(counter.sum());
        System.out.println("LongAdder耗時：" + (end - start));
    }

    private static class Task implements Runnable {

        private LongAdder counter;

        public Task(LongAdder counter) {
            this.counter = counter;
        }

        @Override
        public void run() {
            for (int i = 0; i < 10000; i++) {
                counter.increment();
            }
        }
    }
}

爲什麼LongAdder比較快？

因爲AtomicLong需要flush和refresh.讓不同的線程中看見主內存的最新值。

而Adder每個線程只在自己線程中操作。