- JDK Atomic开头的类,是通过 CAS 原理解决并发情况下原子性问题。
- CAS 包含 3 个参数,CAS(V, E, N)。V 表示需要更新的变量,E 表示变量当前期望值,N 表示更新为的值。只有当变量 V 的值等于 E 时,变量 V 的值才会被更新为 N。如果变量 V 的值不等于 E ,说明变量 V 的值已经被更新过,当前线程什么也不做,返回更新失败。
- 当多个线程同时使用 CAS 更新一个变量时,只有一个线程可以更新成功,其他都失败。失败的线程不会被挂起,可以继续重试 CAS,也可以放弃操作。
- CAS 操作的原子性是通过 CPU 单条指令完成而保障的。JDK 中是通过 Unsafe 类中的 API 完成的。
- 在并发量很高的情况,会有大量 CAS 更新失败,所以需要慎用。
未使用原子类,测试代码
package constxiong.interview; /** * JDK 原子类测试 * @author ConstXiong * @date 2019-06-11 11:22:01 */public class TestAtomic { private int count = 0;public int getAndIncrement() {return count++;}//private AtomicInteger count = new AtomicInteger(0);////public int getAndIncrement() {//return count.getAndIncrement();//}public static void main(String[] args) {final TestAtomic test = new TestAtomic();for (int i = 0; i <3; i++) {new Thread(){@Overridepublic void run() {for (int j = 0; j <10; j++) {try {Thread.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + " 获取递增值:" + test.getAndIncrement());}}}.start();}}}
打印结果中,包含重复值
Thread-0 获取递增值:1Thread-2 获取递增值:2Thread-1 获取递增值:0Thread-0 获取递增值:3Thread-2 获取递增值:3Thread-1 获取递增值:3Thread-2 获取递增值:4Thread-0 获取递增值:5Thread-1 获取递增值:5Thread-1 获取递增值:6Thread-2 获取递增值:8Thread-0 获取递增值:7Thread-1 获取递增值:9Thread-0 获取递增值:10Thread-2 获取递增值:10Thread-0 获取递增值:11Thread-2 获取递增值:13Thread-1 获取递增值:12Thread-1 获取递增值:14Thread-0 获取递增值:14Thread-2 获取递增值:14Thread-1 获取递增值:15Thread-2 获取递增值:15Thread-0 获取递增值:16Thread-1 获取递增值:17Thread-0 获取递增值:19Thread-2 获取递增值:18Thread-0 获取递增值:20Thread-1 获取递增值:21Thread-2 获取递增值:22
测试代码修改为原子类
package constxiong.interview; import java.util.concurrent.atomic.AtomicInteger; /** * JDK 原子类测试 * @author ConstXiong * @date 2019-06-11 11:22:01 */public class TestAtomic { //private int count = 0;////public int getAndIncrement() {//return count++;//}private AtomicInteger count = new AtomicInteger(0);public int getAndIncrement() {return count.getAndIncrement();}public static void main(String[] args) {final TestAtomic test = new TestAtomic();for (int i = 0; i <3; i++) {new Thread(){@Overridepublic void run() {for (int j = 0; j <10; j++) {try {Thread.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + " 获取递增值:" + test.getAndIncrement());}}}.start();}}}
打印结果中,不包含重复值
Thread-0 获取递增值:1Thread-2 获取递增值:2Thread-1 获取递增值:0Thread-0 获取递增值:3Thread-1 获取递增值:4Thread-2 获取递增值:5Thread-0 获取递增值:6Thread-1 获取递增值:7Thread-2 获取递增值:8Thread-0 获取递增值:9Thread-2 获取递增值:10Thread-1 获取递增值:11Thread-0 获取递增值:12Thread-1 获取递增值:13Thread-2 获取递增值:14Thread-0 获取递增值:15Thread-1 获取递增值:16Thread-2 获取递增值:17Thread-0 获取递增值:18Thread-1 获取递增值:19Thread-2 获取递增值:20Thread-0 获取递增值:21Thread-2 获取递增值:23Thread-1 获取递增值:22Thread-0 获取递增值:24Thread-1 获取递增值:25Thread-2 获取递增值:26Thread-0 获取递增值:27Thread-2 获取递增值:28Thread-1 获取递增值:29