使用 MVC 和 DAO 模式在 JSP 页面的 HTML 中显示 JDBC ResultSet

2022-01-30 00:00:00 model-view-controller jdbc java jsp servlets

我正在使用 JSP 和 JDBC 实现 MVC.我已经将一个数据库类文件导入到我的 JSP 文件中，我想显示一个 DB 表的数据.我不知道我应该如何将 Java 类中的 ResultSet 返回到 JSP 页面并将其嵌入到 HTML 中.

I'm implementing MVC using JSP and JDBC. I have imported a database class file to my JSP file and I would like to show the data of a DB table. I don't know how I should return the ResultSet from the Java class to the JSP page and embed it in HTML.

我怎样才能做到这一点?

How can I achieve this?

推荐答案

在设计良好的 MVC 方法中，JSP 文件不应包含任何 Java 代码，servlet 类不应包含任何 JDBC 代码.

In a well designed MVC approach, the JSP file should not contain any line of Java code and the servlet class should not contain any line of JDBC code.

p>

假设您要在网上商店中显示产品列表，则需要创建以下代码.

Assuming that you want to show a list of products in a webshop, the following code needs to be created.

  • 代表产品真实世界实体的 Product 类，它应该只是一个 Javabean.

  • A Product class representing a real world entity of a product, it should be just a Javabean.

public class Product {

    private Long id; 
    private String name;
    private String description;
    private BigDecimal price;

    // Add/generate getters/setters/c'tors/equals/hashcode boilerplate.
}

  • 一个 DAO 类所有讨厌的 JDBC 工作并返回一个不错的 List<Product>.

  • A DAO class which does all the nasty JDBC work and returns a nice List<Product>.

    public class ProductDAO {

    private DataSource dataSource;

    public ProductDAO(DataSource dataSource) {
        this.dataSource = dataSource;
    }

    public List<Product> list() throws SQLException {
        List<Product> products = new ArrayList<Product>();

        try (
            Connection connection = dataSource.getConnection();
            PreparedStatement statement = connection.prepareStatement("SELECT id, name, description, price FROM product");
            ResultSet resultSet = statement.executeQuery();
        ) {
            while (resultSet.next()) {
                Product product = new Product();
                product.setId(resultSet.getLong("id"));
                product.setName(resultSet.getString("name"));
                product.setDescription(resultSet.getString("description"));
                product.setPrice(resultSet.getBigDecimal("price"));
                products.add(product);
            }
        }

        return products;
    }

}

  • 一个 servlet 类，它获取列表并将其放入请求范围内.

  • A servlet class which obtains the list and puts it in the request scope.

    @WebServlet("/products")
public class ProductsServlet extends HttpServlet {

    @Resource(name="jdbc/YourDB") // For Tomcat, define as <Resource> in context.xml and declare as <resource-ref> in web.xml.
    private DataSource dataSource;
    private ProductDAO productDAO;

    @Override
    public void init() {
        productDAO = new ProductDAO(dataSource);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        try {
            List<Product> products = productDAO.list();
            request.setAttribute("products", products); // Will be available as ${products} in JSP
            request.getRequestDispatcher("/WEB-INF/products.jsp").forward(request, response);
        } catch (SQLException e) {
            throw new ServletException("Cannot obtain products from DB", e);
        }
    }

}

  • 最后是 /WEB-INF/products.jsp 中的 JSP 文件它使用 JSTL <c:forEach> 来迭代 List<Product> 由 ${products} 在 EL 中提供，并使用 JSTL<c:out> 转义字符串属性，以避免在涉及用户时出现 XSS 漏洞-受控输入.

  • Finally a JSP file in /WEB-INF/products.jsp which uses JSTL <c:forEach> to iterate over List<Product> which is made available in EL by ${products}, and uses JSTL <c:out> to escape string properties in order to avoid XSS holes when it concerns user-controlled input.

    <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/format" prefix="fmt" %>
...
<table>
    <c:forEach items="${products}" var="product">
        <tr>
            <td>${product.id}</td>
            <td><c:out value="${product.name}" /></td>
            <td><c:out value="${product.description}" /></td>
            <td><fmt:formatNumber value="${product.price}" type="currency" currencyCode="USD" /></td>
        </tr>
    </c:forEach>
</table>

    • 要让它工作，只需通过它的 URL 调用 servlet.前提是 servlet 被注释为 @WebServlet("/products") 或在 web.xml 中使用 <url-pattern>/products</url-pattern>，那么就可以通过http://example.com/contextname/products

    To get it to work, just call the servlet by its URL. Provided that the servlet is annotated @WebServlet("/products") or mapped in web.xml with <url-pattern>/products</url-pattern>, then you can call it by http://example.com/contextname/products

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    • 如何将列数未知的 ResultSet 映射到 List 并在 HTML 表格中显示?
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