(Please keep my CC'd, I'm not subscribed to the list) Hi folks,
while going over the Java keybinding code (trying to solve a different
problem), I found some strangeness in how KeyboardManager and JComponent
handle keystrokes. In particular, they try to match any registered
keybindings to both the normal keycode, as well as the extended keycode
from the KeyEvent.
According to the documentation, the normal keycode relates to the
physical position of the key on the keyboard and is not influenced by
the keyboard layout. The extended keycode relates to the key pressed,
according to the current keyboard layout. When a US QWERTY layout is
used, both keycodes will be identical.
Matching the extended keycode makes sense, if a keybinding is CTRL-Q,
then you want it to work whenever you press the key labeled "Q" on the
keyboard. Matching the normal keycode makes less sense to me - why would
the "key-that-is-Q-on-a-QWERTY-keyboard" trigger a Ctrl-Q binding? I
presume this is a sort of backward compatibility feature, but I wonder
if the application should be in control of this? In any case, in the
current implementation, this approach causes a problem, which I will
describe below.
Handling of keybindings happens by letting each JComponent track a
number of "input maps", mapping keystrokes to actions. There is a map
for when the component is focused, when it is the ancestor of a focused
component, and when the component is in a focused window. The first two
mappings are resolved by traversing the component hierarchy and letting
each component check its maps, the latter map is resolved by
KeyboardManager, which keeps one big map of bindings for each window.
In summary, resolving keypress works like this:
1. SwingUtilities.processKeyBindings() / JComponent.processKeyBindings():
Starting with the focused component, going upwards, each component
checks its keybindings for the current keyEvent. Each component first
checks against the extended keycode, then the normal keycode.
2. KeyboardManager.fireKeyboardAction(), via
JComponent.processKeyBindingsForAllComponents():
In the map of WHEN_IN_FOCUSED_WINDOW bindings kept by
KeyboardManager for the current window, the key event is looked up
and, if found, forwarded to the right JComponent. Again, this checks
the extended keycode first, then the normal keycode.
3. KeyboardManager.fireKeyboardAction() / JMenuBar.processKeyBinding():
For each JMenuBar in the current window (tracked in a separate map
by KeyboardManager), JMenuBar.processKeyBinding() is called, which
recurses through the menu hierarchy to check the
WHEN_IN_FOCUSED_WINDOW keybindings of each menu item. I assume this
is needed because children of JMenus are not added to the hierarchy
directly, but through a (detached) JPopupMenu instance.
Note that in step 1, it actually seems like the component hierarchy is
traversed in both SwingUtilities.processKeyBindings() as well as in
JComponent.processKeyBindings(), which seems like it is doing double
work (and applies the WHEN_FOCUSED maps even to ancestors of the focused
component). However, this doesn't seem relevant to the subject of this
mail, so I'll not investigate this further here.
In my examples below, I will be using the Belgian AZERTY layout. For
these examples, the only relevant changes are that the Q is swapped with
the A, and the W is swapped with the Z.
This resolution process has, AFAICS, two problems:
1. A given shortcut can be triggered by two keys. For example, when
using the Belgian AZERTY layout, a Ctrl-Q shortcut will be triggered
by both CTRL-Q (matching the extended keycode) as well as by CTRL-A
(the key in the same place as the Q on a QWERTY keyboard, by
matching the normal keycode).
I guess this is really intended as a feature, but it might be
confusing. One unintended side effect seems to be that, on OSX, some
keyboard shortcuts (such as Ctrl-, to open preferences) are handled
by the windowing system and passed to the application using a
seperate notification (e.g. "Open your preferences"), in *addition*
to passing the original keystroke to the application. This was
reported in https://bugs.openjdk.java.net/browse/JDK-8019498 where
a keybinding for Ctrl-M was triggered by pressing Ctrl-, in
*addition* to opening the preferences. Note that the preferences
event handling uses some OSX specific code, e.g.
http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/c021b855f51e/src/java.desktop/macosx/classes/com/apple/eawt/_AppEventHandler.java#l192
2. The second problem is that a component that is checked earlier in
the resolution order can "hijack" keystrokes from later components.
Normally, this is intentional - WHEN_FOCUSED and
WHEN_ANCESTOR_OF_FOCUSED_COMPONENTS favor the focused component, or
components close to them, and for WHEN_IN_FOCUSED_WINDOW bindings,
no duplicate bindings should exist.
However, because each step in the resolution matches both the
extended and normal keycode, a component can hijack the even using
the "backward compatible" normal keycode matching, even when a later
component would have succesfully matched against the extended
keycode.
For example, when using a Belgian AZERTY layout, a button in the
window binds Ctrl-Z, and a menu item binds Ctrl-W, you would expect
both keypresses to trigger the corresponding action. However, in
practice the button will match the Ctrl-W keypress too (by matching
the normal keycode), preventing the menu bar (which comes later in
the resolution procedure) from matching the keypress, so both
Ctrl-W and Ctrl-Z trigger the button.
The latter problem is of course the bigger one. The obvious solution is
to run through the entire resolution twice, once matching the extended
keycode, then once more matching the normal keycode. To fix the first
problem too, the last step could perhaps be made optional, or perhaps
KeyStroke can be modified to match the normal keycode, extended keycode,
or both. Not sure if this can be done in a compatible way, though, since
some of the methods involved are public or protected.
To confirm my analisis, I wrote a small testcase (inline below, and at
https://gist.github.com/matthijskooijman/4d016e7a9e3fb07d0699). It shows
both of the problems outlined above. It contains a menu item bound to
Ctrl-Q, which triggers on both Ctrl-Q and Ctrl-A when using the Belgian
AZERTY layout. It also contains a button binding to Ctrl-Z, which
prevents a menu item bound to Ctrl-W from working when using the Belgian
AZERTY layout. I have tested this on Linux, by selecting a different
keyboard layout ("Input source") in the Gnome keyboard settings (without
actually switching the keyboard).
There seem to be a few related bug reports already. I suspect that most
or all of these are caused by this problem, though not all of them have
enough details to be sure.
https://bugs.openjdk.java.net/browse/JDK-8087915
https://bugs.openjdk.java.net/browse/JDK-8022079
https://bugs.openjdk.java.net/browse/JDK-8019498
https://bugs.openjdk.java.net/browse/JDK-8141096
Interestingly, all of these reports are about OSX, but I'm pretty sure
the problem I'm describing will occur on all platforms, being caused by
platform-independent code. There might be additional OSX-specific
problems, of course, but I do not have access to OSX to test.
Gr.
Matthijs
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
class Item extends JMenuItem {
public void addNotify() {
System.out.println("Add\n");
}
public Item(String s) { super(s); }
}
public class Test {
public static void main(String[] args) {
SwingUtilities.invokeLater(() -> {
JFrame frame = new JFrame();
JMenuBar bar = new JMenuBar();
JButton button = new JButton("Button Ctrl-Z");
JTextArea text = new JTextArea("");
JMenu menu = new JMenu("Menu");
JMenuItem item1 = new Item( "Item 1" );
JMenuItem item2 = new JMenuItem( "Item 2" );
item1.addActionListener((ActionEvent e) -> text.append("Menuitem
1\n"));
item2.addActionListener((ActionEvent e) -> text.append("Menuitem
2\n"));
button.addActionListener((ActionEvent e) -> text.append("Button
click\n"));
item1.setAccelerator(KeyStroke.getKeyStroke('Q',
KeyEvent.CTRL_MASK));
item2.setAccelerator(KeyStroke.getKeyStroke('W',
KeyEvent.CTRL_MASK));
KeyStroke keyStroke = KeyStroke.getKeyStroke('Z',
KeyEvent.CTRL_MASK);
ActionListener action = (ActionEvent) -> text.append("Button
key\n");
button.registerKeyboardAction(action, "action", keyStroke,
JComponent.WHEN_IN_FOCUSED_WINDOW);
menu.add(item1);
menu.add(item2);
bar.add(menu);
text.setEnabled(false);
frame.setLayout(new BorderLayout());
frame.add(button, BorderLayout.SOUTH);
frame.add(text, BorderLayout.CENTER);
frame.setJMenuBar(bar);
frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
frame.setSize(200,200);
frame.setVisible(true);
});
}
}
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