JDBC的DBUtil 和 xxxDAO 示例

DBUtil

java 复制代码

package common;

import com.mysql.jdbc.jdbc2.optional.MysqlDataSource;

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DBUtil {
    // 需要封装和数据库之间的连接操作.
    private static final String URL = "jdbc:mysql://127.0.0.1:3306/xxx?characterEncoding=utf8&useSSL=false";
    private static final String USERNAME = "xxx";
    private static final String PASSWORD = "xxx";

    private static volatile DataSource dataSource = null;

    private static DataSource getDataSource() {
        if (dataSource == null) {
            synchronized (DBUtil.class) {
                if (dataSource == null) {
                    MysqlDataSource mysqlDataSource = new MysqlDataSource();
                    mysqlDataSource.setURL(URL);
                    mysqlDataSource.setUser(USERNAME);
                    mysqlDataSource.setPassword(PASSWORD);
                    dataSource = mysqlDataSource;
                }
            }
        }
        return dataSource;
    }

    public static Connection getConnection() throws SQLException {
        return getDataSource().getConnection();
    }

    public static void close(Connection connection, PreparedStatement statement, ResultSet resultSet) {
        if (resultSet != null) {
            try {
                resultSet.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (statement != null) {
            try {
                statement.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

xxxDAO

java 复制代码

public class ProblemDAO {
    public void insert(Problem problem) {
        Connection connection = null;
        PreparedStatement statement = null;
        try {
            // 1. 和数据库建立连接
            connection = DBUtil.getConnection();
            // 2. 构造 SQL 语句
            String sql = "insert into xxx values(null, ?, ?, ?, ?, ?)";
            statement = connection.prepareStatement(sql);
            statement.setString(1, problem.getTitle());
            statement.setString(2, problem.getLevel());
            statement.setString(3, problem.getDescription());
            statement.setString(4, problem.getTemplateCode());
            statement.setString(5, problem.getTestCode());
            // 3. 执行 SQL
            int ret = statement.executeUpdate();
            if (ret != 1) {
                System.out.println("题目新增失败!");
            } else {
                System.out.println("题目新增成功!");
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.close(connection, statement, null);
        }
    }

    public void delete(int id) {
        Connection connection = null;
        PreparedStatement statement = null;
        try {
            // 1. 和数据库建立连接
            connection = DBUtil.getConnection();
            // 2. 拼装 SQL 语句
            String sql = "delete from xxx where id = ?";
            statement = connection.prepareStatement(sql);
            statement.setInt(1, id);
            // 3. 执行 SQL
            int ret = statement.executeUpdate();
            if (ret != 1) {
                System.out.println("删除题目失败!");
            } else {
                System.out.println("删除题目成功!");
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        } finally {
            DBUtil.close(connection, statement, null);
        }
    }

    // 这个操作是把当前题目列表中的所有题都查出来了
    // 万一数据库中的题目特别多, 咋办? 只要实现 "分页查询" 即可. 后台实现分页查询, 非常容易.
    // 前端传过来一个当前的 "页码" , 根据页码算一下, 依据 sql limit offset 语句, 要算出来 offset 是 几
    // 但是前端这里实现一个分页器稍微麻烦一些(比后端要麻烦很多). 此处暂时不考虑分页功能.
    public List<Problem> selectAll() {
        List<Problem> problems = new ArrayList<>();
        Connection connection = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try {
            // 1. 和数据库建立连接
            connection = DBUtil.getConnection();
            // 2. 拼装 SQL
            String sql = "select id, title, level from xxx";
            statement = connection.prepareStatement(sql);
            // 3. 执行 SQL
            resultSet = statement.executeQuery();
            // 4. 遍历 resultSet
            while (resultSet.next()) {
                // 每一行都是一个 Problem 对象
                Problem problem = new Problem();
                problem.setId(resultSet.getInt("id"));
                problem.setTitle(resultSet.getString("title"));
                problem.setLevel(resultSet.getString("level"));
                problems.add(problem);
            }
            return problems;
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        } finally {
            DBUtil.close(connection, statement, resultSet);
        }
        return null;
    }

    public Problem selectOne(int id) {
        Connection connection = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try {
            // 1. 和数据库建立连接
            connection = DBUtil.getConnection();
            // 2. 拼接 SQL 语句
            String sql = "select * from oj_table where id = ?";
            statement = connection.prepareStatement(sql);
            statement.setInt(1, id);
            // 3. 执行 SQL
            resultSet = statement.executeQuery();
            // 4. 遍历查询结果. (由于 id 是主键, 按照 id 查找的结果一定是唯一的)
            if (resultSet.next()) {
                Problem problem = new Problem();
                problem.setId(resultSet.getInt("id"));
                problem.setTitle(resultSet.getString("title"));
                problem.setLevel(resultSet.getString("level"));
                problem.setDescription(resultSet.getString("description"));
                problem.setTemplateCode(resultSet.getString("templateCode"));
                problem.setTestCode(resultSet.getString("testCode"));
                return problem;
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        } finally {
            DBUtil.close(connection, statement, resultSet);
        }
        return null;
    }