Transparent JFrame background

Yes, it's possible in many ways. This is one of them:

setUndecorated(true);
setBackground(new Color(1.0f,1.0f,1.0f,0.5f));

4th float (which I set to 0.5f) in Color's constructor is alpha channel. It can be 0.0f - 1.0f depend on transparency you want.

It is possible.

If your JFrame is a local variable or field:

myJFrame.setUndecorated(true);

If your class extends JFrame:

setUndecorated(true);

You should make content pane transparent too.

frame.setUndecorated(true);
frame.getContentPane().setBackground(new Color(1.0f,1.0f,1.0f,0.0f));
frame.setBackground(new Color(1.0f,1.0f,1.0f,0.0f));

See Translucent and Shaped Swing Windows by Kirill Grouchnikov.