Java Web 动态查询商品：使用 DAO 和 JDBC 实现

为了实现动态查询商品的功能，我们需要使用 Java Web 的 DAO 和 JDBC 技术。下面是一个简单的示例，包含了所有需要的文件的全部代码和数据库。

首先，我们需要创建一个数据库表来存储商品信息。可以使用以下 SQL 语句在数据库中创建一个名为'products' 的表：

CREATE TABLE products (
  id INT PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(100),
  price DECIMAL(10, 2),
  category VARCHAR(50)
);

接下来，我们创建一个 Product 类来表示商品对象，包含 id、name、price 和 category 属性，以及对应的 getter 和 setter 方法。

public class Product {
    private int id;
    private String name;
    private double price;
    private String category;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    public String getCategory() {
        return category;
    }

    public void setCategory(String category) {
        this.category = category;
    }
}

接下来，我们创建一个 ProductDAO 接口来定义商品数据访问的方法。

import java.util.List;

public interface ProductDAO {
    void addProduct(Product product);
    void updateProduct(Product product);
    void deleteProduct(int id);
    Product getProductById(int id);
    List<Product> getAllProducts();
    List<Product> getProductsByCategory(String category);
}

然后，我们创建一个 ProductDAOImpl 类来实现 ProductDAO 接口，使用 JDBC 技术与数据库进行交互。

import java.sql.*;
import java.util.ArrayList;
import java.util.List;

public class ProductDAOImpl implements ProductDAO {
    private static final String URL = "jdbc:mysql://localhost:3306/mydb";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "password";

    @Override
    public void addProduct(Product product) {
        try (Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
             PreparedStatement stmt = conn.prepareStatement("INSERT INTO products (name, price, category) VALUES (?, ?, ?)")) {
            stmt.setString(1, product.getName());
            stmt.setDouble(2, product.getPrice());
            stmt.setString(3, product.getCategory());
            stmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void updateProduct(Product product) {
        try (Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
             PreparedStatement stmt = conn.prepareStatement("UPDATE products SET name = ?, price = ?, category = ? WHERE id = ?")) {
            stmt.setString(1, product.getName());
            stmt.setDouble(2, product.getPrice());
            stmt.setString(3, product.getCategory());
            stmt.setInt(4, product.getId());
            stmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void deleteProduct(int id) {
        try (Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
             PreparedStatement stmt = conn.prepareStatement("DELETE FROM products WHERE id = ?")) {
            stmt.setInt(1, id);
            stmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override
    public Product getProductById(int id) {
        try (Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
             PreparedStatement stmt = conn.prepareStatement("SELECT * FROM products WHERE id = ?")) {
            stmt.setInt(1, id);
            ResultSet rs = stmt.executeQuery();
            if (rs.next()) {
                Product product = new Product();
                product.setId(rs.getInt("id"));
                product.setName(rs.getString("name"));
                product.setPrice(rs.getDouble("price"));
                product.setCategory(rs.getString("category"));
                return product;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return null;
    }

    @Override
    public List<Product> getAllProducts() {
        List<Product> products = new ArrayList<>();
        try (Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
             Statement stmt = conn.createStatement()) {
            ResultSet rs = stmt.executeQuery("SELECT * FROM products");
            while (rs.next()) {
                Product product = new Product();
                product.setId(rs.getInt("id"));
                product.setName(rs.getString("name"));
                product.setPrice(rs.getDouble("price"));
                product.setCategory(rs.getString("category"));
                products.add(product);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return products;
    }

    @Override
    public List<Product> getProductsByCategory(String category) {
        List<Product> products = new ArrayList<>();
        try (Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
             PreparedStatement stmt = conn.prepareStatement("SELECT * FROM products WHERE category = ?")) {
            stmt.setString(1, category);
            ResultSet rs = stmt.executeQuery();
            while (rs.next()) {
                Product product = new Product();
                product.setId(rs.getInt("id"));
                product.setName(rs.getString("name"));
                product.setPrice(rs.getDouble("price"));
                product.setCategory(rs.getString("category"));
                products.add(product);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return products;
    }
}

最后，我们创建一个 Servlet 来处理用户的请求，并使用 ProductDAOImpl 类来实现动态查询商品的功能。

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;

public class ProductServlet extends HttpServlet {
    private ProductDAO productDAO = new ProductDAOImpl();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String category = req.getParameter("category");
        if (category != null) {
            List<Product> products = productDAO.getProductsByCategory(category);
            req.setAttribute("products", products);
        } else {
            List<Product> products = productDAO.getAllProducts();
            req.setAttribute("products", products);
        }
        req.getRequestDispatcher("products.jsp").forward(req, resp);
    }
}

在上述代码中，我们假设存在一个名为'products.jsp' 的 JSP 页面来展示查询结果。

希望这个简单的示例能帮助到你实现动态查询商品的功能。请注意，这只是一个基本的示例，你可以根据实际需求进行修改和扩展。