三：ArrayList的理解

ArrayList 線程不安全的案例和解決方案

• 故障現象

  public class ContainerDemo {
      public static void main(String[] args) {
          List<Integer> list = new ArrayList<>();
          Random random = new Random();
          for (int i = 0; i < 100; i++) {
              new Thread(() -> {
                  list.add(random.nextInt(10));
                  System.out.println(list);
              }).start();
          }
      }
  }
  

  發現報 java.util.ConcurrentModificationException

• 導致原因

  • 併發修改導致的異常

• 解決方案

  • new Vector();

  • Collections.synchronizedList(new ArrayList<>());

  • new CopyOnWriteArrayList<>();

  •   /**  CopyOnWriteArrayList
         * 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();
            }
        }
    

• 優化建議

  • 在讀多寫少的時候推薦使用 CopeOnWriteArrayList 這個類