Servlet中的java.util.concurrentModificationException

Question

@WebServlet(name = "AddCart", urlPatterns = {"/AddCart"})
public class AddCart extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession();
        ArrayList<Product> previousItems = (ArrayList<Product>) session.getAttribute("previousItems");
        Product p = (Product) session.getAttribute("currentProduct");
        if (previousItems == null) {
            previousItems = new ArrayList<Product>();
        }

            if (p != null) {
                if (previousItems.size()>0) {
                    for (Product p1 : previousItems) {
                        if (p1.getId() == p.getId()) {
                            p1.addQuantity();
                        } else { //This
                            previousItems.add(p);
                        }
                    }
                } else {
                    previousItems.add(p);
                }
            }

        session.setAttribute("previousItems", previousItems);
        response.sendRedirect("cart.jsp");
    }
}

ConcurrentModificationException

java.util.arrayList$itr.checkforcoModification(arrayList.java:901)java.util.arrayList$itr.next(arrayList.java:851)servlets.addcart.doget

Answer 1

不能用增强的for循环将项添加到正在迭代的列表中。这是因为您正在修改列表的内部状态；虽然可以处理这一点，但大多数迭代器实现不处理底层集合的状态更改，以便在大量、绝大多数的用例中保持简单。

而不是这样：

for (Product p1 : previousItems) {
    previousItems.add(p); // Simplified
}

如果您希望p在之后的列表中，请将其放入另一个列表中，然后在迭代之后添加该列表：

List<Product> other = new ArrayList<>();
for (Product p1 : previousItems) {
    other.add(p);
}
previousItems.addAll(other);