JUC学习-3-不安全的集合类-CopyOnWriteArrayList-CopyOnWriteArraySet-ConcurrentHashMap

不安全的集合类

CopyOnWriteArrayList

/*
 * 并发下 ArrayList 是不安全的
 * java.util.ConcurrentModificationException 并发修改异常
 * 解决方案
 * List<String> list = new Vector<>();
 * List<String> list = Collections.synchronizedList(new ArrayList<>());
 * List<String> list = new CopyOnWriteArrayList<>();  写入时复制 COW 计算机程序设计领域的一种优化策略
 * 多个线程调用的时候 list 读取的时候是固定的,  写入(覆盖)
 * 在写入的时候避免覆盖 造成数据问题
 * CopyOnWriteArrayList 比 Vector 好在哪里
 *
 * */
public class ListTest {
    public static void main(String[] args) {
//        List<String> list = new ArrayList<>();
        List<String> list = new CopyOnWriteArrayList<>();

        for (int i = 1; i <= 10; i++) {
            new Thread(() -> {
                list.add(UUID.randomUUID().toString().substring(0, 5));
                System.out.println(list);
            }, String.valueOf(i)).start();
        }

    }
}

CopyOnWriteArraySet

/*
* 并发下 HashSet 是不安全的
* 解决方案
* Set<String> set = Collections.synchronizedSet(new HashSet<>()) ;
* Set<String> set = new CopyOnWriteArraySet<>();
* */
public class SetTest {
    public static void main(String[] args) throws InterruptedException {
//        Set<String> set = new HashSet<>();
        Set<String> set = new CopyOnWriteArraySet<>();
        for (int i = 1; i <= 30; i++) {
            new Thread(() -> {
                set.add(UUID.randomUUID().toString().substring(0, 5));
                System.out.println(set);
            }, String.valueOf(i)).start();
        }
    }
}

HashSet的底层就是HashMap

ConcurrentHashMap

/*
 * 并发下 HashMap 是不安全的
 * 解决方案
 * Map<String, String> map = Collections.synchronizedMap(new HashMap<>());
 * Map<String, String> map = new ConcurrentHashMap<>();
 *
 * */
public class MapTest {
    public static void main(String[] args) {
        //new HashMap<>(); 默认等价为 new HashMap<>(16,0.75);
//        Map<String, String> map = new HashMap<>();
        Map<String, String> map = new ConcurrentHashMap<>();
        for (int i = 1; i <= 30; i++) {
            new Thread(() -> {
                map.put(Thread.currentThread().getName(), UUID.randomUUID().toString().substring(0, 5));
                System.out.println(map);
            }, String.valueOf(i)).start();
        }
    }
}

Callable

• 可以有返回值
• 可以抛出异常
• 方法不同 run()/ call()

public class CallableTest {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        // new Thread(new Runnable() {}).start();
        //  new Thread(new FutureTask()).start();
        //  new Thread(new FutureTask(Callable)).start();

        MyThread myThread = new MyThread();
        FutureTask integerFutureTask = new FutureTask(myThread); // 适配类
        new Thread(integerFutureTask, "A").start();
        new Thread(integerFutureTask, "B").start(); // 结果会被缓存 提升效率
        Object o = integerFutureTask.get(); // 获得callable的返回结果 这个方法可能产生阻塞
        System.out.println(o);
    }
}

class MyThread implements Callable<Integer> {

    @Override
    public Integer call() throws Exception {
        System.out.println("call()");
        return 1024;
    }
}