private static Connection conectar(){
    try {
        Class.forName("com.mysql.jdbc.Driver");
        return DriverManager.getConnection("jdbc:mysql://localhost/fiscal", "jp", "");
    } catch (ClassNotFoundException ex) {
        Logger.getLogger(Concorrencia.class.getName()).log(Level.SEVERE, null, ex);
    } catch (SQLException ex) {
        Logger.getLogger(Concorrencia.class.getName()).log(Level.SEVERE, null, ex);
    }
    return null;
}

private static int getProximoId(Connection con, int cupomId) throws SQLException{
    PreparedStatement stmt = con.prepareStatement("select max(nr_item) from item_cupom where cupom_id = ?");
    stmt.setInt(1, cupomId);
    ResultSet rs = stmt.executeQuery();
    try{
        if(rs.next()){
            return rs.getInt(1) + 1;
        }else{
            return 1;
        }
    }finally{
        rs.close();
        stmt.close();
    }
}

public static void inserirItem(Connection con, int cupomId, boolean demorar) throws SQLException, InterruptedException{
    int nrItem = getProximoId(con, cupomId);
    if(demorar){
        Thread.sleep(200);
    }
    PreparedStatement pstmt = con.prepareStatement("insert into item_cupom(cupom_id, nr_item) values(?, ?);");
    pstmt.setInt(1, cupomId);
    pstmt.setInt(2, nrItem);
    pstmt.execute();
    pstmt.close();
}

private static boolean seraQueVaiDemorar(){
    double valor = Math.random();
    return valor > 0.8;
}

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    Connection con = conectar();

    for (int i = 0; i < 2; i++) {
        new Thread(){
            @Override
            public void run() {
                for(int i = 0; i < 300; i++){
                    try {
                        inserirItem(con, 5, seraQueVaiDemorar());
                    } catch (Exception ex) {
                        Logger.getLogger(Concorrencia.class.getName()).log(Level.SEVERE, null, ex);
                    }
                }
            }

        }.start();
    }
}