java 原子量Atomic举例（AtomicReference）

Posted 2020-08-09

java并发库提供了很多原子类来支持并发访问的数据安全性，除了常用的

AtomicInteger、AtomicBoolean、AtomicLong 外还有

AtomicReference 用以支持对象的原子操作：AtomicReference<V> 可以封装引用一个V实例，
通过

public final boolean compareAndSet(V expect, V update) 

可以支持并发访问，set的时候进行对比判断，如果当前值和操作之前一样则返回false，否则表示数据没有 变化，例如下面的代码
使用 AtomicReference 实现了并发计数：

package test;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicReference;

public class TS {
    public static void main(String[] args) throws InterruptedException {
        dfasd111();
    }

    private static AtomicReference<Integer> ar = new AtomicReference<Integer>(0);

    public static void dfasd111() throws InterruptedException {
        int t = 100;
        final int c = 100;
        final CountDownLatch latch = new CountDownLatch(t);
        for (int i = 0; i < t; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    for (int i = 0; i < c; i++) {
                        while (true) {
                            Integer temp = ar.get();
                            if (ar.compareAndSet(temp, temp + 1)) {
                                break;
                            }
                        }
                    }
                    latch.countDown();
                }
            }).start();
        }
        latch.await();
        System.out.println(ar.get()); //10000000
    }

    public final void test() {
        System.out.println(this.getClass());
    }
}

一、原子量实现的计数器

import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;
public class AtomicCounter {

    private AtomicInteger value = new AtomicInteger();

    public int getValue() {
        return value.get();
    }

    public int increase() {
        return value.incrementAndGet();// 内部使用死循环for(;;)调用compareAndSet(current, next)
        //        return value.getAndIncrement();
    }

    public int increase(int i) {
        return value.addAndGet(i);// 内部使用死循环for(;;)调用compareAndSet(current, next)
        //        return value.getAndAdd(i);
    }

    public int decrease() {
        return value.decrementAndGet();// 内部使用死循环for(;;)调用compareAndSet(current, next)
        //        return value.getAndDecrement();
    }

    public int decrease(int i) {
        return value.addAndGet(-i);// 内部使用死循环for(;;)调用compareAndSet(current, next)
        //        return value.addAndGet(-i);
    }

    public static void main(String[] args) {
        final AtomicCounter counter = new AtomicCounter();
        ExecutorService service = Executors.newCachedThreadPool();
        for (int i = 0; i < 10; i++) {
            service.execute(new Runnable() {
                @Override
                public void run() {
                    System.out.println(counter.increase());
                }
            });
        }
        service.shutdown();
    }
}

二、原子量实现的银行取款

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicLong;
public class Account {

    private AtomicLong balance;

    public Account(long money) {
        balance = new AtomicLong(money);
        System.out.println("Total Money:" + balance);
    }

    public void deposit(long money) {
        balance.addAndGet(money);
    }

    public void withdraw(long money) {
        for (; ; ) {//保证即时同一时间有人也在取款也可以再次尝试取款，如果不需要并发尝试取款，可以去掉这句
            long oldValue = balance.get();
            if (oldValue < money) {
                System.out.println(Thread.currentThread().getName() + " 余额不足！ 余额：" + balance);
                break;
            }
            try {Thread.sleep(new Random().nextInt(1000));} catch (Exception e) { }// 模拟取款时间
            if (balance.compareAndSet(oldValue, oldValue - money)) {
                System.out.println(Thread.currentThread().getName() + " 取款 " + money + " 成功！ 余额：" + balance);
                break;
            }
            System.out.println(Thread.currentThread().getName() + " 遇到并发，再次尝试取款！");
        }
    }

    public static void main(String[] args) {
        final Account account = new Account(1000);
        ExecutorService pool = Executors.newCachedThreadPool();
        int i = 0;
        while (i++ < 13) {
            pool.execute(new Runnable() {
                @Override
                public void run() {
                    account.withdraw(100);
                }
            });
        }
        pool.shutdown();
    }
}