That's a system setting, and I don't even know if it applies to Java.

[EDIT] I tested, and the frame is still updated when resizing the window:

1. void setup() {
2.   size(400, 400);
3.   frame.setResizable(true);
4.   textFont(createFont("Arial", 48));
5.   fill(0);
6. }
7.  
8. void draw() {
9.   background(255);
10.   text("FC: " + frameCount, 50, 50);
11.   line(0, 0, width, height);
12. }

But the frame cannot be updated in real time. Or it would be costly, as it would imply to do a full reinit on each pixel size, to create a new pixels[] array, a new PGraphics, etc.
Swing applications can be resized in real time (the layout is updated permanently), so I suppose it is possible, it might be that Processing authors discarded (or didn't tried) the option.