List集合线程不安全

List集合线程不安全

Exception in thread "64" java.util.ConcurrentModificationException

/**
 *List集合线程不安全
 */
public class ThreadDemo4 {
    public static void main(String[] args) {
        //创建ArrayList集合
        List<String> list = new ArrayList<>();
        for (int i= 0;i<100;i++){
            new Thread(()->{
                list.add(UUID.randomUUID().toString().substring(0,8));
                System.out.println(list);
            },String.valueOf(i)).start();
        }
    }
}

运行结果：

 查看ArrayList 添加方法源码

  /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

没有synchronized，所以是线程不安全

解决方案3种

第一种  Vector

List<String> list = new Vector<>();

第二种 Collections解决

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

第三种CopyOnWriteArrayList写时复制技术

List<String> list = new CopyOnWriteArrayList<>();

HashSet线程不安全

解决方案

Set<String> set = new CopyOnWriteArraySet<>();

HashMap线程不安全

解决方案

Map<String,String> set = new ConcurrentHashMap<>();