已有数据库jdbc,表名为user,表中已有数据id、name、email、password、birthday，如何使用JDBC完成对数据库中用户信息的增删改查操作

使用JDBC进行数据库操作的基本步骤如下：

1. 加载数据库驱动程序

2. 建立数据库连接

3. 创建Statement对象或PrepareStatement对象

4. 执行SQL语句

5. 处理结果集

6. 关闭连接

下面是对用户信息的增删改查操作的示例代码：

import java.sql.*;

public class UserDAO {

    // 建立数据库连接
    public Connection getConnection() {
        Connection conn = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=utf8";
            String username = "root";
            String password = "123456";
            conn = DriverManager.getConnection(url, username, password);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return conn;
    }

    // 添加用户
    public void addUser(User user) {
        Connection conn = getConnection();
        PreparedStatement pstmt = null;
        String sql = "insert into user(name, email, password, birthday) values(?,?,?,?)";
        try {
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getName());
            pstmt.setString(2, user.getEmail());
            pstmt.setString(3, user.getPassword());
            pstmt.setDate(4, new java.sql.Date(user.getBirthday().getTime()));
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (pstmt != null) pstmt.close();
                if (conn != null) conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    // 删除用户
    public void deleteUser(int id) {
        Connection conn = getConnection();
        PreparedStatement pstmt = null;
        String sql = "delete from user where id=?";
        try {
            pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (pstmt != null) pstmt.close();
                if (conn != null) conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    // 更新用户信息
    public void updateUser(User user) {
        Connection conn = getConnection();
        PreparedStatement pstmt = null;
        String sql = "update user set name=?, email=?, password=?, birthday=? where id=?";
        try {
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getName());
            pstmt.setString(2, user.getEmail());
            pstmt.setString(3, user.getPassword());
            pstmt.setDate(4, new java.sql.Date(user.getBirthday().getTime()));
            pstmt.setInt(5, user.getId());
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (pstmt != null) pstmt.close();
                if (conn != null) conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    // 查询用户信息
    public User findUserById(int id) {
        Connection conn = getConnection();
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        User user = null;
        String sql = "select * from user where id=?";
        try {
            pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            rs = pstmt.executeQuery();
            if (rs.next()) {
                user = new User();
                user.setId(rs.getInt("id"));
                user.setName(rs.getString("name"));
                user.setEmail(rs.getString("email"));
                user.setPassword(rs.getString("password"));
                user.setBirthday(rs.getDate("birthday"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) rs.close();
                if (pstmt != null) pstmt.close();
                if (conn != null) conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return user;
    }
}

其中，User类表示用户信息，包括id、name、email、password、birthday等属性。在上面的代码中，addUser方法用于添加用户，deleteUser方法用于删除用户，updateUser方法用于更新用户信息，findUserById方法用于查询指定id的用户信息。其他操作类似，可以根据需要进行扩展。