用java语言写UsersDao类，该类主要用于程序与数据库的交互，在该类中封装了对数据库表users的添加、查询、删除和更新等操作。

下面是一个Java语言的UsersDao类的示例代码，该类主要用于与数据库表users进行交互，并封装了添加、查询、删除和更新等操作：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class UsersDao {
    // 数据库连接相关信息
    private String driver = "com.mysql.jdbc.Driver";
    private String url = "jdbc:mysql://localhost:3306/test";
    private String username = "root";
    private String password = "root";

    // 添加用户
    public boolean addUser(User user) {
        Connection conn = null;
        PreparedStatement pstmt = null;
        int result = 0;
        try {
            Class.forName(driver);
            conn = DriverManager.getConnection(url, username, password);
            String sql = "insert into users(username, password, email) values(?, ?, ?)";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getUsername());
            pstmt.setString(2, user.getPassword());
            pstmt.setString(3, user.getEmail());
            result = pstmt.executeUpdate();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return result > 0;
    }

    // 删除用户
    public boolean deleteUser(int id) {
        Connection conn = null;
        PreparedStatement pstmt = null;
        int result = 0;
        try {
            Class.forName(driver);
            conn = DriverManager.getConnection(url, username, password);
            String sql = "delete from users where id=?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            result = pstmt.executeUpdate();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return result > 0;
    }

    // 更新用户
    public boolean updateUser(User user) {
        Connection conn = null;
        PreparedStatement pstmt = null;
        int result = 0;
        try {
            Class.forName(driver);
            conn = DriverManager.getConnection(url, username, password);
            String sql = "update users set username=?, password=?, email=? where id=?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getUsername());
            pstmt.setString(2, user.getPassword());
            pstmt.setString(3, user.getEmail());
            pstmt.setInt(4, user.getId());
            result = pstmt.executeUpdate();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return result > 0;
    }

    // 查询用户
    public User getUser(int id) {
        Connection conn = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        User user = null;
        try {
            Class.forName(driver);
            conn = DriverManager.getConnection(url, username, password);
            String sql = "select * from users where id=?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            rs = pstmt.executeQuery();
            if (rs.next()) {
                user = new User();
                user.setId(rs.getInt("id"));
                user.setUsername(rs.getString("username"));
                user.setPassword(rs.getString("password"));
                user.setEmail(rs.getString("email"));
            }
        } catch (Exception 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;
    }
}

在以上代码中，我们使用了JDBC进行数据库连接，并封装了添加、查询、删除和更新等操作。其中，我们使用了预处理语句PreparedStatement进行操作，这样可以有效地防止SQL注入攻击。同时，在每个方法中，我们都进行了try-catch-finally的异常处理和资源释放操作。