注意:原子操作的意义是说线程不能被中断,本身并不是线程安全的,但是调用atomic的设值方法:
compareAndSet()就是线程安全的了。
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();
- }
- }
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