Set opacity of a decorated JFrame in Java 8

问题

I would like to know how to get a transparent JFrame in the latest version of Java.

Currently, you can only use

<JFrame>.setOpacity();

if the frame is not decorated.

I have no use for an undecorated frame, so I'd like to know how to go around this restriction and set the opacity of the frame to 0.5f while still keeping the title bar, resize options etc.

I have read the docs here: http://docs.oracle.com/javase/tutorial/uiswing/misc/trans_shaped_windows.html. The code only worked on Java 6 and no longer runs. The error, as I said, is:

Exception in thread "AWT-EventQueue-0" java.awt.IllegalComponentStateException: The frame is decorated
    at java.awt.Frame.setOpacity(Frame.java:960)
    at TranslucentWindowDemo$1.run(TranslucentWindowDemo.java:53)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    ...

I have also tried setting the background (setBackground : Color) using a Color with custom Alpha value (new Color(int, int, int, Alpha)) but it throws the exact same error. Setting the transaprency of a JPanel this way won't work, as it will still lay on the JFrame, which is not transparent.

I could find no other answer on Stack Overflow that correctly addressed this issue. In fact, a few suggested that this could be fixed with:

JFrame.setDefaultLookAndFeelDecorated(true);

But they were misinformed of perhaps referring to Java 7, as I have tested it and the result is just the same.

I have also tried to manually set the Look And Feel:

try {
    for (final LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
        if ("Nimbus".equals(info.getName())) {
            UIManager.setLookAndFeel(info.getClassName());
            break;
        }
    }

} catch [...] 

And combining this with the solution suggested above also did not work.

Please refer to the code over to the example I linked above (Oracle doc) for a MCVE, as that is the one I'm using.

Any way around this?

回答1:

As far as I can tell, the basic answer is: it is not possible, at least with the System look and feel. As indicated in Is The Java Tutorials Translucent Window example giving trouble to those playing with jdk7?, the JavaDocs clearly indicate that “the window must be undecorated” for setOpacity() to work.

It is however possible to do it with the (ugly) Cross-platform look and feel, that you can progrmmatically set as follows:

UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());

In fact, as the cross-platform look and feel could be overridden through configuration, the safest would actually be to set it explicitly to Metal as follows:

UIManager.setLookAndFeel(new MetalLookAndFeel());

The reason this works, is that the JDK implementation of Frame.setOpacity() throws an exception when !isUndecorated(), and JFrame.frameInit() sets itself as undecorated when the look and feel's getSupportsWindowDecorations() returns true. It then calls getRootPane().setWindowDecorationStyle() with JRootPane.FRAME, indicating that the decorations will be provided by the root pane instead of the frame.

From what I can see in the JDK, the Metal look and feel is the only one for which getSupportsWindowDecorations() returns true, as it is the only one which overrides it, and the default implementation simply returns false.

However, some third-party look and feels support it too. This is the case for instance for the Tiny Look and Feel, as I just tried:

(Note that I took this screenshot on Ubuntu, TinyLAF just so happens to have a default theme that looks like Windows XP!)

See also this question for a list of known third-party look and feels.

回答2:

Try adding this line before creating the JFrame window:

JFrame.setDefaultLookAndFeelDecorated(true);

Exactly that line, don't replace JFrame in the beggining, it needs to be JFrame.

(You can also spot this line in the tutorials you mentioned precicely placed before creating the window).

回答3:

Actually this is possible, using the dirty solution of reflect. If we dig into setOpacity method (which is inherited from java.awt.Frame class) we will see the following code:

@Override
public void setOpacity(float opacity) {
    synchronized (getTreeLock()) {
        if ((opacity < 1.0f) && !isUndecorated()) {
            throw new IllegalComponentStateException("The frame is decorated");
        }
        super.setOpacity(opacity);
    }
}

where isUndecorated is a simple getter to the field named undecorated (inside java.awt.Frame class).

Changing the value of this field will do the trick and this exception won't be thrown.

Check this example i have made:

public class JFrameOpacity {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            setSystemLookAndFeel();
            JFrame frame = new JFrame("Opacity to decorated Frame");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setLayout(new FlowLayout());

            JButton decreaseOpacity = new JButton("Reduce Opacity");
            decreaseOpacity.addActionListener(e -> {
                if (frame.getOpacity() - 0.1f <= 0.1f)
                    frame.setOpacity(0.1f);
                else
                    frame.setOpacity(frame.getOpacity() - 0.1f);
            });
            frame.add(decreaseOpacity);

            JButton increaseOpacity = new JButton("Increase Opacity");
            increaseOpacity.addActionListener(e -> {
                if (frame.getOpacity() + 0.1f >= 1f)
                    frame.setOpacity(1f);
                else
                    frame.setOpacity(frame.getOpacity() + 0.1f);
            });
            frame.add(increaseOpacity);

            frame.setSize(300, 300);
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
            try {
                undecorate(frame); //Change it after frame is visible
            } catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
        });
    }

    private static void undecorate(Frame frame) throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {
        Field undecoratedField = Frame.class.getDeclaredField("undecorated");
        undecoratedField.setAccessible(true);
        undecoratedField.set(frame, true);
    }

    private static void setSystemLookAndFeel() {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException e) {
            e.printStackTrace();
        }
    }

}

Preview:

---

I have tested this in The Microsoft Windows Look and Feel (Windows 7 x64) and it works. Note the comment i added when i call undecorate method. I made some tests on that and i realized that if you undecorate the frame before it gets at least one time visible, when you will make it visible it will be undecorated - it will not have this title bar and stuff.

I am not sure though if this is going to give other problems to the application but you can always change the value of the field, change its opacity and the set it back.

来源：https://stackoverflow.com/questions/39538731/set-opacity-of-a-decorated-jframe-in-java-8