Java Id（）=空）{ 抛出新的IllegalArgumentException（“用户尚未创建，用户ID为null”）； } 对象[]值={ user.getEmail（）， user.getFirstname（）， user.getLastname（）， toSqlDate（user.getBirthdate（））， user.getId（） }; 试一试( Connection Connection=daoFactory.getConnection（）； PreparedStatement=prepareStatement（连接、SQL\u更新、false、值）； ) { int affectedRows=语句.executeUpdate（）； if（affectedRows==0）{ 抛出新的DAOException（“更新用户失败，没有受影响的行”）； } }捕获（SQLE异常）{ 抛出新的异常（e）； } } @凌驾 公共void delete（用户）引发异常{ 对象[]值={ user.getId（） }; 试一试( Connection Connection=daoFactory.getConnection（）； PreparedStatement=prepareStatement（连接、SQL\u删除、false、值）； ) { int affectedRows=语句.executeUpdate（）； if（affectedRows==0）{ 抛出新的异常（“删除用户失败，没有受影响的行”）； }否则{ user.setId（null）； } }捕获（SQLE异常）{ 抛出新的异常（e）； } } @凌驾 public boolean existEmail（字符串电子邮件）引发异常{ 对象[]值={ 电子邮件 }; 布尔存在=假； 试一试( Connection Connection=daoFactory.getConnection（）； PreparedStatement=prepareStatement（连接、SQL存在、电子邮件、false、值）； ResultSet ResultSet=statement.executeQuery（）； ) { exist=resultSet.next（）； }捕获（SQLE异常）{ 抛出新的异常（e）； } 回归存在； } @凌驾 public void changePassword（用户）引发异常{ if（user.getId（）==null）{ 抛出新的IllegalArgumentException（“用户尚未创建，用户ID为null”）； } 对象[]值={ user.getPassword（）， user.getId（） }; 试一试( Connection Connection=daoFactory.getConnection（）； PreparedStatement语句=prepareStatement（连接、SQL\u更改\u密码、false、值）； ) { int affectedRows=语句.executeUpdate（）； if（affectedRows==0）{ 抛出新的DAOException（“更改密码失败，没有受影响的行”）； } }捕获（SQLE异常）{ 抛出新的异常（e）； } } //助手------------------------------------------------------------------------------------ /** *将给定结果集的当前行映射到用户。 *@param resultSet当前行要映射到用户的结果集。 *@从给定结果集的当前行返回映射用户。 *@在数据库级别出现故障时抛出SQLException。 */ 私有静态用户映射（ResultSet ResultSet）引发SQLException{ 用户=新用户（）； user.setId（resultSet.getLong（“id”）； user.setEmail（resultSet.getString（“电子邮件”）； user.setFirstname（resultSet.getString（“firstname”）； user.setLastname（resultSet.getString（“lastname”）； user.setBirthdate（resultSet.getDate（“生日”）； 返回用户； } }

我发现帖子中的一条评论有类似的问题，但我猜巴卢斯没有时间去看它。

巴卢斯正在使用语句，它会自动关闭在try块括号内打开的连接（以及语句和结果集）。

！。我错过了。如何将连接返回到池？关闭是否会自动返回到池的连接？是。池连接重写close（）方法，以便只返回池。

public void close() throws SQLException {
        if (this.connection is still eligible for reuse) {
            do not close this.connection, but just return it to pool for reuse;
        } else {
            actually invoke this.connection.close();
        }
    }

package com.example.dao;

import static com.example.dao.DAOUtil.*;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

import com.example.model.User;

/**
 * This class represents a concrete JDBC implementation of the {@link UserDAO} interface.
 *
 * @author BalusC
 * @link http://balusc.blogspot.com/2008/07/dao-tutorial-data-layer.html
 */
public class UserDAOJDBC implements UserDAO {

    // Constants ----------------------------------------------------------------------------------

    private static final String SQL_FIND_BY_ID =
        "SELECT id, email, firstname, lastname, birthdate FROM User WHERE id = ?";
    private static final String SQL_FIND_BY_EMAIL_AND_PASSWORD =
        "SELECT id, email, firstname, lastname, birthdate FROM User WHERE email = ? AND password = MD5(?)";
    private static final String SQL_LIST_ORDER_BY_ID =
        "SELECT id, email, firstname, lastname, birthdate FROM User ORDER BY id";
    private static final String SQL_INSERT =
        "INSERT INTO User (email, password, firstname, lastname, birthdate) VALUES (?, MD5(?), ?, ?, ?)";
    private static final String SQL_UPDATE =
        "UPDATE User SET email = ?, firstname = ?, lastname = ?, birthdate = ? WHERE id = ?";
    private static final String SQL_DELETE =
        "DELETE FROM User WHERE id = ?";
    private static final String SQL_EXIST_EMAIL =
        "SELECT id FROM User WHERE email = ?";
    private static final String SQL_CHANGE_PASSWORD =
        "UPDATE User SET password = MD5(?) WHERE id = ?";

    // Vars ---------------------------------------------------------------------------------------

    private DAOFactory daoFactory;

    // Constructors -------------------------------------------------------------------------------

    /**
     * Construct an User DAO for the given DAOFactory. Package private so that it can be constructed
     * inside the DAO package only.
     * @param daoFactory The DAOFactory to construct this User DAO for.
     */
    UserDAOJDBC(DAOFactory daoFactory) {
        this.daoFactory = daoFactory;
    }

    // Actions ------------------------------------------------------------------------------------

    @Override
    public User find(Long id) throws DAOException {
        return find(SQL_FIND_BY_ID, id);
    }

    @Override
    public User find(String email, String password) throws DAOException {
        return find(SQL_FIND_BY_EMAIL_AND_PASSWORD, email, password);
    }

    /**
     * Returns the user from the database matching the given SQL query with the given values.
     * @param sql The SQL query to be executed in the database.
     * @param values The PreparedStatement values to be set.
     * @return The user from the database matching the given SQL query with the given values.
     * @throws DAOException If something fails at database level.
     */
    private User find(String sql, Object... values) throws DAOException {
        User user = null;

        try (
            Connection connection = daoFactory.getConnection();
            PreparedStatement statement = prepareStatement(connection, sql, false, values);
            ResultSet resultSet = statement.executeQuery();
        ) {
            if (resultSet.next()) {
                user = map(resultSet);
            }
        } catch (SQLException e) {
            throw new DAOException(e);
        }

        return user;
    }

    @Override
    public List<User> list() throws DAOException {
        List<User> users = new ArrayList<>();

        try (
            Connection connection = daoFactory.getConnection();
            PreparedStatement statement = connection.prepareStatement(SQL_LIST_ORDER_BY_ID);
            ResultSet resultSet = statement.executeQuery();
        ) {
            while (resultSet.next()) {
                users.add(map(resultSet));
            }
        } catch (SQLException e) {
            throw new DAOException(e);
        }

        return users;
    }

    @Override
    public void create(User user) throws IllegalArgumentException, DAOException {
        if (user.getId() != null) {
            throw new IllegalArgumentException("User is already created, the user ID is not null.");
        }

        Object[] values = {
            user.getEmail(),
            user.getPassword(),
            user.getFirstname(),
            user.getLastname(),
            toSqlDate(user.getBirthdate())
        };

        try (
            Connection connection = daoFactory.getConnection();
            PreparedStatement statement = prepareStatement(connection, SQL_INSERT, true, values);
        ) {
            int affectedRows = statement.executeUpdate();
            if (affectedRows == 0) {
                throw new DAOException("Creating user failed, no rows affected.");
            }

            try (ResultSet generatedKeys = statement.getGeneratedKeys()) {
                if (generatedKeys.next()) {
                    user.setId(generatedKeys.getLong(1));
                } else {
                    throw new DAOException("Creating user failed, no generated key obtained.");
                }
            }
        } catch (SQLException e) {
            throw new DAOException(e);
        }
    }

    @Override
    public void update(User user) throws DAOException {
        if (user.getId() == null) {
            throw new IllegalArgumentException("User is not created yet, the user ID is null.");
        }

        Object[] values = {
            user.getEmail(),
            user.getFirstname(),
            user.getLastname(),
            toSqlDate(user.getBirthdate()),
            user.getId()
        };

        try (
            Connection connection = daoFactory.getConnection();
            PreparedStatement statement = prepareStatement(connection, SQL_UPDATE, false, values);
        ) {
            int affectedRows = statement.executeUpdate();
            if (affectedRows == 0) {
                throw new DAOException("Updating user failed, no rows affected.");
            }
        } catch (SQLException e) {
            throw new DAOException(e);
        }
    }

    @Override
    public void delete(User user) throws DAOException {
        Object[] values = { 
            user.getId()
        };

        try (
            Connection connection = daoFactory.getConnection();
            PreparedStatement statement = prepareStatement(connection, SQL_DELETE, false, values);
        ) {
            int affectedRows = statement.executeUpdate();
            if (affectedRows == 0) {
                throw new DAOException("Deleting user failed, no rows affected.");
            } else {
                user.setId(null);
            }
        } catch (SQLException e) {
            throw new DAOException(e);
        }
    }

    @Override
    public boolean existEmail(String email) throws DAOException {
        Object[] values = { 
            email
        };

        boolean exist = false;

        try (
            Connection connection = daoFactory.getConnection();
            PreparedStatement statement = prepareStatement(connection, SQL_EXIST_EMAIL, false, values);
            ResultSet resultSet = statement.executeQuery();
        ) {
            exist = resultSet.next();
        } catch (SQLException e) {
            throw new DAOException(e);
        }

        return exist;
    }

    @Override
    public void changePassword(User user) throws DAOException {
        if (user.getId() == null) {
            throw new IllegalArgumentException("User is not created yet, the user ID is null.");
        }

        Object[] values = {
            user.getPassword(),
            user.getId()
        };

        try (
            Connection connection = daoFactory.getConnection();
            PreparedStatement statement = prepareStatement(connection, SQL_CHANGE_PASSWORD, false, values);
        ) {
            int affectedRows = statement.executeUpdate();
            if (affectedRows == 0) {
                throw new DAOException("Changing password failed, no rows affected.");
            }
        } catch (SQLException e) {
            throw new DAOException(e);
        }
    }

    // Helpers ------------------------------------------------------------------------------------

    /**
     * Map the current row of the given ResultSet to an User.
     * @param resultSet The ResultSet of which the current row is to be mapped to an User.
     * @return The mapped User from the current row of the given ResultSet.
     * @throws SQLException If something fails at database level.
     */
    private static User map(ResultSet resultSet) throws SQLException {
        User user = new User();
        user.setId(resultSet.getLong("id"));
        user.setEmail(resultSet.getString("email"));
        user.setFirstname(resultSet.getString("firstname"));
        user.setLastname(resultSet.getString("lastname"));
        user.setBirthdate(resultSet.getDate("birthdate"));
        return user;
    }

}