java will this threading setup work or what can i be doing wrong

Question

Im a bit unsure and have to get advice.
I have the:

public class MyApp extends JFrame{

And from there i do;

MyServer = new MyServer (this);
MyServer.execute();

MyServer is a:

public class MyServer extends SwingWorker<String, Object> {   

MyServer is doing listen_socket.accept() in the doInBackground()

and on connection it create a new

class Connection implements Runnable {

I have the belove DbHelper that are a singleton.

It holds an Sqlite connection. Im initiating it in the above MyApp
and passing references all the way in to my runnable:

class Connection implements Runnable {

My question is what will happen if there are two simultaneous read or `write?
My thought here was the all methods in the singleton are synchronized and
would put all calls in the queue waiting to get a lock on the synchronized method.

Will this work or what can i change?

public final class DbHelper {

    private boolean initalized = false;
    private String HomePath = "";
    private File DBFile;

    private static final String SYSTEM_TABLE = "systemtable";   
    Connection con = null;
    private Statement stmt;
    private static final DbHelper instance = new DbHelper ();

    public static DbHelper getInstance() {

        return instance;

    }

    private DbHelper () {

        if (!initalized)
        {
            initDB();

            initalized = true;
        }
    }

    private void initDB()
    {
        DBFile = locateDBFile();
        try {

            Class.forName("org.sqlite.JDBC");

            // create a database connection
            con = DriverManager.getConnection("jdbc:sqlite:J:/workspace/workComputer/user_ptpp");

        } catch (SQLException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    private File locateDBFile()
    {
        File f = null;
        try{
            HomePath = System.getProperty("user.dir");
            System.out.println("HomePath: " + HomePath);
            f = new File(HomePath + "/user_ptpp");
            if (f.canRead())
                return f;
            else
            {
                boolean success = f.createNewFile();
                if (success) {
                    System.out.println("File did not exist and was created " + HomePath);
                    // File did not exist and was created
                } else {
                    System.out.println("File already exists " + HomePath);

                    // File already exists

                }
            }
        } catch (IOException e) {
             System.out.println("Maybe try a new directory. " + HomePath);
            //Maybe try a new directory.
        }
        return f;
    }

    public String getHomePath()
    {
        return HomePath;
    }



    public synchronized String getSelectedSystemTableColumn( String column) {

        String query = "select "+ column + " from " + SYSTEM_TABLE ;
        try {
            stmt = con.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
            ResultSet rs = stmt.executeQuery(query);
            while (rs.next()) {

                String value = rs.getString(column);

                if(value == null || value == "")
                    return "";
                else
                    return value;
            }

        } catch (SQLException e ) {
            e.printStackTrace();
            return "";
        } finally {

        }
        return "";

    }

}

Answer 1

Classes in java.sql are required to be thread-safe, according to the spec:

http://docs.oracle.com/javase/1.3/docs/guide/jdbc/spec/jdbc-spec.frame9.html

We require that all operations on all the java.sql objects be multi-thread safe and able to cope correctly with having several threads simultaneously calling the same object. Some drivers may allow more concurrent execution than others. Developers can assume fully concurrent execution; if the driver requires some form of synchronization, it will provide it. The only difference visible to the developer will be that applications will run with reduced concurrency.

This is unlikely to be a good idea. You ought to reconsider this design. I think pooling connections and closing them in the narrowest scope possible would be a better idea.