I use Robot class to paste strings from clipboard to file chooser dialog window in a loop. Strings are file paths, so these are 100-180 characters long. After the first few strings it is unable to paste more. The same solution is working well with shorter strings.
StringSelection ssss = new StringSelection(fileSource);
Toolkit.getDefaultToolkit().getSystemClipboard().setContents(ssss, null);
Robot probot = new Robot();
probot.keyPress(KeyEvent.VK_CONTROL);
Thread.sleep(2000);
probot.keyPress(KeyEvent.VK_V);
Thread.sleep(2000);
probot.keyRelease(KeyEvent.VK_V);
Thread.sleep(2000);
probot.keyRelease(KeyEvent.VK_CONTROL);
Thread.sleep(2000);
probot.keyPress(KeyEvent.VK_ENTER);
probot.keyRelease(KeyEvent.VK_ENTER);