Question

i have an sql class that connects to the database and updates the information from my game server. I was wondering if anything here would cause a deadlock and what are good ways to prevent deadlocks. I am new to alot of things so any tips would be great 🙂

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

You need to worry about synchronization on the Java side, because java.sql implementations are not thread safe. And you need to think about isolation on the database server, balancing responsiveness with ACID.

I’d recommend a few other things for your Java class:

1. Use a connection pool. Destroying and creating connections is very expensive. Your SQL class should not be handling such chores.
2. Use PreparedStatement and bind variables. Creating a query string that way is a bad idea.
3. Close resources in a finally block.

All those static strings…I’m reading this on a mobile phone, but what I’m seeing is not good. You may have a serious violation of 1st normal form.