互聯網面試題之集合類不安全的併發修改異常

ArrayList

/**
 * 故障現象
 *  java.util.ConcurrentModificationException
 *
 * 導致原因
 *  併發爭搶修改導致，一個線程正在寫人，另一個線程過來搶奪，導致數據不一致異常，併發修改異常。
 *
 * 解決方案
 *  new Vector()
 *  Collections.synchronizedList(new ArrayList<>())
 *  new CopyOnWriteArrayList<>()
 *
 * 優化建議
 */
public class ArrayListNotSafeDemo {

    public static void main(String[] args) {

//        List<String> list = new ArrayList();
//        List<String> list = new Vector();
//        List<String> list = Collections.synchronizedList(new ArrayList<>());
        List<String> list = new CopyOnWriteArrayList<>();

        final int threadNum = 300;
        CountDownLatch latch = new CountDownLatch(threadNum);
        for(int i = 0; i < threadNum; i++) {
            // 啓動新的線程
            Thread t = new Thread(()-> {
                try {
                    list.add(UUID.randomUUID().toString().replace("-", ""));
                    System.out.println(Thread.currentThread().getName() + ", " + list);
                }finally {
                    latch.countDown();
                }
            }, "thread-" + i);
            t.start();
        }

        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        System.out.println("list size = " + list.size());
    }
}

CopyOnWriteArrayList

寫時複製，CopyOnWrite容器即寫時複製容器。往一個容器添加元素的時候，不直接往當前容器Object[] 添加，而是先將容器Object[] 進行copy，複製出一個新的Object[] newElements，然後向新的容器Object[] newElements 裏添加元素，添加完元素後，再將原容器的引用執向新的容器setArray(newElements);，這樣做的好處是可以對CopyOnWrite 容器進行併發的讀，而不需要加鎖，因爲當前容器不會添加任何元素。所以CopyOnWrite 容器也是一種讀寫分離思想，讀和寫不同的容器。

/**
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return {@code true} (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        Object[] newElements = Arrays.copyOf(elements, len + 1);
        newElements[len] = e;
        setArray(newElements);
        return true;
    } finally {
        lock.unlock();
    }
}