jsp二级下拉菜单

CREATE TABLE categories (
    id INT PRIMARY KEY,
    name VARCHAR(255),
    parent_id INT
);

@WebServlet("/categories")
public class CategoryServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String parentId = request.getParameter("parentId");
        List<Category> categories = getCategoryByParentId(parentId);
        request.setAttribute("categories", categories);
        request.getRequestDispatcher("categories.jsp").forward(request, response);
    }

    private List<Category> getCategoryByParentId(String parentId) {
        // JDBC code to fetch categories from database based on parentId
        // ...
    }
}

<form action="someAction" method="post">
    <select name="mainCategory" onchange="this.form.submit()">
        <option value="">Select Main Category</option>
        <!-- Populate main categories -->
        <c:forEach items="${mainCategories}" var="category">
            <option value="${category.id}">${category.name}</option>
        </c:forEach>
    </select>

    <select name="subCategory" disabled>
        <option value="">Select Sub Category</option>
        <!-- Populate sub categories based on main category selection -->
        <c:forEach items="${categories}" var="category">
            <option value="${category.id}">${category.name}</option>
        </c:forEach>
    </select>
</form>