并发容器之CopyOnWriteArrayList

 

测试案例：

import lombok.extern.slf4j.Slf4j;
import net.jcip.annotations.ThreadSafe;

import java.util.List;
import java.util.concurrent.*;

@ThreadSafe
@Slf4j
public class CopyOnWriteListExample {

    // 请求总数
    public static int requestTotal = 2000;

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

    public static List<Integer> list = new CopyOnWriteArrayList<>();

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(requestTotal);

        for (int i=0;i<requestTotal;i++) {
            final int num = i;
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    add(num);
                    semaphore.release();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                countDownLatch.countDown();
            });
        }

        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        executorService.shutdown();

        log.info("{}", list.size());
    }

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

}

 

add方法源码：

public boolean add(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        // 数组copy
        Object[] newElements = Arrays.copyOf(elements, len + 1);
        // 新增对象添加到新copy的数组
        newElements[len] = e;
        // 引用指向新数据
        setArray(newElements);
        return true;
    } finally {
        lock.unlock();
    }
}

 

由此上可知，CopyOnWriteArrayList适合多读少写的场景