死锁预防(Java+MySQL)

Question

我有一个 sql 类，它连接到数据库并更新游戏服务器中的信息。我想知道这里是否会导致死锁以及防止死锁的好方法是什么。我对很多事情都很陌生，所以任何建议都会很棒:)

package server.util;

import java.sql.*;
import java.security.MessageDigest;

import server.model.players.Client;

public class SQL {

public static Connection con = null;
public static Statement stmt;
public static boolean connectionMade;
public static void createConnection() {
    try {
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        con = DriverManager.getConnection("jdbc:mysql://URL/DATABASE", "USERNAME", "PASS");
        stmt = con.createStatement();
    } catch (Exception e) {
        e.printStackTrace();
    }
}
public static ResultSet query(String s) throws SQLException {
    try {
        if (s.toLowerCase().startsWith("select")) {
            ResultSet rs = stmt.executeQuery(s);
            return rs;
        } else {
            stmt.executeUpdate(s);
        }
        return null;
    } catch (Exception e) {
        destroyConnection();
        createConnection();
        e.printStackTrace();
    }
    return null;
}

public static void destroyConnection() {
    try {
        stmt.close();
        con.close();
        connectionMade = false;
    } catch (Exception e) {
        e.printStackTrace();
    }
}

public static boolean saveHighScore(Client clientToSave) {
    try {
        query("DELETE FROM `skills` WHERE playerName = '"+clientToSave.playerName+"';");
        query("DELETE FROM `skillsoverall` WHERE playerName = '"+clientToSave.playerName+"';");
        //query("DELETE FROM `scores` WHERE playerName = '"+clientToSave.playerName+"';");
        query("INSERT INTO `skills` (`playerName`,`Attacklvl`,`Attackxp`,`Defencelvl`,`Defencexp`,`Strengthlvl`,`Strengthxp`,`Hitpointslvl`,`Hitpointsxp`,`Rangelvl`,`Rangexp`,`Prayerlvl`,`Prayerxp`,`Magiclvl`,`Magicxp`,`Cookinglvl`,`Cookingxp`,`Woodcuttinglvl`,`Woodcuttingxp`,`Fletchinglvl`,`Fletchingxp`,`Fishinglvl`,`Fishingxp`,`Firemakinglvl`,`Firemakingxp`,`Craftinglvl`,`Craftingxp`,`Smithinglvl`,`Smithingxp`,`Mininglvl`,`Miningxp`,`Herblorelvl`,`Herblorexp`,`Agilitylvl`,`Agilityxp`,`Thievinglvl`,`Thievingxp`,`Slayerlvl`,`Slayerxp`,`Farminglvl`,`Farmingxp`,`Runecraftlvl`,`Runecraftxp`,`Hunterlvl`,`Hunterxp`,`Summonlvl`,`Summonxp`) VALUES ('"+clientToSave.playerName+"',"+clientToSave.playerLevel[0]+","+clientToSave.playerXP[0]+","+clientToSave.playerLevel[1]+","+clientToSave.playerXP[1]+","+clientToSave.playerLevel[2]+","+clientToSave.playerXP[2]+","+clientToSave.playerLevel[3]+","+clientToSave.playerXP[3]+","+clientToSave.playerLevel[4]+","+clientToSave.playerXP[4]+","+clientToSave.playerLevel[5]+","+clientToSave.playerXP[5]+","+clientToSave.playerLevel[6]+","+clientToSave.playerXP[6]+","+clientToSave.playerLevel[7]+","+clientToSave.playerXP[7]+","+clientToSave.playerLevel[8]+","+clientToSave.playerXP[8]+","+clientToSave.playerLevel[9]+","+clientToSave.playerXP[9]+","+clientToSave.playerLevel[10]+","+clientToSave.playerXP[10]+","+clientToSave.playerLevel[11]+","+clientToSave.playerXP[11]+","+clientToSave.playerLevel[12]+","+clientToSave.playerXP[12]+","+clientToSave.playerLevel[13]+","+clientToSave.playerXP[13]+","+clientToSave.playerLevel[14]+","+clientToSave.playerXP[14]+","+clientToSave.playerLevel[15]+","+clientToSave.playerXP[15]+","+clientToSave.playerLevel[16]+","+clientToSave.playerXP[16]+","+clientToSave.playerLevel[17]+","+clientToSave.playerXP[17]+","+clientToSave.playerLevel[18]+","+clientToSave.playerXP[18]+","+clientToSave.playerLevel[19]+","+clientToSave.playerXP[19]+","+clientToSave.playerLevel[20]+","+clientToSave.playerXP[20]+","+clientToSave.playerLevel[21]+","+clientToSave.playerXP[21]+","+clientToSave.playerLevel[22]+","+clientToSave.playerXP[22]+");");
        query("INSERT INTO `skillsoverall` (`playerName`,`lvl`,`xp`) VALUES ('"+clientToSave.playerName+"',"+(clientToSave.getLevelForXP(clientToSave.playerXP[0]) + clientToSave.getLevelForXP(clientToSave.playerXP[1]) + clientToSave.getLevelForXP(clientToSave.playerXP[2]) + clientToSave.getLevelForXP(clientToSave.playerXP[3]) + clientToSave.getLevelForXP(clientToSave.playerXP[4]) + clientToSave.getLevelForXP(clientToSave.playerXP[5]) + clientToSave.getLevelForXP(clientToSave.playerXP[6]) + clientToSave.getLevelForXP(clientToSave.playerXP[7]) + clientToSave.getLevelForXP(clientToSave.playerXP[8]) + clientToSave.getLevelForXP(clientToSave.playerXP[9]) + clientToSave.getLevelForXP(clientToSave.playerXP[10]) + clientToSave.getLevelForXP(clientToSave.playerXP[11]) + clientToSave.getLevelForXP(clientToSave.playerXP[12]) + clientToSave.getLevelForXP(clientToSave.playerXP[13]) + clientToSave.getLevelForXP(clientToSave.playerXP[14]) + clientToSave.getLevelForXP(clientToSave.playerXP[15]) + clientToSave.getLevelForXP(clientToSave.playerXP[16]) + clientToSave.getLevelForXP(clientToSave.playerXP[17]) + clientToSave.getLevelForXP(clientToSave.playerXP[18]) + clientToSave.getLevelForXP(clientToSave.playerXP[19]) + clientToSave.getLevelForXP(clientToSave.playerXP[20]) + clientToSave.getLevelForXP(clientToSave.playerXP[21]) + clientToSave.getLevelForXP(clientToSave.playerXP[22]))+","+((clientToSave.playerXP[0]) + (clientToSave.playerXP[1]) + (clientToSave.playerXP[2]) + (clientToSave.playerXP[3]) + (clientToSave.playerXP[4]) + (clientToSave.playerXP[5]) + (clientToSave.playerXP[6]) + (clientToSave.playerXP[7]) + (clientToSave.playerXP[8]) + (clientToSave.playerXP[9]) + (clientToSave.playerXP[10]) + (clientToSave.playerXP[11]) + (clientToSave.playerXP[12]) + (clientToSave.playerXP[13]) + (clientToSave.playerXP[14]) + (clientToSave.playerXP[15]) + (clientToSave.playerXP[16]) + (clientToSave.playerXP[17]) + (clientToSave.playerXP[18]) + (clientToSave.playerXP[19]) + (clientToSave.playerXP[20]) + (clientToSave.playerXP[21]) + (clientToSave.playerXP[22]))+");");
        //query("INSERT INTO `scores` (`playerName`,`killcount`,`pkpoints`,`pcpoints`) VALUES ('"+clientToSave.playerName+"',"+clientToSave.KC+","+clientToSave.pkPoints+","+clientToSave.pcPoints+");");
        } catch (Exception e) {
        e.printStackTrace();
        return false;
    }
    return true;
}
}

Answer 1

您需要担心 Java 端的同步，因为 java.sql 实现不是线程安全的。您需要考虑数据库服务器上的隔离，平衡响应性与 ACID。

我会为您的 Java 类推荐一些其他内容：

1. 使用连接池。破坏和创建连接的成本非常昂贵。您的 SQL 类不应该处理此类琐事。
2. 使用PreparedStatement 并绑定变量。以这种方式创建查询字符串是一个坏主意。
3. 在finally 块中关闭资源。

所有这些静态字符串......我正在手机上阅读这篇文章，但我看到的并不好。您可能严重违反了第一范式。

Answer 2

死锁通常是由同步不良的代码引起的。最典型的情况是线程 A 获取资源 X 的锁，线程 B 获取资源 Y 的锁，然后两个线程都等待，同时尝试获取尚未锁定的资源。

由于您的代码根本没有同步，所以应该没问题。

为了避免死锁问题，请尝试在同步块内执行尽可能少的工作。在同步块内调用类外部的代码时要非常小心，因为该外部代码可能会尝试获取锁并产生死锁。请记住，执行数据库查询是对外部代码的调用，并且数据库查询锁定数据库内部的资源并不罕见。

您是否真的看到了僵局，或者只是出于谨慎？