How to find real display density (DPI) from Java code?

Question

I'm going to do some low-level rendering stuff, but I need to know real display DPI for making everything of correct size.

I've found one way to do this: java.awt.Toolkit.getDefaultToolkit().getScreenResolution() — but it returns incorrect result on OS X with "retina" display, it's 1/2 of the real DPI. (In my case it should be 220, but it's 110)

So either some other, more correct API must be available, or alternatively I need to implement a hack just for OS X — somehow find if the current display is "retina". But I couldn't find any way to query for this information too. There's this answer but on my machine Toolkit.getDefaultToolkit().getDesktopProperty("apple.awt.contentScaleFactor") just returns null.

How can I do it?

Answer 1

Looks like it's currently possible to get it from java.awt.GraphicsEnvironment. Here's commented code example which does work on latest JDK (8u112).

// find the display device of interest
final GraphicsDevice defaultScreenDevice = GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice();

// on OS X, it would be CGraphicsDevice
if (defaultScreenDevice instanceof CGraphicsDevice) {
    final CGraphicsDevice device = (CGraphicsDevice) defaultScreenDevice;

    // this is the missing correction factor, it's equal to 2 on HiDPI a.k.a. Retina displays
    final int scaleFactor = device.getScaleFactor();

    // now we can compute the real DPI of the screen
    final double realDPI = scaleFactor * (device.getXResolution() + device.getYResolution()) / 2;
}

Answer 2

Here's an example adopted from @sarge-borsch that won't throw compile errors on Windows and Linux.

public static int getScaleFactor() {
    try {
        // Use reflection to avoid compile errors on non-macOS environments
        Object screen = Class.forName("sun.awt.CGraphicsDevice").cast(GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice());
        Method getScaleFactor = screen.getClass().getDeclaredMethod("getScaleFactor");
        Object obj = getScaleFactor.invoke(screen);
        if (obj instanceof Integer) {
            return ((Integer)obj).intValue();
        }
    } catch (Exception e) {
        System.out.println("Unable to determine screen scale factor.  Defaulting to 1.");
    }
    return 1;
}