Shell shell = new Shell(display);

        Text t = new Text(shell, SWT.BORDER);
        t.setBounds(0, 0, 400, 400);

        t.setData("org.eclipse.rap.rwt.activeKeys", new String[] { "a", "b" });

        t.addKeyListener(new KeyAdapter() {
            @Override
            public void keyPressed(KeyEvent e) {
                System.out.println(e.character);

                if( e.character == 'a' ) {
                    t.setData("org.eclipse.rap.rwt.activeKeys", null);
                }
            }
        });

        shell.open();