java - 关闭 DAO 层中 jdbc 中结果集的连接

Question

我正在读这个excellent post来自 BalusC ，享有盛誉且活跃的 StackOverflow 成员。

在帖子中，我发现数据库访问完成后，connection 和resultSet 都关闭了。我很确定我遗漏了一些东西或被忽略了。我只想知道连接是如何关闭的。

事实上，在帖子的最后，他提到了代码如何处理连接返回池。但我不明白这是怎么发生的。

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();
        }
    }

这里是相关类(尊重 BalusC)

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;
    }

}

我发现帖子中的一条评论有类似的问题，但我猜，BalusC 没有时间看它。

Answer 1

BalusC 使用 try-with-resources语句，它会自动关闭在 try block 的括号内打开的连接(以及语句和结果集)。