详细写出employee的dao

Employee DAO（Data Access Object）是一种用于访问数据库的设计模式，用于处理相关实体的持久化数据。下面是一个简单的Employee DAO示例，它使用JDBC连接到数据库。

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

public class EmployeeDAO {
    private Connection connection;

    public EmployeeDAO() {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/test";
            String username = "root";
            String password = "password";
            connection = DriverManager.getConnection(url, username, password);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }

    public List<Employee> getAllEmployees() {
        List<Employee> employees = new ArrayList<>();
        try {
            Statement statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery("SELECT * FROM employees");
            while (resultSet.next()) {
                int id = resultSet.getInt("id");
                String name = resultSet.getString("name");
                String department = resultSet.getString("department");
                double salary = resultSet.getDouble("salary");
                employees.add(new Employee(id, name, department, salary));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return employees;
    }

    public Employee getEmployeeById(int id) {
        Employee employee = null;
        try {
            PreparedStatement preparedStatement = connection.prepareStatement("SELECT * FROM employees WHERE id = ?");
            preparedStatement.setInt(1, id);
            ResultSet resultSet = preparedStatement.executeQuery();
            if (resultSet.next()) {
                String name = resultSet.getString("name");
                String department = resultSet.getString("department");
                double salary = resultSet.getDouble("salary");
                employee = new Employee(id, name, department, salary);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return employee;
    }

    public void addEmployee(Employee employee) {
        try {
            PreparedStatement preparedStatement = connection.prepareStatement("INSERT INTO employees (name, department, salary) VALUES (?, ?, ?)");
            preparedStatement.setString(1, employee.getName());
            preparedStatement.setString(2, employee.getDepartment());
            preparedStatement.setDouble(3, employee.getSalary());
            preparedStatement.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public void updateEmployee(Employee employee) {
        try {
            PreparedStatement preparedStatement = connection.prepareStatement("UPDATE employees SET name=?, department=?, salary=? WHERE id=?");
            preparedStatement.setString(1, employee.getName());
            preparedStatement.setString(2, employee.getDepartment());
            preparedStatement.setDouble(3, employee.getSalary());
            preparedStatement.setInt(4, employee.getId());
            preparedStatement.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public void deleteEmployee(int id) {
        try {
            PreparedStatement preparedStatement = connection.prepareStatement("DELETE FROM employees WHERE id=?");
            preparedStatement.setInt(1, id);
            preparedStatement.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public void closeConnection() {
        try {
            connection.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

上面的代码演示了如何使用JDBC API连接到数据库，执行SQL查询和更新语句，并从结果集中检索数据。Employee DAO中的方法用于从数据库中检索，添加，更新和删除Employee对象。每个方法都使用PreparedStatement对象执行SQL语句，该对象支持参数化查询，可以防止SQL注入攻击。此外，Employee DAO在构造函数中初始化数据库连接，并提供了一个关闭连接的方法，以确保资源在不再需要时被正确释放。