Question

I am trying to insert into a XpressMP database, It’s a new database. I wrote a multithreaded program for that to insert into the database with multiple threads.

I am trying to insert into the below tables with two columns- ID and ACCOUNT

ID      String Primary Key
ACCOUNT String

So currently I have ID as a Primary key. And I need to generate a new ID every time whenever I am inserting in the database otherwise it will throw an exception showing-

duplicate row in unique index:

So for that I made a class Immutable like this in which I have user_id which gets generated everytime a new thread will ask for-

public final class ConstantsFiles {

    private static Random rnd = new Random();
    private final static int NO_OF_DIGITS = 10;

    public static String user_id;

    public final static String ACCOUNT = "{\"lv\":null,\"lmd\":13597}";

    public final static String INSERT_SQL = "INSERT INTO COPY"
        + "("
        + "ID, ACCOUNT) VALUES"
        + "(?, ?)";

    /**
     * Generates a Random user id
     * 
     */

    public static String getUser_id() {
        StringBuilder sb = new StringBuilder(NO_OF_DIGITS);
        for (int i = 0; i < NO_OF_DIGITS; i++)
            sb.append((char) ('0' + rnd.nextInt(10)));

        return sb.toString();
    }

}

Below is my multithreaded code which gets the new ID everytime from the get method I have in my immutable class above-

@Override
public void run() {

    try {
        dbConnection = getDBConnection();
        preparedStatement = dbConnection.prepareStatement(ConstantsFiles.INSERT_SQL);

        preparedStatement.setString(1, ConstantsFiles.getUser_id());
        preparedStatement.setString(2, ConstantsFiles.ACCOUNT);

        preparedStatement.executeUpdate();

    } catch (Exception e) {
        LOG.error(e);
    } finally {
        // close connection
    }
    }

In which each thread will generate a new USER ID every time and use the same ACCOUNT value every time and insert into the database.

But somehow at some stage it is throwing me an exception-

duplicate row in unique index:COPY_PRIMARY_KEY_

That means at some point it is using the same USER ID to insert into the database and because of that it is throwing the exception.

Any thoughts why it is happening. I have already made the class immutable and added the get method. Then what’s the problem? May be my class is not immutable?

I just need to insert into database with a NEW USER ID getting generated for each new thread but I can use the same ACCOUNT value every time.

Answer 1

Random does not guarantee unique. You need to open a transaction with the database and make sure it’s unused before you insert or this is guaranteed to happen eventually.

Edit: I don’t know XpressMP at all, but in pseudo-SQL you’d need to do something like:

BEGIN
    DECLARE @free = NULL
    LOCK COPY
    SELECT INTO @free ID FROM COPY WHERE ID = ?
    IF free IS NULL
        INSERT ....
    UNLOCK COPY
END