java.util.concurrent包

1. 线程计数

有时候我们需要对线程间做计数,或者是统计.这时候我们需要这些类

public class Test {

    public static void invoke(){
        // 可以理解为停车位 , 有三个停车位,十个线程是车
        // 只有车离开才会进下一辆车
        Semaphore available = new Semaphore(3, true);
        try {
            // 获取许可进入
            available.acquire();
            System.out.println(Thread.currentThread().getName()+" : 进入invoke()");
            Thread.sleep(3000);
            // 释放许可
            available.release();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        // countDownLatchDemo();
        // cyclicBarrierDemo();

        for (int i = 1; i <= 10; i++) {
            new Thread(() -> {
                invoke();
            },"test"+i).start();
        }
    }

    private static void cyclicBarrierDemo() {
        // 线程屏障,如果没有满足条件 ,线程会一直在await()阻塞
        CyclicBarrier cyclicBarrier = new CyclicBarrier(50);
        Map<String,String> map = new ConcurrentHashMap<>();
        for (int i = 1; i <= 50; i++) {
            int a = i;
            System.out.println("a : "+a);
            new Thread(() -> {
                UUID uuid = UUID.randomUUID();
                map.put(a+"",uuid.toString());
                try {
                    cyclicBarrier.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
                map.entrySet().forEach(System.out::println);
            },"test"+i).start();
        }
    }

    private static void countDownLatchDemo() throws InterruptedException {
        // 计数器
        // 设置50个累计数
        // 每调用一次countDownLatch.countDown()就会累计+1
        // countDownLatch.await() : 如果计数未到指定值,它会阻塞
        CountDownLatch countDownLatch = new CountDownLatch(50);
        Map<String,String> map = new ConcurrentHashMap<>();
        for (int i = 1; i <= 50; i++) {
            int a = i;
            new Thread(() -> {
                UUID uuid = UUID.randomUUID();
                map.put(a+"",uuid.toString());
                countDownLatch.countDown();
            },"test"+i).start();
        }
        countDownLatch.await();
        map.entrySet().forEach(System.out::println);
    }

}

2. 集合安全

平时使用的集合大部分都是不安全的,当我们需要线程安全的集合时,java已经为我们提供好了!

/**
 * 安全集合框架
 *
 * @Author: blaaair
 **/
public class Test {

    public static void main(String[] args) {
        //safeListDemo();
        // safeMap();

    }

    private static void safeMap() {
        Map<String,String> map = new ConcurrentHashMap<>();
        for (int i = 1; i <= 50 ; i++) {
            new Thread(() -> {
                map.put(Thread.currentThread().getName(), UUID.randomUUID().toString());
                System.out.println(map);
            },i+"").start();

        }
    }

    private static void safeListDemo() {
        // 不安全List
        // List<String> list = new ArrayList<>();
        // 安全List
        // list set 同理,CopyOnWriteArraySet<>();
        List<String> list =new CopyOnWriteArrayList<>();

        for (int i = 1; i <= 50 ; i++) {
            new Thread(() -> {
                list.add(UUID.randomUUID().toString());
                list.forEach(System.out::println);
            },i+"").start();

        }
    }
}