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Is the MySQL procedure in this Minecraft plugin correct?

To get an idea of what the basic structure looks like, I downloaded a money system including MySQL from Spigot and looked at the code.
public static boolean playerExists(String uuid) {
try {
ResultSet rs = Simplecoinsystem.mysql.query("SELECT * FROM CoinData WHERE UUID= '" + uuid + "'");
if (rs.next())
return (rs.getString("UUID") != null);
return false;
} catch (SQLException e) {
e.printStackTrace();
return false;
}
}
public static void createPlayer(String uuid) {
if (!playerExists(uuid))
Simplecoinsystem.mysql.update("INSERT INTO CoinData (UUID, COINS) VALUES ('" + uuid +
"', '" + Simplecoinsystem.getInstance().getConfig().getInt("startcoins") + "');");
}
public static Integer getCoins(String uuid) {
Integer i = Integer.valueOf(0);
if (playerExists(uuid)) {
try {
ResultSet rs = Simplecoinsystem.mysql.query("SELECT * FROM CoinData WHERE UUID= '" + uuid + "'");
if (rs.next())
Integer.valueOf(rs.getInt("COINS"));
i = Integer.valueOf(rs.getInt("COINS"));
} catch (SQLException e) {
e.printStackTrace();
}
} else {
createPlayer(uuid);
}
return i;
}
public static void setCoins(String uuid, Integer coins) {
if (playerExists(uuid)) {
Simplecoinsystem.mysql.update("UPDATE CoinData SET COINS= '" + coins + "' WHERE UUID= '" + uuid + "';");
} else {
createPlayer(uuid);
}
}
Am I correct that it is actually impractical to create a new entry with the uuid of the non-existent player after each query of the coins if the player does not exist?
Wouldn't this make it possible to flood the database with thousands of unnecessary entries by issuing, for example, a "/money (player)" command as an evil player/admin?
Couldn't I just ask when entering the server if the uuid is already stored and if not, just enter it? This way there would only be entries from players who have already been on the server before. Whether this needs great server performance, I'm not sure.
This is my first own MySQL class.
public class MySQL {
private String host, database, user, password;
private int port;
private Connection con;
public MySQL(String host, int port, String database, String user, String password) {
this.host = host;
this.port = port;
this.database = database;
this.user = user;
this.password = password;
connect();
}
public void connect() {
try {
con = DriverManager.getConnection("jdbc:mysql://" + host + ":" + port + "/" + database + "?autoReconnect=true", user, password);
System.out.println("&cDie MySQL Verbindung wurde erfolgreich aufgebaut!");
} catch (SQLException e) {
e.printStackTrace();
}
}
public void disconnect() {
try {
if(this.con != null) {
this.con.close();
System.out.println("§cDie MySQL Verbindung wurde erfolgreich beendet!");
}
} catch (Exception e) {
e.printStackTrace();
}
}
public void update(String query) {
try {
Statement st = con.createStatement();
st.executeUpdate(query);
st.close();
} catch (Exception e) {
e.printStackTrace();
connect();
}
}
public ResultSet qry(String query) {
ResultSet rs = null;
try {
Statement st = con.createStatement();
rs = st.executeQuery(query);
} catch (Exception e) {
e.printStackTrace();
connect();
}
return rs;
}
public Connection getConnection() {
return this.con;
}
}
Except for this part, both MySQL classes are built relatively the same.
This is the part that is in the MySQL class of the Spigot plugin.

Your code have multiple issues.
When the connection will be closed, next time you will have an error. In your Mysql class, I suggest you to do:
public Connection getConnection() {
if(con == null || con.isClosed())
connect();
return con;
}
Then, use it in all method like getConnection().prepareStatement().
You can be attacked with SQL Injection. To fix this, try to do something like:
PreparedStatement st = con.prepareStatement("SELECT * FROM CoinData WHERE UUID = ?");
st.setString(1, uuid.toString()); // Yes it start at 1 !!
st.executeUpdate();
With this, even with all values, you can't be attacked with injections.
You will have an error while getting coins:
if (rs.next()) // go to good line
Integer.valueOf(rs.getInt("COINS")); // useless convertion
i = Integer.valueOf(rs.getInt("COINS")); // error if no line.
You can just do:
if(rs.next())
i = rs.getInt("COINS");
If the column "UUID" is unique, you will not have duplicated lines.
Finally, about performance, it's better to do it one time: at login, instead of all time. You can also create an object stored in an hashmap to easier access to it, without using SQL, like that:
public static HashMap<UUID, Integer> coinsByPlayer = new HashMap<>();
OR:
public static HashMap<UUID, MyObject> coinsByPlayer = new HashMap<>();
public class MyObject {
private int coins = 0;
public MyObject(UUID uuid) {
// make SQL request to get data
}
public int getCoins() {
return coins;
}
public void setCoins(int next){
coins = next;
// here make "UPDATE" sql query
}
}

What do you say? Is it ok with the try/catch function? #Elikill58
public Connection getConnection() {
try {
if(con == null || con.isClosed()) {
connect();
}
} catch (SQLException e) {
e.printStackTrace();
}
return con;
}
edit:
public Connection getConnection_one() throws SQLException {
if(con == null || con.isClosed()) {
connect();
return con;
} else {
return con;
}
}

Discord bot unusually repeats

So I'm trying to create a discord bot that has simple access to a database for printing out values, my code currently will print the values to the discord server but it repeats them 5 times.
Bot functionality class:
private MySQLAccess sql = new MySQLAccess();
public static void main(String[] args) throws Exception {
JDABuilder ark = new JDABuilder(AccountType.BOT);
ark.setToken("insert_discord_token_here");
ark.addEventListener(new MessageListener());
ark.buildAsync();
}
#Override
public void onMessageReceived(MessageReceivedEvent e) {
if (e.getAuthor().isBot()) return;
Message msg = e.getMessage();
String str = msg.getContentRaw();
//Ping pong
if (str.equalsIgnoreCase("!ping")) {
e.getChannel().sendMessage("Pong!").queue();
}
//Bal check
if (str.contains("!bal")) {
String user = str.substring(5);
System.out.println(user);
try {
sql.readDataBase(e.getChannel(), user);
} catch (Exception e1) {
}
}
}
Database Access Class:
private Connection connect = null;
private Statement statement = null;
private ResultSet resultSet = null;
private final String user = "pass";
private final String pass = "user";
public void readDataBase(MessageChannel msg, String username) throws Exception {
//Retrieve data and search for username
try {
Class.forName("com.mysql.cj.jdbc.Driver");
connect = DriverManager.getConnection("jdbc:mysql://localhost/serverusers?allowPublicKeyRetrieval=true&useSSL=false", user, pass);
statement = connect.createStatement();
resultSet = statement
.executeQuery("select * from serverusers.userinfo where user=\"" + username + "\"");
writeResultSet(resultSet, msg);
} catch (Exception e) {
throw e;
} finally {
close();
}
}
private void writeResultSet(ResultSet resultSet, MessageChannel msg) throws SQLException {
// Check resultSet and print its contents
if (resultSet.next()) {
String user = resultSet.getString(2);
Double website = resultSet.getDouble(3);
msg.sendMessage("User: " + user).queue();
msg.sendMessage("Bank Amount: " + website).queue();
}
}
private void close() {
try {
if (resultSet != null) {
resultSet.close();
}
if (statement != null) {
statement.close();
}
if (resultSet != null) {
resultSet.close();
}
if (connect != null) {
connect.close();
}
} catch (Exception e) {
}
}
When the program is run it finds the correct data that I'm looking for and the search function is fine, but for some odd reason the program will spit the same username and balance out 5 times.
Screenshot of Discord Bot

The common mistake here is that you run the program multiple times, each instance then responds accordingly with the same thing. You can check if that is the case by opening the task manager and looking for java processes. This often occurs with developers using the Eclipse IDE because of the console hiding other processes behind a drop-down menu on the console.

Unable to catch onFailure() GWT AsyncCallback

I am creating an appliaction which requires user authentication. I have one problem when I'm trying to log in. When I type a correct username and password, the onSuccess method is called. But when I type a wrong one, or empty fields, then the onFailure() method is NOT called.
I really want to know why this is happening. Because I wan't to display some sort of dialogbox when the username or password is incorrect.
This is the ClickHandler, which takes the username and password from the fields:
loginButton.addClickHandler(new ClickHandler() {
#Override
public void onClick(ClickEvent event) {
String username = usernameBox.getText();
String password = passwordBox.getText();
performUserConnection(username, password);
}
});
And this is the method that performs the user conenction, which as I said, works if I have a correct username/ password. It displays my alert message, but it does not display any alert message if it's not correct.
private static void performUserConnection(String username, String password) {
DBConnectionAsync rpcService = (DBConnectionAsync) GWT.create(DBConnection.class);
ServiceDefTarget target = (ServiceDefTarget) rpcService;
String moduleRelativeURL = GWT.getModuleBaseURL() + "DBConnectionImpl";
target.setServiceEntryPoint(moduleRelativeURL);
rpcService.authenticateUser(username, password, new AsyncCallback<User>() {
#Override
public void onSuccess(User user) {
Window.alert("TRALALA. Username: " + user.getUsername());
}
#Override
public void onFailure(Throwable caught) {
Window.alert("LALALALAL");
// DialogBox dialogBox = createDialogBox();
// dialogBox.setGlassEnabled(true);
// dialogBox.setAnimationEnabled(true);
// dialogBox.center();
// dialogBox.show();
}
});
}
UPDATE Server Part.
public class DBConnectionImpl extends RemoteServiceServlet implements DBConnection {
private static final long serialVersionUID = 1L;
private String URL = new String("jdbc:mysql://localhost:3306");
private String user = "root";
private String pass = "andrei";
private String schema = "administrare_bloc";
public DBConnectionImpl() {
}
private Connection getConnection() throws Exception {
Properties props = new Properties();
props.setProperty("user", user);
props.setProperty("password", pass);
props.setProperty("zeroDateTimeBehavior", "convertToNull");
Class.forName("com.mysql.jdbc.Driver").newInstance();
Connection conn = DriverManager.getConnection(URL + "/" + schema, props);
return conn;
}
#Override
public User authenticateUser(String username, String password) throws Exception {
User returnUser = null;
Connection conn = null;
PreparedStatement stmt = null;
ResultSet rs = null;
try {
conn = getConnection();
try {
String q = "select * from users where username=? and password=?";
stmt = conn.prepareStatement(q);
stmt.setString(1, username);
stmt.setString(2, password);
rs = stmt.executeQuery();
while (rs.next()) {
int id = rs.getInt("id");
String user = rs.getString("username");
String pass = rs.getString("password");
String type = rs.getString("type");
returnUser = new User(id, user, pass, type);
}
} catch (Exception ex) {
ex.printStackTrace();
}
} catch (SQLException ex) {
ex.printStackTrace();
} finally {
rs.close();
stmt.close();
conn.close();
}
return returnUser;
}
}
Thanks in advance

The onFailure method will only be Called if you throw an exception on the server. Now you just return a null object if no user is found.

simple login form using java and db2 . everything ok but still resultset empty

I am trying to make a simple login form. Every thing is working fine, connection is established, query is executed but ResultSet is still empty so always getting redirected to fail.jsp. No error no warning at all.
Servlet code:
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String name = request.getParameter("name");
String password = request.getParameter("password");
modelclass md = new modelclass();
daoclass dao = new daoclass();
md.setName(name);
md.setPassword(password);
System.out.println("this just before the sql query on the main servlet page");
String sql = "Select * from USERADD where name = ? and password= ?";
String result = dao.guser(md, sql);
if (result.equals("success")) {
response.sendRedirect("welcome.jsp");
} else {
response.sendRedirect("fail.jsp");
}
}
This is the DAO class which makes connection.
Data Access code(dao.java):
public class daoclass {
public static String username = "NickNeo";
public static String password = "123123";
public static String driver = "com.ibm.db2.jcc.DB2Driver";
public static String url = "jdbc:db2://localhost:50000/CITYLIFE";
public static Connection con = null;
public static PreparedStatement ps = null;
static {
try {
Class.forName(driver);
System.out.println("before connection");
con = DriverManager.getConnection(url, username, password);
System.out.println("Connection Successfullll......!!!!!!");
} catch(Exception e) {
e.printStackTrace();
}
}
public String guser(modelclass obj, String sql) {
try {
System.out.println("entry into try block");
ps=con.prepareStatement(sql);
ps.setString(1, obj.getName());
ps.setString(2, obj.getPassword());
System.out.println("before query");
ResultSet rs = ps.executeQuery();
System.out.println("query executed");
int i = 0;
while(rs.next()) {
System.out.println("entered into while loop");
++i;
}
if (i >= 1) {
return "success";
} else {
System.out.println("this is inside else of while block");
return "fail";
}
} catch(Exception e) {
e.printStackTrace();
}
System.out.println("this is the most outer fail statement");
return "fail";
}
}
the rs is always empty. tried many things but still getting rs as empty. please help

Failed to Start Derby Database

So I'm quite new to Java and Derby. I'm using both with my Flex app on Tomcat 7.
When I make a call to Java from Flex the login function works fine but my getUserByUsername function does not.
public Boolean loginUser(String username, String password) throws Exception
{
Connection c = null;
String hashedPassword = new String();
try
{
c = ConnectionHelper.getConnection();
PreparedStatement ps = c.prepareStatement("SELECT password FROM users WHERE username=?");
ps.setString(1, username);
ResultSet rs = ps.executeQuery();
if(rs.next())
{
hashedPassword = rs.getString("password");
}
else
{
return false;
}
if(Password.check(password, hashedPassword))
{
return true;
}
else
{
return false;
}
}
catch (SQLException e)
{
e.printStackTrace();throw new DAOException(e);
}
finally
{
ConnectionHelper.closeConnection(c);
}
}
public User getUserByUsername(String username) throws DAOException
{
//System.out.println("Executing DAO.getUserByName:" + username);
User user = new User();
Connection c = null;
try
{
c = ConnectionHelper.getConnection();
PreparedStatement ps = c.prepareStatement("SELECT * FROM users WHERE username = ?");
ps.setString(1, username);
ResultSet rs = ps.executeQuery();
while(rs.next())
{
user.setId(rs.getInt("id"));
user.setUsername(rs.getString("username"));
user.setPassword(rs.getString("password"));
user.setTeam(rs.getString("team"));
user.setScore(rs.getInt("score"));
}
}
catch (SQLException e)
{
e.printStackTrace();
throw new DAOException(e);
}
finally
{
ConnectionHelper.closeConnection(c);
}
return user;
}
The stack I get in Flex is useless as far as I can tell:
Flex Message (flex.messaging.messages.ErrorMessage) clientId = 8EB6D37B-7E0B-01B0->AA55-457722B9036C correlationId = A39E574F-CFC6-51FE-6CBE-451AF329E2F8 destination >= service messageId = 8EB6DF4C-650B-BDD7-7802-B813A61C8DC8 timestamp = >1401318734645 timeToLive = 0 body = null code = Server.Processing message = >services.DAOException : java.sql.SQLException: Failed to start database >'/Applications/blazeds/tomcat/webapps/testdrive/WEB-INF/database/game_db', see the next >exception for details. details = null rootCause = ASObject(23393258)>>{message=java.sql.SQLException: Failed to start database >'/Applications/blazeds/tomcat/webapps/testdrive/WEB-INF/database/game_db', see the next >exception for details., suppressed=[], localizedMessage=java.sql.SQLException: Failed to >start database '/Applications/blazeds/tomcat/webapps/testdrive/WEB->INF/database/game_db', see the next exception for details., cause=java.sql.SQLException} >body = null extendedData = null
My first thought was that it was just an error in my function (maybe someone else will notice it) but I've been looking through it for a couple hours and I can't see anything.
After that I thought maybe Derby had a problem with concurrent connections. I saw somewhere that Embedded JDBC can only handle one connection so I changed the driver from Embedded to Client which once again resulted in the login function working and the other an error saying the url in the connection was null. Any thoughts? Thanks ahead of time for any ideas.
EDIT:
package services;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.net.URLDecoder;
public class ConnectionHelper
{
private String url;
private static ConnectionHelper instance;
public String getUrl()
{
return url;
}
private ConnectionHelper()
{
try
{
Class.forName("org.apache.derby.jdbc.EmbeddedDriver").newInstance();
String str = URLDecoder.decode(getClass().getClassLoader().getResource("services").toString(),"UTF-8");
str= str.substring(0, str.indexOf("classes/services"));
if ( str.startsWith("file:/C:",0)){
str=str.substring(6);
}
else{
str=str.substring(5);
}
url = "jdbc:derby:" + str + "database/game_db";
System.out.println("Database url "+url);
}
catch (Exception e)
{
e.printStackTrace();
}
}
public static ConnectionHelper getInstance()
{
if (instance == null)
instance = new ConnectionHelper();
return instance;
}
public static Connection getConnection() throws java.sql.SQLException
{
return DriverManager.getConnection(getInstance().getUrl());
}
public static void closeConnection(Connection c)
{
try
{
if (c != null)
{
c.close();
}
}
catch (SQLException e)
{
e.printStackTrace();
}
}
}

There is NO problem with multiple connections in embedded mode. Full stop.
That said, what you may have come across, is that only one jvm process can access the Derby database files at a time. But that jvm may well have 1000s of threads each with their own connection to Derby (resources permitting, of course).