Develop Reference

H2 Database, problems getting data from tables

I'm creating a JavaFX application, I've connected to the database fine. However when i look to get data from the tables i get the error
org.h2.jdbc.JdbcSQLException: Table "LECTURE" not found; SQL
statement: SELECT NAME FROM Lecture [42102-192]
and I'm 100% sure i'm connected to the database and the table is definitely there, any suggestions on why this is?
hear is my connection code and the code i am running just so you can see
public class ConnectionFactory {
//static reference to itself
private static ConnectionFactory instance = new ConnectionFactory();
public static final String URL = "jdbc:h2:file:~/db\\.";
public static final String USER = "notepad";
public static final String PASSWORD = "password";
public static final String DRIVER_CLASS = "org.h2.Driver";
//private constructor
private ConnectionFactory() {
try {
Class.forName(DRIVER_CLASS);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
private Connection createConnection() {
Connection connection = null;
try {
connection = DriverManager.getConnection(URL, USER, PASSWORD);
} catch (SQLException e) {
System.out.println("ERROR: Unable to Connect to Database.");
}
return connection;
}
public static Connection getConnection() {
return instance.createConnection();
}
}
And the query being run
private void onLoadYearSelect() {
try {
Connection con = ConnectionFactory.getConnection();
Statement stat = con.createStatement();
String query = "SELECT NAME FROM Lecture";
ResultSet years = stat.executeQuery(query);
while(years.next()){
yearSelect.setValue(years.getString("NAME"));
System.out.println(years.getString("NAME"));
}
} catch (SQLException e) {
e.printStackTrace();
}
}
public void initialize(){
onLoadYearSelect();
}

If it says the table doesn't exist, then it really doesn't exist.
Most likely, you are not actually connecting to the correct database. In fact, by default, if the connection string points to a non-existent database, it just creates an empty database on the fly, which would explain your error.
It's probably too late now (because there is probably a 2nd database created already somewhere), but to avoid this confusion, it's not a bad idea to include IFEXISTS=TRUE in the connection string so that it fails if the database doesn't exist, rather than creating an empty one that will mask the true problem.
public static final String URL = "jdbc:h2:file:~/db\\.;IFEXISTS=TRUE";
However, one thing you can still try to debug the problem, is to add IFEXISTS=TRUE to the connection string. Then move or rename the database you think it should be connecting to so as to make the connection string invalid. Basically, force it to fail. If the code still connects to the database successfully, then you'll know the connection string is not pointing to the location you think it is.

MYSQL shows too many processes from java application

i have a java application which connects to mysql database using MYSQL connector. problem is when application started, MYSQL process list shows many connections than i requested in process list (attached image).
i have two threads running which connects to database within 5 seconds and 11 seconds. but, when i refresh mysql process list, it shows server's host ports are changing rapidely than threads are running. normally its changing 3-5 ports per second. can someone please guide me any optimizing issues or any changes to test with this?
thanks
P.S.
I have created a class which connects to DB at initialization and that class's object is in a places where needs DB connectivity. and that class having all methods which using to query from DB.
EDIT
my database connectivity class code is
public class Data{
static Connection con; //create connection
static Statement stmt; //create statement
static ResultSet rs; //create result set
static HostRead hr = new HostRead();
static int db_port = 3306;
static String db_root = "127.0.0.1";
static String db_name = "chsneranew";
static String db_user = "root";
static String db_pass = "";
/**Constructer method*/
public Data(){
this(db_root,db_port,db_name,db_user,db_pass);
if(getConnection()==null){
System.out.println("error in database connection");
}
else{
con = getConnection();
}
}
protected void finalize() throws Throwable {
try {
System.out.println("desctroyed");
con.close();
} finally {
super.finalize();
}
}
public static Connection getConnection(){
try{
Class.forName("com.mysql.jdbc.Driver").newInstance();
Connection conn = DriverManager.getConnection("jdbc:mysql://"+db_root+":"+db_port+"/"+db_name, db_user, db_pass);
stmt = conn.createStatement();
return conn;
}
catch(ClassNotFoundException er){
JOptionPane.showMessageDialog(null,"Error found ...\nDataBase Driver error (Invalid Drivers)\nUsers Cant login to system without database\n\nContact System Administrator","Error",JOptionPane.ERROR_MESSAGE);
return null;
}
catch(Exception er){
JOptionPane.showMessageDialog(null,"Error found ...\nDataBase Access error (Invalid Authentication)\nOr\nDataBase not found. Details are not be loaded \n\nUsers Cant login to system without database\n\nContact System Administrator","Error",JOptionPane.ERROR_MESSAGE);
return null;
}
}
public String getUserName(){
try{
Statement stmt2 = getConnection().createStatement();
ResultSet rss2;
String sql = "SELECT name FROM gen";
rss2 = stmt2.executeQuery(sql);
if(rss2.next()){
return rss2.getString("name");
}
}
catch(Exception er){
er.printStackTrace();
}
return null;
}
}
i am calling getUserName()method in my threads. using
Data d = new Data();
d.getUserName();

conn.close();
You need to close the connection, the connection is not closed that is why it is still there in the list. You need to Connection conn above so that it may be visible to rest of the code.

You are calling the getConnection() method three times when you want to read the data via the getUserName() method. Two times in the constructor when your constructor of the Data class is called (one for the if(...) check, one for the con = getConnection() line) and one time when you actually want to read the data at the getConnection().createStatement() line. So you have three connections to the database, and that is just the getUserName method...
Rewrite your code that only one connection is established and this connection is reused for any further execution.

How can I write string to container to be used after a loop?

I have an aplication which create a number of query (update or insert) and then each query is executed.
The whole code is working fine but I've saw that my server IO latency is too much during this proccess.
The code execute a loop which is taking arround 1 minute.
Then what I wanted to do is write each query in memory instead to execute it, and then, once I have the whole list of query to execute, use "LOAD DATA LOCAL INFILE" from mysql, which will take less time.
My question is: How can I write all my query (String object) in a "File" or "any other container" in java to use it after the loop?.
#user3283548 This is my example code:
Class1:
import java.util.ArrayList;
public class Class1 {
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
ArrayList<String> Staff=new ArrayList<String>();
Staff.add("tom");
Staff.add("Laura");
Staff.add("Patricia");
for (int x = 0; x < Staff.size(); x++) {
System.out.println(Staff.get(x));
Class2 user = new Class2 (Staff.get(x));
user.checkUser();
}
}
}
Class2:
public class Class2 {
private String user;
public Class2(String user){
this.user=user;
}
public void checkUser() throws Exception{
if (user.equals("tom")){
String queryUser="update UsersT set userStatus='2' where UserName='"+user+"';";
Class3 updateUser = new Class3(queryUser);
updateUser.UpdateQuery();;
}else{
String queryUser="Insert into UsersT (UserName,userStatus)Values('"+user+"','1');";
Class3 updateUser = new Class3(queryUser);
updateUser.InsertQuery();
System.out.println(user+" is not ton doing new insert");
}
}
}
Class3:
public class Class3 {
public String Query;
public Class3(String Query){
this.Query = Query;
}
public void UpdateQuery() throws Exception{
/*// Accessing Driver From Jar File
Class.forName("com.mysql.jdbc.Driver");
//DB Connection
Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/default","root","1234567");
String sql =Query;
PreparedStatement pst = con.prepareStatement(sql);*/
System.out.println(Query); //Just to test
//pst.execute();
}
public void InsertQuery() throws Exception{
/*// Accessing Driver From Jar File
Class.forName("com.mysql.jdbc.Driver");
//DB Connection
Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/default","root","1234567");
String sql =Query;
PreparedStatement pst = con.prepareStatement(sql);*/
System.out.println(Query); //Just to test
//pst.execute();
}
}
Then, what I wanted to do is create an ArraList in Class1 and use it in Class3 to collect all the queries which has to be executed.
The idea is to execute the list of queries in one time, once the main process is finished, istead to do it for each element within in loop of the Class1. I wanted to do it, because I think it will be take less resource IO from the server HD

Your loop is probably too slow because you're building up Strings using String
I'd hazard a guess you're doing things like
String query = "SELECT * FROM " + variablea + " WHERE + variableb + " = " ...
If you're doing a lot of string concatenation then use StringBuilder as every time you change a string it is actually re-created which is expensive. Simply changing your code to use StringBuilder instead of string will probably cut your loop executed time to a couple of MS. Simply call .toString() method of StringBuilder obj to get the string.
Storing objects
If you want to store anything for later use you should store it in a Collection. If you want a a key-value relationship then use a Map (HashMap would suit you fine). If you just want the values use an List (ArrayList is most popular).
So for example if I wanted to store query strings for later use I would...
Construct the string using StringBuilder.
Put the string (by calling .toString() into a HashMap
Get the query string from the HashMap...
You should never store things on disk if you don't need them to be persistent over application restarts and even then I'd store them in a database not in a file.
Hope this helps.
Thanks
David
EDIT: UPDATE BASED ON YOU POSTING YOUR CODE:
OK this needs some major re-factoring!
I've kept it really simple because I don't have a lot of time to re-write comprehensively.
I've commented where I have made corrections.
Your major issue here is creating objects in loops. You should just create the object once as creating objects is expensive.
I've also corrected other coding issues and replaced the for loop as you shouldn't be writing it like that.I've also renamed the classes to something useful.
I've not tested this so you may need to do some work to get it to work. But this should be a lot faster.
OLD CLASS 1
import java.util.ArrayList;
import java.util.List;
public class StaffChecker {
public static void main(String[] args) throws Exception {
// Creating objects is expensive, you should do this as little as possible
StaffCheckBO staffCheckBO = new StaffCheckBO();
// variables should be Camel Cased and describe what they hold
// Never start with ArrayList start with List you should specific the interface on the left side.
List<String> staffList = new ArrayList<String>();
staffList.add("tom");
staffList.add("Laura");
staffList.add("Patricia");
// use a foreach loop not a (int x = 0 ... ) This is the preffered method.
for (String staffMember : staffList) {
// You now dont need to use .get() you can access the current variable using staffMember
System.out.println(staffMember);
// Do the work
staffCheckBO.checkUser(staffMember);
}
}
}
OLD CLASS 2
/**
* Probably not really any need for this class but I'll assume further business logic may follow.
*/
public class StaffCheckBO {
// Again only create our DAO once...CREATING OBJECTS IS EXPENSIVE.
private StaffDAO staffDAO = new StaffDAO();
public void checkUser(String staffMember) throws Exception{
boolean staffExists = staffDAO.checkStaffExists(staffMember);
if(staffExists) {
System.out.println(staffMember +" is not in database, doing new insert.");
staffDAO.insertStaff(staffMember);
} else {
System.out.println(staffMember +" has been found in the database, updating user.");
staffDAO.updateStaff(staffMember);
}
}
}
OLD CLASS 3
import java.sql.*;
/**
* You will need to do some work to get this class to work fully and this is obviously basic but its to give you an idea.
*/
public class StaffDAO {
public boolean checkStaffExists(String staffName) {
boolean staffExists = false;
try {
String query = "SELECT * FROM STAFF_TABLE WHERE STAFF_NAME = ?";
PreparedStatement preparedStatement = getDBConnection().prepareStatement(query);
// Load your variables into the string in order to be safe against injection attacks.
preparedStatement.setString(1, staffName);
ResultSet resultSet = preparedStatement.executeQuery();
// If a record has been found the staff member is in the database. This obviously doesn't account for multiple staff members
if(resultSet.next()) {
staffExists = true;
}
} catch (SQLException e) {
System.out.println("SQL Exception in getStaff: " + e.getMessage());
}
return staffExists;
}
// Method names should be camel cased
public void updateStaff(String staffName) throws Exception {
try {
String query = "YOUR QUERY";
PreparedStatement preparedStatement = getDBConnection().prepareStatement(query);
// Load your variables into the string in order to be safe against injection attacks.
preparedStatement.setString(1, staffName);
ResultSet resultSet = preparedStatement.executeQuery();
} catch (SQLException e) {
System.out.println("SQL Exception in getStaff: " + e.getMessage());
}
}
public void insertStaff(String staffName) throws Exception {
try {
String query = "YOUR QUERY";
PreparedStatement preparedStatement = getDBConnection().prepareStatement(query);
// Load your variables into the string in order to be safe against injection attacks.
preparedStatement.setString(1, staffName);
ResultSet resultSet = preparedStatement.executeQuery();
} catch (SQLException e) {
System.out.println("SQL Exception in getStaff: " + e.getMessage());
}
}
/**
* You need to abstract the connection logic away so you avoid code reuse.
*
* #return
*/
private Connection getDBConnection() {
Connection connection = null;
try {
Class.forName("com.mysql.jdbc.Driver");
connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/default", "root", "1234567");
} catch (ClassNotFoundException e) {
System.out.println("Could not find class. DB Connection could not be created: " + e.getMessage());
} catch (SQLException e) {
System.out.println("SQL Exception. " + e.getMessage());
}
return connection;
}
}

Singleton in JSP, how to properly tidy up on close?

I am just getting started with jsp and my question is this - when I have a singleton class, how do I tidy up after it?
In particular:
public class DBConnection {
private static Connection connection = null;
private static Statement statement = null;
public static ResultSet executeQuery(String query){
if (connection == null) { /*initConnection*/ }
if (statement == null) { /*initStatement*/ }
// do some stuff
}
}
Now, I use this class in several pages to get results from jdbc. However, I need to eventually call statement.close(); and connection.close(); - when should I call those?
I am using singleton, because it felt wrong to call for connection to a database over and over whenever I needed to make a query.

The Connection must be closed always, and after you have executed all your database statements for the desired operations. Two examples:
Case 1: You must show a list of products to user filtered by criteria from database. Solution: get a connection, retrieve a list of products using the filter criteria, close the connection.
Case 2: The client selects some of these products and updates the minimum stock to get an alert and restock them. Solution: get a connection, update all the products, close the connection.
Based on these cases, we can learn lot of things:
You can execute more than a single statement while having/maintaining a single connection open.
The connection should live only in the block where it is used. It should not live before or after that.
Both cases can happen at the same time since they are in a multi threaded environment. So, a single database connection must not be available to be used by two threads at the same time, in order to avoid result problems. For example, user A searches the products that are in category Foo and user B searches the products that are in category Bar, you don't want to show the products in category Bar to user A.
From last sentence, each database operation ((or group of similar operations like Case 2) should be handled in an atomic operation. To assure this, the connection must not be stored in a singleton object, instead it must be live only in the method being used.
In consequence:
Do not declare the Connection nor the Statement nor the ResultSet nor other JDBC resource as static. It will simply fail. Instead, declare only the Connection as field of your DBConnection class. Let each method decide to handle each Statement (or PreparedStatement) and ResultSet and specific JDBC resources.
Since you must close the connection after its usage, then add two more methods: void open() and void close(). These methods will handle the database connection retrieval and closing that connection.
Additional, since the DBConnection looks like a wrapper class for Connection class and database connection operations, I would recommend to have at least three more methods: void setAutoCommit(boolean autoCommit), void commit() and void rollback(). These methods will be plain wrappers for Connection#setAutoCommit Connection#close and Connection#rollback respectively.
Then you can use the class in this way:
public List<Product> getProducts(String categoryName) {
String sql = "SELECT id, name FROM Product WHERE categoryName = ?";
List<Product> productList = new ArrayList<Product>();
DBConnection dbConnection = new DBConnection();
try {
dbConnection.open();
ResultSet resultSet = dbConnection.executeSelect(sql, categoryName); //execute select and apply parameters
//fill productList...
} catch (Exception e) {
//always handle your exceptions
...
} finally {
//don't forget to also close other resources here like ResultSet...
//always close the connection
dbConnection.close();
}
}
Note that in this example the PreparedStatement is not in the getProducts method, it will be a local variable of the executeSelect method.
Additional notes:
When working in an application server, you should not open connections naively e.g. using Class.forName("..."), instead use a database connection pool. You can roll on some database connection pooling libraries like C3P0 as explained here: How to establish a connection pool in JDBC?. Or configure one in your application server, as I explain here: Is it a good idea to put jdbc connection code in servlet class?
If this is for learning purposes, then roll on your own classes to handle the communication with your database. In real world applications, this is not recommended (doesn't mean you should not do it). Instead, use a database connectivity framework like ORMs e.g. JPA (Java official ORM framework) or Hibernate; there are no ORM frameworks that handles database communication like Spring JDBC and MyBatis. The choice is yours.
More info:
Should a database connection stay open all the time or only be opened when needed?
How do servlets work? Instantiation, sessions, shared variables and multithreading. Not directly related to your question, but it will help you understand why to not maintain state in resources that are used in multithreaded environments.

Define connection resource in mywebapp/META-INF/context.xml file
<Resource name="jdbc/mydb" auth="Container" type="javax.sql.DataSource"
maxActive="10" maxIdle="2" maxWait="20000"
driverClassName="com.mysql.jdbc.Driver"
username="myuser" password="mypwd"
url="jdbc:mysql://localhost:3306/mydb?useUnicode=true&characterEncoding=utf8"
validationQuery="SELECT 1" />
Create DB.java helper class to minimize code in other parts of app
import java.sql.*;
import javax.sql.DataSource;
import javax.naming.Context;
import javax.naming.InitialContext;
public class DB {
public static Connection createConnection() throws SQLException {
try {
Context ctx = new InitialContext();
DataSource ds = (DataSource)ctx.lookup("java:comp/env/jdbc/mydb");
return ds.getConnection();
} catch (SQLException ex) {
throw ex;
} catch (Exception ex) {
SQLException sqex = new SQLException(ex.getMessage());
sqex.initCause(ex);
throw sqex;
}
}
public static void close(ResultSet rs, Statement stmt, Connection conn) {
if (rs != null) try { rs.close(); } catch (Exception e) { }
if (stmt != null) try { stmt.close(); } catch (Exception e) { }
if (conn != null) try { conn.close(); } catch (Exception e) { }
}
public static void close(ResultSet rs, boolean closeStmtAndConn) {
if (rs==null) return;
try {
Statement stmt = rs.getStatement();
close(rs, stmt, stmt!=null ? stmt.getConnection() : null);
} catch (Exception ex) { }
}
}
And somewhere in your app DAO code use DB helper.
public List<MyBean> getBeans() throws SQLException {
List<MyBean> list = new ArrayList<MyBean>();
ResultSet rs=null;
try {
Connection con = DB.createConnection();
String sql = "Select * from beantable where typeid=?";
PreparedStatement stmt = con.prepareStatement(sql, Statement.NO_GENERATED_KEYS);
stmt.setInt(1, 101);
rs = stmt.executeQuery();
while(rs.next()
list.add( createBean(rs) );
} finally {
DB.close(rs, true); // or DB.close(rs, stmt, conn);
}
return list;
}
private MyBean createBean(ResultSet rs) throws SQLException {
MyBean bean = new MyBean();
bean.setId( rs.getLong("id") );
bean.setName( rs.getString("name" );
bean.setTypeId( rs.getInt("typeid") );
return bean;
}

I would add two methods to the class:
public static void open() throws SomeException;
public static void close() throws SomeException;
then your calling code looks something like this{
try {
DBConnection.open();
... code to use the connection one or more times ...
} finally {
DBConnection.close();
}
Wrap all your database calls inside that and it will take care of closing whether there is an exception thrown or not.
Of course, this isn't much different than having a regular class, which I might recommend:
try {
DBConnection conn = new DBConnection();
conn.open();
... all the code to use the database (but you pass 'conn' around) ...
} finally {
conn.close();
}
And you might want to look at the java.lang.AutoCloseable and java.io.Closeable to see if that helps you.
2
If you are keeping it open across page loads, there isn't any place to put the try ... finally stuff so you can open it and close it when the servlet closes or the server closes or something like that.
If you are going to leave it open, you need to make sure and add code to verify it doesn't close when you aren't looking. A short network glitch, for example, could close it down. In that case, you need to reopen it when it gets closed. Otherwise, all database access from that point will fail.
You might want to look into the concept of a DataBase Pool. Apache has one -- DBCP. Tomcat has its own that's quite good. Other containers, like JBOSS, WebSphere, WebLogic all have them. There's a couple that can be used with the Spring Framework. What it does is manage one or more database connections. Your code asks it for one and it returns an open one, unless none is available and then it opens one and returns it. You call close when your code gets through with it but it doesn't really close the connection, it just returns it to the pool.
You can usually configure the pool to check for shut down connections and reopen if needed.

Executing JDBC MySQL query with this custom method

I've been doing my homework and I decided to re-write my vote4cash class which manages the mysql for my vote4cash reward system into a new class called MysqlManager. The MysqlManager class I've made needs to allow the Commands class to connect to mysql - done and it needs to allow the Commands class to execute a query - I need help with this part. I've had a lot more progress with the new class that I've made but I'm stuck on one of the last, most important parts of the class, again, allowing the commands class to execute a query.
In my MysqlManager class I have put the code to connects to MySql under
public synchronized static void createConnection() {
Now I just need to put the code that allows the Commands class to execute a query under this as well. I've researched and tried to do this for a while now, but I've had absolutely no luck.
The entire MysqlManager class:
package server.util;
/*
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
*/
import java.sql.*;
import java.net.*;
import server.model.players.Client;//Will be needed eventually so that I can reward players who have voted.
/**
* MySQL and Vote4Cash Manager
* #author Cloudnine
*
*/
public class MysqlManager {
/** MySQL Connection */
public static Connection conn = null;
public static Statement statement = null;
public static ResultSet results = null;
public static Statement stmt = null;
public static ResultSet auth = null;
public static ResultSet given = null;
/** MySQL Database Info */
public static String DB = "vote4gold";
public static String URL = "localhost";
public static String USER = "root";
public static String PASS = "";
public static String driver = "com.mysql.jdbc.Driver"; //Driver for JBDC(Java and MySQL connector)
/** Connects to MySQL Database*/
public synchronized static void createConnection() {
try {
Class.forName(driver);
conn = DriverManager.getConnection(URL + DB, USER, PASS);
conn.setAutoCommit(false);
stmt = conn.createStatement();
Misc.println("Connected to MySQL Database");
}
catch(Exception e) {
//e.printStackTrace();
}
}
public synchronized static void destroyConnection() {
try {
statement.close();
conn.close();
} catch (Exception e) {
//e.printStackTrace();
}
}
public synchronized static ResultSet query(String s) throws SQLException {
try {
if (s.toLowerCase().startsWith("select")) {
ResultSet rs = statement.executeQuery(s);
return rs;
} else {
statement.executeUpdate(s);
}
return null;
} catch (Exception e) {
destroyConnection();
createConnection();
//e.printStackTrace();
}
return null;
}
}
The snippet of my command:
if (playerCommand.equals("claimreward")) {
try {
PreparedStatement ps = DriverManager.getConnection().createStatement("SELECT * FROM votes WHERE ip = hello AND given = '1' LIMIT 1");
//ps.setString(1, c.playerName);
ResultSet results = ps.executeQuery();
if(results.next()) {
c.sendMessage("You have already been given your voting reward.");
} else {
ps.close();
ps = DriverManager.getConnection().createStatement("SELECT * FROM votes WHERE ip = hello AND given = '0' LIMIT 1");
//ps.setString(1, playerCommand.substring(5));
results = ps.executeQuery();
if(results.next()) {
ps.close();
ps = DriverManager.getConnection().createStatement("UPDATE votes SET given = '1' WHERE ip = hello");
//ps.setString(1, playerCommand.substring(5));
ps.executeUpdate();
c.getItems().addItem(995, 5000000);
c.sendMessage("Thank you for voting! You've recieved 5m gold!");
} else {
c.sendMessage("You haven't voted yet. Vote for 5m gold!");
}
}
ps.close();
} catch (Exception e) {
e.printStackTrace();
}
}
return;
How the command works:
When a player types ::commandname(in this case, claimreward), the commands function will be executed. This isn't the entire commands class, just the part that I feel is needed to be posted for my question to be detailed enough for a helpful answer.
Note: I have all my imports.
Note: Mysql connects successfully.
Note: I need to make the above command code snippet able to execute mysql queries.
Note: I prefer the query to be executed straight from the command, instead of from the MysqlManager, but I will do whatever I need to resolve this problem.
I feel that I've described my problem detailed and relevantly enough, but if you need additional information or understanding on anything, tell me and I'll try to be more specific.
Thank you for taking the time to examine my problem. Thanks in advance if you are able to help.
-Alex

Your approach is misguided on many different levels, I can't even start to realize what should be done how here.
1) Don't ever use static class variables unless you know what you do there (and I'm certain, you don't)
2) I assume there is a reason you create your own jdbc connection (e.G. its part of your homework) if not, you shouldn't do that. I see you use DriverManager and PreparedStatement in one part, you should continue to use them.
3) Your approach seems to intend to start with a relative good code base (your command part) and then goes to a very low-level crude approach on database connections (your MysqlManager) unless really necessary and you know what you do, you should stay on the same level of abstraction and aim for the most abstract that fits your needs. (In this case, write MysqlManager the way you wrote Command)
4) In your previous question (that you just assumed everybody here has read, which is not the case) you got the suggestion to redesign your ideas, you should do that. Really, take a class in coding principles learn about anti-patterns and then start from scratch.
So in conclusion: Write at least the MysqlManager again, its fatally broken beyond repair. I'm sorry. Write me an email if you have further questions, I will take my time to see how I can help you. (an#steamnet.de)