动态jsp树形结构

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

<%@ page import="java.sql.*" %>
<%@ page import="java.util.*" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<%
    // 连接数据库
    Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/yourdb", "username", "password");
    Statement stmt = conn.createStatement();
    ResultSet rs = stmt.executeQuery("SELECT * FROM categories");

    // 将数据存储在Map中
    Map<Integer, List<Category>> categoryMap = new HashMap<>();
    while (rs.next()) {
        int id = rs.getInt("id");
        String name = rs.getString("name");
        int parentId = rs.getInt("parent_id");

        Category category = new Category(id, name, parentId);
        categoryMap.computeIfAbsent(parentId, k -> new ArrayList<>()).add(category);
    }

    // 关闭资源
    rs.close();
    stmt.close();
    conn.close();

    request.setAttribute("categoryMap", categoryMap);
%>

<c:set var="rootCategories" value="${categoryMap[null]}" />

<ul>
    <c:forEach items="${rootCategories}" var="category">
        <li>
            ${category.name}
            <c:if test="${not empty categoryMap[category.id]}">
                <ul>
                    <c:forEach items="${categoryMap[category.id]}" var="subCategory">
                        <li>${subCategory.name}</li>
                    </c:forEach>
                </ul>
            </c:if>
        </li>
    </c:forEach>
</ul>

class Category {
    int id;
    String name;
    int parentId;

    public Category(int id, String name, int parentId) {
        this.id = id;
        this.name = name;
        this.parentId = parentId;
    }
}