并发容器 J.U.C（二）CopyOnWriteArraySet & ConcurrentSkipListSet

与线程不安全的集合类的对应关系

• HashSet -> CopyOnWriteArraySet
• TreeSet -> ConcurrentSkipListSet

CopyOnWriteArraySet概述

• 底层实现依赖于CopyOnWriteArrayList，因此也适合于数据量很小的，读操作大于写操作的集合；
• 迭代器不支持可变的remove操作，使用迭代器遍历时速度很快；

CopyOnWriteArraySet示例

import com.example.concurrency.annotations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;

import java.util.Set;
import java.util.concurrent.CopyOnWriteArraySet;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

@Slf4j
@ThreadSafe
public class CopyOnWriteArraySetExample {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    private static Set<Integer> set = new CopyOnWriteArraySet<>();

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal; i++) {
            final int count = i;
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update(count);
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("size:{}", set.size());
    }

    private static void update(int i) {
        set.add(i);
    }
}

输出：

14:16:42.468 [main] INFO com.example.concurrency.example.concurrent.CopyOnWriteArraySetExample - size:5000

ConcurrentSkipListSet概述

• 和TreeSet一样，支持自然排序，可以在构造的时候定义比较器；
• 其中的contains, add, remove操作都是线程安全的，但对于批量操作，比如addAll removeAll, containsAll并不能保证原子性执行，因为其底层还是调用contains, add, remove方法，在批量操作时，只能保证每一个的add等操作是原子性的，对于批量操作在调用时，还是要手动加上锁保证原子性；
• 不允许存储空元素；

ConcurrentSkipListSet示例

import com.example.concurrency.annotations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;

import java.util.Set;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

@Slf4j
@ThreadSafe
public class ConcurrentSkipListSetExample {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    private static Set<Integer> set = new ConcurrentSkipListSet<>();

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal; i++) {
            final int count = i;
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update(count);
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("size:{}", set.size());
    }

    private static void update(int i) {
        set.add(i);
    }
}

输出：

14:17:17.908 [main] INFO com.example.concurrency.example.concurrent.ConcurrentSkipListSetExample - size:5000