无法获取剪贴板的内容（Java）

Question

我正在运行以下代码：

String clipBoardText;

public String getAllTextOfPage() {
    StringSelection stringSelection = new StringSelection(null);
    Toolkit.getDefaultToolkit().getSystemClipboard().setContents(
            stringSelection, null);

    Robot r = null;
    try {
        r = new Robot();
    } catch (AWTException e) {
        e.printStackTrace();
    }
    r.keyPress(KeyEvent.VK_CONTROL);
    r.keyPress(KeyEvent.VK_A);
    r.keyRelease(KeyEvent.VK_A);
    try {
        Thread.sleep(1000);
    }
    catch (InterruptedException ie) {
        System.out.print(ie.getLocalizedMessage());
    }

    r.keyPress(KeyEvent.VK_C);
    r.keyRelease(KeyEvent.VK_CONTROL);
    r.keyRelease(KeyEvent.VK_C);

    Clipboard clipboard = Toolkit.getDefaultToolkit().getSystemClipboard();

        try {
            clipBoardText = (String)clipboard.getContents(null).getTransferData(DataFlavor.stringFlavor);
            System.out.println(clipBoardText);
        }
        catch (UnsupportedFlavorException ex){
            System.out.println(ex);
            ex.printStackTrace();
        }
        catch (IOException ex) {
            System.out.println(ex);
            ex.printStackTrace();
        }
    return clipBoardText;
}

控制台输出如下：

null

Process finished with exit code 0

但是当我将剪贴板的内容粘贴到记事本中时，它有我正在寻找的内容。因此，我在检索剪贴板内容时似乎存在问题。