写一个代码实现webjavaweb增删改查

当然可以！以下是一个简单的示例代码，演示了如何使用Java Web实现增删改查功能：

// 导入必要的包
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/UserServlet")
public class UserServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    // 数据库连接参数
    private final String URL = "jdbc:mysql://localhost:3306/test";
    private final String USERNAME = "root";
    private final String PASSWORD = "password";

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doPost(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String action = request.getParameter("action");

        // 根据 action 参数执行相应的操作
        if (action.equals("add")) {
            addUser(request, response);
        } else if (action.equals("delete")) {
            deleteUser(request, response);
        } else if (action.equals("update")) {
            updateUser(request, response);
        } else if (action.equals("query")) {
            queryUser(request, response);
        }
    }

    // 添加用户
    private void addUser(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        Connection conn = null;
        PreparedStatement pstmt = null;
        PrintWriter out = response.getWriter();

        try {
            // 建立数据库连接
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);

            // 执行插入语句
            String sql = "INSERT INTO users (username, password) VALUES (?, ?)";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, username);
            pstmt.setString(2, password);
            pstmt.executeUpdate();

            out.println("添加用户成功！");
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        } finally {
            // 关闭数据库连接和资源
            try {
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    // 删除用户
    private void deleteUser(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String username = request.getParameter("username");

        Connection conn = null;
        PreparedStatement pstmt = null;
        PrintWriter out = response.getWriter();

        try {
            // 建立数据库连接
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);

            // 执行删除语句
            String sql = "DELETE FROM users WHERE username = ?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, username);
            pstmt.executeUpdate();

            out.println("删除用户成功！");
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        } finally {
            // 关闭数据库连接和资源
            try {
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    // 更新用户
    private void updateUser(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        Connection conn = null;
        PreparedStatement pstmt = null;
        PrintWriter out = response.getWriter();

        try {
            // 建立数据库连接
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);

            // 执行更新语句
            String sql = "UPDATE users SET password = ? WHERE username = ?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, password);
            pstmt.setString(2, username);
            pstmt.executeUpdate();

            out.println("更新用户成功！");
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        } finally {
            // 关闭数据库连接和资源
            try {
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    // 查询用户
    private void queryUser(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String username = request.getParameter("username");

        Connection conn = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        PrintWriter out = response.getWriter();

        try {
            // 建立数据库连接
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);

            // 执行查询语句
            String sql = "SELECT * FROM users WHERE username = ?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, username);
            rs = pstmt.executeQuery();

            while (rs.next()) {
                out.println("Username: " + rs.getString("username") + ", Password: " + rs.getString("password"));
            }
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        } finally {
            // 关闭数据库连接和资源
            try {
                if (rs != null) {
                    rs.close();
                }
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

上述代码演示了如何通过Servlet来实现用户的增删改查功能。根据传递的 `action` 参数，选择相应的操作。具体的数据库连接和操作使用了JDBC来实现。

请注意，上述代码仅为示例，实际应用中还需要进行错误处理、表单验证、权限控制等一系列安全性和可靠性的考虑。此外，代码中使用的数据库连接方式是基于JDBC，你需要根据实际情况修改数据库连接参数和驱动类。

希望以上代码对你有所帮助！如果还有其他问题，请随时提问。