JUC-List

  ArrayList构造函数：构造一个初始容量为10的空列表。

/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
    DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

  ArrayList是线程不安全的，add方法未加锁，会发生java.util.ConcurrentModificationException（并发修改异常），并发争抢修改导致。分析思路：故障现象->导致原因->解决方案->优化建议。
  解决方法：

• Vector
• Collections.synchronizedList()
• CopyOnWriteArrayList（采用）

/**
 * @author luffy
 **/
public class ArrayListDemo {
    public static void main(String[] args){
        //List<String> list = new ArrayList<>();
        List<String> list = new CopyOnWriteArrayList<>();
        //java.util.ConcurrentModificationException-并发修改异常
        //1、Vector add方法加锁-但性能急剧下降
        //2、List<String> list = Collections.synchronizedList(new ArrayList<>());
        //3、CopyOnWriteArrayList-写时复制List
        /**
         * 故障现象-导致原因-解决方案-优化建议
         */
        for(int i =0 ;i< 30;i++){
            new Thread(()->{
                list.add(UUID.randomUUID().toString().substring(0,10));
                System.out.println(list);
            },String.valueOf(i)).start();
        }
    }
}

  CopyOnWriteArrayList类add方法源码（写时复制思想，读写分离）：

  /**
     * 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();
        }
    }