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<h1>The AWT Focus Subsystem</h1>
<p>
Prior to Java 2 Standard Edition, JDK 1.4, the AWT focus subsystem
was inadequate. It suffered from major design and API problems,
as well as over a hundred open bugs. Many of these bugs were caused by
platform inconsistencies, or incompatibilities between the native
focus system for heavyweights and the Java focus system for
lightweights.
<p>
The single worst problem with the AWT focus implementation was the
inability to query for the currently focused Component. Not only was
there no API for such a query, but also, because of an insufficient
architecture, such information was not even maintained by the code.
<p>
Almost as bad was the inability of lightweight children of a Window
(not a Frame or a Dialog) to receive keyboard input. This problem
existed because Windows never received <code>WINDOW_ACTIVATED</code>
events and thus could never be activated, and only active Windows
could contain focused Components.
<p>
In addition, many developers noted that the APIs for FocusEvent and
WindowEvent were insufficient because they did not provide a way for
determining the "opposite" Component involved in the focus or
activation change. For example, when a Component received a FOCUS_LOST
event, it had no way of knowing which Component was gaining
focus. Since Microsoft Windows provides this functionality for free,
developers migrating from Microsoft Windows C/C++ or Visual Basic to
Java had been frustrated by the omission.
<p>
To address these and other deficiencies, we have designed a new focus
model for the AWT in JDK 1.4. The primary design changes were the
construction of a new centralized KeyboardFocusManager class, and a
lightweight focus architecture. The amount of focus-related,
platform-dependent code has been minimized and replaced by fully
pluggable and extensible public APIs in the AWT. While we have
attempted to remain backward compatible with the existing
implementation, we were forced to make minor incompatible changes in
order to reach an elegant and workable conclusion. We anticipate that
these incompatibilities will have only a trivial impact on existing
applications.
<p>
This document is a formal specification both of the new APIs and of
existing APIs which remain relevant in the new model. Combined with
the javadoc for focus-related classes and methods, this document
should enable developers to create substantial AWT and Swing
applications with a focus behavior that is customized yet consistent
across platforms. This document has the following sections:
<ul>
<li><a href=#Overview>Overview of KeyboardFocusManager</a>
<li><a href=#BrowserContexts>KeyboardFocusManager and Browser Contexts</a>
<li><a href=#KeyEventDispatcher>KeyEventDispatcher</a>
<li><a href=#FocusEventAndWindowEvent>FocusEvent and WindowEvent</a>
<li><a href=#EventDelivery>Event Delivery</a>
<li><a href=#OppositeComponents>Opposite Components and Windows</a>
<li><a href=#TemporaryFocusEvents>Temporary FocusEvents</a>
<li><a href=#FocusTraversal>Focus Traversal</a>
<li><a href=#FocusTraversalPolicy>Focus Traversal Policy</a>
<li><a href=#FocusTraversalPolicyProviders>Focus Traversal Policy Providers</a>
<li><a href=#ProgrammaticTraversal>Programmatic Traversal</a>
<li><a href=#Focusability>Focusability</a>
<li><a href=#FocusableWindows>Focusable Windows</a>
<li><a href=#RequestingFocus>Requesting Focus</a>
<li><a href=#FocusAndPropertyChangeListener>Focus and PropertyChangeListener</a>
<li><a href=#FocusAndVetoableChangeListener>Focus and VetoableChangeListener</a>
<li><a href=#ZOrder>Z-Order</a>
<li><a href=#ReplacingDefaultKeyboardFocusManager>Replacing DefaultKeyboardFocusManager</a>
<li><a href=#Incompatibilities>Incompatibilities with Previous Releases</a>
</ul>
<a id="Overview"></a>
<h2>Overview of KeyboardFocusManager</h2>
<p>
The focus model is centralized around a single class,
KeyboardFocusManager, that provides a set of APIs for client code to
inquire about the current focus state, initiate focus changes, and
replace default focus event dispatching with a custom dispatcher.
Clients can inquire about the focus state directly, or can register a
PropertyChangeListener that will receive PropertyChangeEvents when a
change to the focus state occurs.
<p>
KeyboardFocusManager introduces the following main concepts and their
terminology:
<ol>
<li>The "focus owner" -- the Component which typically receives
keyboard input.
<li>The "permanent focus owner" -- the last Component to receive
focus permanently. The "focus owner" and the "permanent focus
owner" are equivalent unless a temporary focus change is
currently in effect. In such a situation, the "permanent focus
owner" will again be the "focus owner" when the temporary focus
change ends.
<li>The "focused Window" -- the Window which contains the "focus
owner".
<li>The "active Window" -- the Frame or Dialog that is either the
"focused Window", or the first Frame or Dialog that is an owner
of the "focused Window".
<li>"Focus traversal" -- the user's ability to change the "focus
owner" without moving the cursor. Typically, this is done using
the keyboard (for example, by using the TAB key), or an
equivalent device in an accessible environment. Client code can
also initiate traversal programmatically. Normal focus traversal
can be either "forward" to the "next" Component, or "backward" to
the "previous" Component.
<li>"Focus traversal cycle" -- a portion of the Component hierarchy,
such that normal focus traversal "forward" (or "backward") will
traverse through all of the Components in the focus cycle, but no
other Components. This cycle provides a mapping from an arbitrary
Component in the cycle to its "next" (forward traversal) and
"previous" (backward traversal) Components.
<li>"Traversable Component" -- Component that is in the focus traversal
cycle.
<li>"Non-traversable Component" -- Component that is not in the focus
traversal cycle. Note that a non-traversable Component can nevertheless
be focused in other way (e.g. by direct focus request).
<li>"Focus cycle root" -- Container that is the root of the Component
hierarchy for a particular "focus traversal cycle". When the
"focus owner" is a Component inside a particular cycle, normal
forward and backward focus traversal cannot move the "focus
owner" above the focus cycle root in the Component hierarchy.
Instead, two additional traversal operations, "up cycle" and
"down cycle", are defined to allow keyboard and programmatic
navigation up and down the focus traversal cycle hierarchy. </li>
<li>"Focus traversal policy provider" - Container which has
"FocusTraversalPolicyProvider" property as true. This Container will
be used to acquire focus traversal policy. This container doesn't
define new focus cycle but only modifies the order by which its
children are traversed "forward" and "backward". Focus traversal
policy provider can be set using
<code>setFocusTraversalPolicyProvider</code> on the Container.
</ol>
<p>
Every Window and JInternalFrame is, by default, a "focus cycle
root". If it's the only focus cycle root, then all of its
focusable descendants should be in its focus cycle, and its focus
traversal policy should enforce that they are by making sure that
all will be reached during normal forward (or backward)
traversal. If, on the other hand, the Window or JInternalFrame
has descendants that are also focus cycle roots, then each such
descendant is a member of two focus cycles: the one that it is
the root of, and the one of its nearest focus-cycle-root
ancestor. In order to traverse the focusable components belonging
to the focus cycle of such a "descendant" focus cycle root, one
first traverses (forward or backward) to reach the descendant,
and then uses the "down cycle" operation to reach, in turn, its
descendants.
<p>
Here is an example:<br> <img src="FocusCycle.gif"
alt="Three groups as described below: ABCF BDE and DGH. "><br>
<p>Assume the following:
<ul>
<li><b>A</b> is a <code>Window</code>, which means that it
must be a focus cycle root.
<li><b>B</b> and <b>D</b> are <code>Container</code>s that
are focus cycle roots.
<li><b>C</b> is a <code>Container</code> that is not a focus cycle root.
<li><b>G</b>, <b>H</b>, <b>E</b>, and <b>F</b> are all
<code>Component</code>s.
</ul>
There are a total of three focus cycle roots in this example:
<ol>
<li><b>A</b> is a root, and <b>A</b>, <b>B</b>, <b>C</b>,
and <b>F</b> are members of <b>A</b>'s cycle.
<li><b>B</b> is a root, and <b>B</b>, <b>D</b>, and
<b>E</b> are members of <b>B</b>'s cycle.
<li><b>D</b> is a root, and <b>D</b>, <b>G</b>,
and <b>H</b> are members of <b>D</b>'s cycle.
</ol>
Windows are the only Containers which, by default, are focus cycle
roots.
<code>KeyboardFocusManager</code> is an abstract class. AWT provides a default
implementation in the <code>DefaultKeyboardFocusManager</code> class.
<a id="BrowserContexts"></a>
<h2>KeyboardFocusManager and Browser Contexts</h2>
<p>
Some browsers partition applets in different code bases into separate
contexts, and establish walls between these contexts. Each thread and
each Component is associated with a particular context and cannot
interfere with threads or access Components in other contexts. In such
a scenario, there will be one KeyboardFocusManager per context. Other
browsers place all applets into the same context, implying that there
will be only a single, global KeyboardFocusManager for all
applets. This behavior is implementation-dependent. Consult your
browser's documentation for more information. No matter how many
contexts there may be, however, there can never be more than one focus
owner, focused Window, or active Window, per ClassLoader.
<a id="KeyEventDispatcher"></a>
<h2>KeyEventDispatcher and KeyEventPostProcessor</h2>
<p>
While the user's KeyEvents should generally be delivered to the focus
owner, there are rare cases where this is not desirable. An input
method is an example of a specialized Component that should receive
KeyEvents even though its associated text Component is and should
remain the focus owner.
<p>
A KeyEventDispatcher is a lightweight interface that allows client
code to pre-listen to all KeyEvents in a particular context. Instances
of classes that implement the interface and are registered with the
current KeyboardFocusManager will receive KeyEvents before they are
dispatched to the focus owner, allowing the KeyEventDispatcher to
retarget the event, consume it, dispatch it itself, or make other
changes.
<p>
For consistency, KeyboardFocusManager itself is a
KeyEventDispatcher. By default, the current KeyboardFocusManager will
be the sink for all KeyEvents not dispatched by the registered
KeyEventDispatchers. The current KeyboardFocusManager cannot be
completely deregistered as a KeyEventDispatcher. However, if a
KeyEventDispatcher reports that it dispatched the KeyEvent, regardless
of whether it actually did so, the KeyboardFocusManager will take no
further action with regard to the KeyEvent. (While it is possible for
client code to register the current KeyboardFocusManager as a
KeyEventDispatcher one or more times, there is no obvious reason why
this would be necessary, and therefore it is not recommended.)
<p>
Client-code may also post-listen to KeyEvents in a particular context
using the KeyEventPostProcessor interface. KeyEventPostProcessors
registered with the current KeyboardFocusManager will receive
KeyEvents after the KeyEvents have been dispatched to and handled by
the focus owner. The KeyEventPostProcessors will also receive
KeyEvents that would have been otherwise discarded because no
Component in the application currently owns the focus. This will allow
applications to implement features that require global KeyEvent post-
handling, such as menu shortcuts.
<p>
Like KeyEventDispatcher, KeyboardFocusManager also implements
KeyEventPostProcessor, and similar restrictions apply to its use in
that capacity.
<a id="FocusEventAndWindowEvent"></a>
<h2>FocusEvent and WindowEvent</h2>
<p>
The AWT defines the following six event types central to the focus
model in two different <code>java.awt.event</code> classes:
<ol>
<li><code>WindowEvent.WINDOW_ACTIVATED</code>: This event is
dispatched to a Frame or Dialog (but never a Window which
is not a Frame or Dialog) when it becomes the active Window.
<li><code>WindowEvent.WINDOW_GAINED_FOCUS</code>: This event is
dispatched to a Window when it becomes the focused Window.
Only focusable Windows can receive this event.
<li><code>FocusEvent.FOCUS_GAINED</code>: This event is dispatched
to a Component when it becomes the focus owner. Only focusable
Components can receive this event.
<li><code>FocusEvent.FOCUS_LOST</code>: This event is dispatched
to a Component when it is no longer the focus owner.
<li><code>WindowEvent.WINDOW_LOST_FOCUS</code>: This event is
dispatched to a Window when it is no longer the focused Window.
<li><code>WindowEvent.WINDOW_DEACTIVATED</code>: This event is
dispatched to a Frame or Dialog (but never a Window which is
not a Frame or Dialog) when it is no longer the active Window.
</ol>
<a id="EventDelivery"></a>
<h2>Event Delivery</h2>
<p>
If the focus is not in java application and the user clicks on a focusable
child Component<b>a</b> of an inactive Frame <b>b</b>, the following events
will be dispatched and handled in order:
<ol>
<li><b>b</b> will receive a <code>WINDOW_ACTIVATED</code> event.
<li>Next, <b>b</b> will receive a <code>WINDOW_GAINED_FOCUS</code> event.
<li>Finally, <b>a</b> will receive a <code>FOCUS_GAINED</code> event.
</ol>
If the user later clicks on a focusable child Component <b>c</b> of another
Frame <b>d</b>, the following events will be dispatched and handled in
order:
<ol>
<li><b>a</b> will receive a <code>FOCUS_LOST</code> event.
<li><b>b</b> will receive a <code>WINDOW_LOST_FOCUS</code> event.
<li><b>b</b> will receive a <code>WINDOW_DEACTIVATED</code> event.
<li><b>d</b> will receive a <code>WINDOW_ACTIVATED</code> event.
<li><b>d</b> will receive a <code>WINDOW_GAINED_FOCUS</code> event.
<li><b>c</b> will receive a <code>FOCUS_GAINED</code> event.
</ol>
Note that each event will be fully handled before the next event is
dispatched. This restriction will be enforced even if the Components
are in different contexts and are handled on different event
dispatching threads.
<p>
In addition, each event type will be dispatched in 1-to-1
correspondence with its opposite event type. For example, if a
Component receives a <code>FOCUS_GAINED</code> event, under no
circumstances can it ever receive another <code>FOCUS_GAINED</code>
event without an intervening <code>FOCUS_LOST</code> event.
<p>
Finally, it is important to note that these events are delivered for
informational purposes only. It is impossible, for example, to prevent
the delivery of a pending <code>FOCUS_GAINED</code> event by requesting
focus back to the Component losing focus while handling the preceding
<code>FOCUS_LOST</code> event. While client code may make such a request,
the pending <code>FOCUS_GAINED</code> will still be delivered,
followed later by the events transferring focus back to the original
focus owner.
<p>
If it is absolutely necessary to suppress the <code>FOCUS_GAINED</code> event,
client code can install a <code>VetoableChangeListener</code> which
rejects the focus change. See <a href="#FocusAndVetoableChangeListener">Focus
and VetoableChangeListener</a>.
<a id="OppositeComponents"></a>
<h2>Opposite Components and Windows</h2>
<p>
Each event includes information about the "opposite" Component or
Window involved in the focus or activation change. For example, for a
<code>FOCUS_GAINED</code> event, the opposite Component is the Component
that lost focus. If the focus or activation change occurs with a native
application, with a Java application in a different VM or context, or
with no other Component, then the opposite Component or Window is
null. This information is accessible using
<code>FocusEvent.getOppositeComponent</code> or
<code>WindowEvent.getOppositeWindow</code>.
<p>
On some platforms, it is not possible to discern the opposite
Component or Window when the focus or activation change occurs between
two different heavyweight Components. In these cases, the opposite
Component or Window may be set to null on some platforms, and to a
valid non-null value on other platforms. However, for a focus change
between two lightweight Components which share the same heavyweight
Container, the opposite Component will always be set correctly. Thus,
a pure Swing application can ignore this platform restriction when
using the opposite Component of a focus change that occurred within a
top-level Window.
<a id="TemporaryFocusEvents"></a>
<h2>Temporary FocusEvents</h2>
<p>
<code>FOCUS_GAINED</code> and <code>FOCUS_LOST</code> events are
marked as either temporary or permanent.
<p>
Temporary <code>FOCUS_LOST</code> events are sent when a Component is
losing the focus, but will regain the focus shortly. These events
can be useful when focus changes are used as triggers for validation
of data. For instance, a text Component may want to commit its
contents when the user begins interacting with another Component,
and can accomplish this by responding to <code>FOCUS_LOST</code> events.
However, if the <code>FocusEvent</code> received is temporary,
the commit should not be done, since the text field will be receiving
the focus again shortly.
<p>
A permanent focus transfer typically occurs as the result of a user
clicking on a selectable, heavyweight Component, focus traversal with
the keyboard or an equivalent input device, or from a call to
<code>requestFocus()</code> or <code>requestFocusInWindow()</code>.
<p>
A temporary focus transfer typically occurs as the result of showing a
Menu or PopupMenu, clicking or dragging a Scrollbar, moving a Window
by dragging the title bar, or making another Window the focused
Window. Note that on some platforms, these actions may not generate
any FocusEvents at all. On others, temporary focus transfers will
occur.
<p>
When a Component receives a temporary <code>FOCUS_LOST</code> event,
the event's opposite Component (if any) may receive a temporary
<code>FOCUS_GAINED</code> event, but could also receive a permanent
<code>FOCUS_GAINED</code> event. Showing a Menu or PopupMenu, or
clicking or dragging a Scrollbar, should generate a temporary
<code>FOCUS_GAINED</code> event. Changing the focused Window,
however, will yield a permanent <code>FOCUS_GAINED</code> event
for the new focus owner.
<p>
The Component class includes variants of <code>requestFocus</code> and
<code>requestFocusInWindow</code> which take a desired temporary state as a
parameter. However, because specifying an arbitrary temporary state
may not be implementable on all native windowing systems, correct
behavior for this method can be guaranteed only for lightweight
Components. This method is not intended for general use, but exists
instead as a hook for lightweight Component libraries, such as Swing.
<a id="FocusTraversal"></a>
<h2>Focus Traversal</h2>
<p>
Each Component defines its own Set of focus traversal keys for a given
focus traversal operation. Components support separate Sets of keys
for forward and backward traversal, and also for traversal up one
focus traversal cycle. Containers which are focus cycle roots also
support a Set of keys for traversal down one focus traversal cycle. If
a Set is not explicitly defined for a Component, that Component
recursively inherits a Set from its parent, and ultimately from a
context-wide default set on the current <code>KeyboardFocusManager</code>.
<p>
Using the <code>AWTKeyStroke</code> API, client code can specify
on which of two specific KeyEvents, <code>KEY_PRESSED</code> or
<code>KEY_RELEASED</code>, the focus traversal operation will occur.
Regardless of which KeyEvent is specified, however, all KeyEvents
related to the focus traversal key, including the associated
<code>KEY_TYPED</code> event, will be consumed, and will not be
dispatched to any Component. It is a runtime error to specify a
<code>KEY_TYPED</code> event as mapping to a focus traversal operation,
or to map the same event to multiple focus traversal operations for any
particular Component or for a <code>KeyboardFocusManager</code>'s defaults.
<p>
The default focus traversal keys are implementation-dependent. Sun
recommends that the all implementations for a particular native
platform use the same keys. For Windows and Unix, the recommendations
are:
<ul>
<li>traverse forward to the next Component:
<br><i>TextAreas</i>: <code>CTRL-TAB</code> on <code>KEY_PRESSED</code>
<br><i>All others</i>: <code>TAB</code> on <code>KEY_PRESSED</code> and
<code>CTRL-TAB</code> on <code>KEY_PRESSED</code>
<li>traverse backward to the previous Component:
<br><i>TextAreas</i>: <code>CTRL-SHIFT-TAB</code> on
<code>KEY_PRESSED</code>
<br><i>All others</i>: <code>SHIFT-TAB</code> on <code>KEY_PRESSED</code>
and <code>CTRL-SHIFT-TAB</code> on
<code>KEY_PRESSED</code>
<li>traverse up one focus traversal cycle : <none>
<li>traverse down one focus traversal cycle : <none>
</ul>
<p>
Components can enable and disable all of their focus traversal keys en
masse using <code>Component.setFocusTraversalKeysEnabled</code>. When focus
traversal keys are disabled, the Component receives all KeyEvents for
those keys. When focus traversal keys are enabled, the Component never
receives KeyEvents for traversal keys; instead, the KeyEvents are
automatically mapped to focus traversal operations.
<p>
For normal forward and backward traversal, the AWT focus
implementation determines which Component to focus next based on the
<a href=#FocusTraversalPolicy><code>FocusTraversalPolicy</code></a> of
the focus owner's focus cycle root or focus traversal policy provider. If the
focus owner is a focus cycle root, then it may be ambiguous as to which
Components represent the next and previous Components to focus during
normal focus traversal. Thus, the current
<code>KeyboardFocusManager</code> maintains a reference to the
"current" focus cycle root, which is global across all contexts. The
current focus cycle root is used to resolve the ambiguity.
<p>
For up-cycle traversal, the focus owner is set to the current focus
owner's focus cycle root, and the current focus cycle root is set to
the new focus owner's focus cycle root. If, however, the current focus
owner's focus cycle root is a top-level window, then the focus owner
is set to the focus cycle root's default component to focus, and the
current focus cycle root is unchanged.
<p>
For down-cycle traversal, if the current focus owner is a focus cycle
root, then the focus owner is set to the current focus owner's default
component to focus, and the current focus cycle root is set to the
current focus owner. If the current focus owner is not a focus cycle
root, then no focus traversal operation occurs.
<a id="FocusTraversalPolicy"></a>
<h2>FocusTraversalPolicy</h2>
<p>
A <code>FocusTraversalPolicy</code> defines the order in which Components within
a particular focus cycle root or focus traversal policy provider are
traversed. Instances of <code>FocusTraversalPolicy</code> can be shared across
Containers, allowing those Containers to implement the same traversal policy.
FocusTraversalPolicies do not need to be reinitialized when the
focus-traversal-cycle hierarchy changes.
<p>
Each <code>FocusTraversalPolicy</code> must define the following
five algorithms:
<ol>
<li>Given a focus cycle root and a Component <b>a</b> in that cycle, the
next Component after <b>a</b>.
<li>Given a focus cycle root and a Component <b>a</b> in that cycle, the
previous Component before <b>a</b>.
<li>Given a focus cycle root, the "first" Component in that cycle.
The "first" Component is the Component to focus when traversal
wraps in the forward direction.
<li>Given a focus cycle root, the "last" Component in that cycle.
The "last" Component is the Component to focus when traversal
wraps in the reverse direction.
<li>Given a focus cycle root, the "default" Component in that cycle.
The "default" Component will be the first to receive focus when
traversing down into a new focus traversal cycle. This may be the
same as the "first" Component, but need not be.
</ol>
<p>
A <code>FocusTraversalPolicy</code> may optionally provide an
algorithm for the following:
<blockquote>
Given a Window, the "initial" Component in that Window. The initial
Component will be the first to receive focus when the Window is
first made visible. By default, this is the same as the "default"
Component.
</blockquote>
In addition, Swing provides a subclass of <code>FocusTraversalPolicy</code>,
<code>InternalFrameFocusTraversalPolicy</code>, which allows developers
to provide an algorithm for the following:
<blockquote>
Given a <code>JInternalFrame</code>, the "initial" Component in that
<code>JInternalFrame</code>. The initial Component is the first to
receive focus when the <code>JInternalFrame</code> is first selected.
By default, this is the same as the <code>JInternalFrame</code>'s
default Component to focus.
</blockquote>
A <code>FocusTraversalPolicy</code> is installed on a Container using
Container.<code>setFocusTraversalPolicy</code>. If a policy is not explicitly
set, then a Container inherits its policy from its nearest focus-cycle-root
ancestor. Top-levels initialize their focus traversal policies using the context
default policy. The context default policy is established by using
KeyboardFocusManager. <code>setDefaultFocusTraversalPolicy</code>.
<p>
AWT provides two standard <code>FocusTraversalPolicy</code>
implementations for use by client code.
<ol>
<li><code>ContainerOrderFocusTraversalPolicy</code>: Iterates across the
Components in a focus traversal cycle in the order they were added
to their Containers. Each Component is tested for fitness using the
accept(Component) method. By default, a Component is fit only if it
is visible, displayable, enabled, and focusable.
<li>By default, ContainerOrderFocusTraversalPolicy implicitly transfers
focus down-cycle. That is, during normal forward focus traversal,
the Component traversed after a focus cycle root will be the
focus-cycle-root's default Component to focus, regardless of whether
the focus cycle root is a traversable or non-traversable Container
(see the pic.1,2 below). Such behavior provides backward compatibility
with applications designed without the concepts of up- and down-cycle
traversal.
<li><code>DefaultFocusTraversalPolicy</code>: A subclass of
<code>ContainerOrderFocusTraversalPolicy</code> which redefines
the fitness test. If client code has explicitly set the
focusability of a Component by either overriding
<code>Component.isFocusTraversable()</code> or
<code>Component.isFocusable()</code>, or by calling
<code>Component.setFocusable(boolean)</code>, then a
<code>DefaultFocusTraversalPolicy</code> behaves exactly
like a <code>ContainerOrderFocusTraversalPolicy</code>. If,
however, the Component is relying on default focusability, then a
<code>DefaultFocusTraversalPolicy</code> will reject all
Components with non-focusable peers.
<br>
The focusability of a peer is implementation-dependent. Sun
recommends that all implementations for a particular native platform
construct peers with the same focusability. The recommendations for
Windows and Unix are that Canvases, Labels, Panels, Scrollbars,
ScrollPanes, Windows, and lightweight Components have non-focusable
peers, and all other Components have focusable peers. These
recommendations are used in the Sun AWT implementations. Note that
the focusability of a Component's peer is different from, and does
not impact, the focusability of the Component itself.
</ol>
<p>
Swing provides two additional, standard FocusTraversalPolicy
implementations for use by client code. Each implementation is an
InternalFrameFocusTraversalPolicy.
<ol>
<li>SortingFocusTraversalPolicy: Determines traversal order by
sorting the Components of a focus traversal cycle based on a given
Comparator. Each Component is tested for fitness using the
accept(Component) method. By default, a Component is fit only if it
is visible, displayable, enabled, and focusable.
<li>By default, SortingFocusTraversalPolicy implicitly transfers focus
down-cycle. That is, during normal forward focus traversal, the
Component traversed after a focus cycle root will be the
focus-cycle-root's default Component to focus, regardless of
whether the focus cycle root is a traversable or non-traversable
Container (see the pic.1,2 below). Such behavior provides backward
compatibility with applications designed without the concepts of
up- and down-cycle traversal.
<li>LayoutFocusTraversalPolicy: A subclass of
SortingFocusTraversalPolicy which sorts Components based on their
size, position, and orientation. Based on their size and position,
Components are roughly categorized into rows and columns. For a
Container with horizontal orientation, columns run left-to-right or
right-to-left, and rows run top-to-bottom. For a Container with
vertical orientation, columns run top-to-bottom and rows run
left-to-right or right-to-left. All columns in a row are fully
traversed before proceeding to the next row.
<br>
In addition, the fitness test is extended to exclude JComponents
that have or inherit empty InputMaps.
</ol>
<p>
The figure below shows an implicit focus transfer:
<br><img src="ImplicitFocusTransfer.gif" alt="Implicit focus transfer."><br>
Assume the following:
<ul>
<li><b>A</b>, <b>B</b> and <b>C</b> are components in some window (a container)
<li><b>R</b> is a container in the window and it is a parent of <b>B</b> and <b>C</b>.
Besides, <b>R</b> is a focus cycle root.
<li><b>B</b> is the default component in the focul traversal cycle of <b>R</b>
<li><b>R</b> is a traversable Container in the pic.1, and it is a non-traversable
Container in the pic.2.
<li>In such a case a forward traversal will look as follows:
<ul>
<li> pic.1 : <b>A</b> -> <b>R</b> -> <b>B</b> -> <b>C</b>
<li> pic.2 : <b>A</b> -> <b>B</b> -> <b>C</b>
</ul>
</ul>
<p>
Swing applications, or mixed Swing/AWT applications, that use one of
the standard look and feels, or any other look and feel derived from
BasicLookAndFeel, will use LayoutFocusTraversalPolicy for all
Containers by default.
<p>
All other applications, including pure AWT applications, will use
<code>DefaultFocusTraversalPolicy</code> by default.
<a id="FocusTraversalPolicyProviders"></a>
<h2>Focus Traversal Policy Providers</h2>
<p>
A Container that isn't a focus cycle root has an option to provide a
FocusTraversalPolicy of its own. To do so, one needs to set Container's focus
traversal policy provider property to <code>true</code> with the call to
<blockquote>
<code>Container.setFocusTraversalPolicyProvider(boolean)</code>
</blockquote>
To determine whether a Container is a focus traversal policy provider, the
following method should be used:
<blockquote>
<code>Container.isFocusTraversalPolicyProvider()</code>
</blockquote>
If focus traversal policy provider property is set on a focus cycle root, it
isn't considered a focus traversal policy provider and behaves just like any
other focus cycle root.
<p>
The main difference between focus cycle roots and focus traversal policy
providers is that the latter allow focus to enter and leave them just as all other
Containers. However, children inside focus traversal policy provider are
traversed in the order determined by provider's FocusTraversalPolicy. In order
to enable focus traversal policy providers to behave this way,
FocusTraversalPolicies treat them in the following manner:
<ul>
<li> Focus traversal policy providers can be passed to FocusTraversalPolicy
methods instead of focus cycle roots.
<li> When calculating next or previous Component in
<code>FocusTraversalPolicy.getComponentAfter</code> or
<code>FocusTraversalPolicy.getComponentBefore</code>,
<ul>
<li>if a Component is a child of a focus traversal policy provider,
the next and previous for this Component are determined using this
focus traversal policy provider's FocusTraversalPolicy. However,
in order for focus to leave the provider, the following rules are
applied:
<ul>
<li> if at some point the <code>next</code> found Component is
the <code>first</code> Component of focus traversal policy
provider, the Component after the focus traversal policy
provider is returned
<li> if at some point the <code>previous</code> found Component is
the <code>last</code> Component of focus traversal policy
provider, the Component before the focus traversal policy
provider is returned
</ul>
<li> When calculating the next Component in
<code>FocusTraversalPolicy.getComponentAfter</code>,
<ul>
<li> if an obtained Component is a non-traversable Container and
it is a focus traversal policy provider, then the default Component
of that provider is returned
<li> if the Component passed to the <code>FocusTraversalPolicy.getComponentAfter</code>
method is a traversable Container and it is a focus
traversal policy provider, then the default Component of this provider
is returned
</ul>
<li> When calculating the previous Component in
<code>FocusTraversalPolicy.getComponentBefore</code>,
<ul>
<li> if an obtained Component is a Container (traversable or
non-traversable) and it is a focus traversal policy provider, then
the last Component of that provider is returned
</ul>
</ul>
<li> When calculating the first Component in FocusTraversalPolicy.getFirstComponent,
<ul>
<li> if an obtained Component is a non-traversable Container and it is a focus
traversal policy provider, then the default Component of that provider is
returned
<li> if an obtained Component is a traversable Container and it is a focus traversal
policy provider, then that Container itself is returned
</ul>
<li> When calculating the last Component in FocusTraversalPolicy.getLastComponent,
<ul>
<li> if an obtained Component is a Container (traversable or non-traversable)
and it is a focus traversal policy provider, then the last Component of
that provider is returned
</ul>
</ul>
<a id="ProgrammaticTraversal"></a>
<h2>Programmatic Traversal</h2>
<p>
In addition to user-initiated focus traversal, client code can
initiate a focus traversal operation programmatically. To client code,
programmatic traversals are indistinguishable from user-initiated
traversals. The preferred way to initiate a programmatic traversal is
to use one of the following methods on <code>KeyboardFocusManager</code>:
<ul>
<li><code>KeyboardFocusManager.focusNextComponent()</code>
<li><code>KeyboardFocusManager.focusPreviousComponent()</code>
<li><code>KeyboardFocusManager.upFocusCycle()</code>
<li><code>KeyboardFocusManager.downFocusCycle()</code>
</ul>
<p>
Each of these methods initiates the traversal operation with the
current focus owner. If there is currently no focus owner, then no
traversal operation occurs. In addition, if the focus owner is not a
focus cycle root, then downFocusCycle() performs no traversal
operation.
<p>
<code>KeyboardFocusManager</code> also supports the following variants
of these methods:
<ul>
<li><code>KeyboardFocusManager.focusNextComponent(Component)</code>
<li><code>KeyboardFocusManager.focusPreviousComponent(Component)</code>
<li><code>KeyboardFocusManager.upFocusCycle(Component)</code>
<li><code>KeyboardFocusManager.downFocusCycle(Container)</code>
</ul>
Each of these methods initiates the traversal operation with the
specified Component rather than the focus owner. That is, the
traversal occurs as though the specified Component is the focus owner,
though it need not be.
<p>
Alternate, but equivalent, APIs are defined on the Component and
Container classes themselves:
<ul>
<li><code>Component.transferFocus()</code>
<li><code>Component.transferFocusBackward()</code>
<li><code>Component.transferFocusUpCycle()</code>
<li><code>Container.transferFocusDownCycle()</code>
</ul>
As with the <code>KeyboardFocusManager</code> variants, each of these methods
initiates the traversal operation as though the Component is the focus
owner, though it need not be.
<p>
Also note that hiding or disabling the focus owner, directly or
indirectly via an ancestor, or making the focus owner non-displayable
or non-focusable, initiates an automatic, forward focus traversal.
While hiding any ancestor, lightweight or heavyweight, will always
indirectly hide its children, only disabling a heavyweight ancestor
will disable its children. Thus, disabling a lightweight ancestor of
the focus owner does not automatically initiate a focus traversal.
<p>
If client code initiates a focus traversal, and there is no other
Component to focus, then the focus owner remains unchanged. If client
code initiates an automatic focus traversal by hiding the focus owner,
directly or indirectly, or by making the focus owner non-displayable or
non-focusable, and there is no other Component to focus, then the
global focus owner is cleared. If client code initiates an automatic
focus traversal by disabling the focus owner, directly or indirectly,
and there is no other Component to focus, then the focus owner remains
unchanged.
<a id="Focusability"></a>
<h2>Focusability</h2>
<p>
A focusable Component can become the focus owner ("focusability") and
participates in keyboard focus traversal ("focus traversability") with
a FocusTraversalPolicy. There is no separation of these two concepts;
a Component must either be both focusable and focus traversable, or
neither.
A Component expresses this state via the isFocusable() method. By
default, all Components return true from this method. Client code can
change this default by calling Component.setFocusable(boolean).
<a id="FocusableWindows"></a>
<h2>Focusable Windows</h2>
<p>
To support palette windows and input methods, client code can prevent
a Window from becoming the focused Window. By transitivity, this
prevents the Window or any of its descendants from becoming the focus
owner. Non-focusable Windows may still own Windows that are
focusable. By default, every Frame and Dialog is focusable. Every
Window which is not a Frame or Dialog, but whose nearest owning Frame
or Dialog is showing on the screen, and which has at least one
Component in its focus traversal cycle, is also focusable by
default. To make a Window non-focusable, use
Window.setFocusableWindowState(false).
<p>
If a Window is non-focusable, this restriction is enforced when the
<code>KeyboardFocusManager</code> sees a <code>WINDOW_GAINED_FOCUS</code>
event for the Window. At this point, the focus change is rejected and
focus is reset to a different Window. The rejection recovery scheme
is the same as if a <code>VetoableChangeListener</code> rejected the
focus change. See <a href="#FocusAndVetoableChangeListener">Focus
and VetoableChangeListener</a>.
<p>
Because the new focus implementation requires that KeyEvents intended
for a Window or its descendants be proxied through a child of the
Window's owner, and because this proxy must be mapped on X11 in order
to receive events, a Window whose nearest owning Frame or Dialog is
not showing could never receive KeyEvents on X11. To support this
restriction, we have made a distinction between a Window's "window
focusability" and its "window focusability state". A Window's
focusability state is combined with the showing state of the Window's
nearest owning Frame or Dialog to determine the Window's focusability.
By default, all Windows have a focusability state of true. Setting a
Window's focusability state to false ensures that it will not become
the focused Window regardless of the showing state of its nearest
owning Frame or Dialog.
<p>
Swing allows applications to create JWindows with null owners. Swing
constructs all such JWindows so that they are owned by a private,
hidden Frame. Because the showing state of this Frame will always be
false, a JWindow constructed will a null owner can never be the
focused Window, even if it has a Window focusability state of true.
<p>
If the focused Window is made non-focusable, then the AWT will attempt
to focus the most recently focused Component of the Window's
owner. The Window's owner will thus become the new focused Window. If
the Window's owner is also a non-focusable Window, then the focus
change request will proceed up the ownership hierarchy recursively.
Since not all platforms support cross-Window focus changes (see
<a href=#RequestingFocus>Requesting Focus</a>), it is possible that
all such focus change requests will fail. In this case, the global
focus owner will be cleared and the focused Window will remain unchanged.
<a id="RequestingFocus"></a>
<h2>Requesting Focus</h2>
<p>
A Component can request that it become the focus owner by calling
<code>Component.requestFocus()</code>. This initiates a permanent
focus transfer to the Component only if the Component is displayable,
focusable, visible and all of its ancestors (with the exception of the
top-level Window) are visible. The request will be immediately denied if
any of these conditions is not met. A disabled Component may be
the focus owner; however, in this case, all KeyEvents will be discarded.
<p>
The request will also be denied if the Component's top-level Window is
not the focused Window and the platform does not support requesting
focus across Windows. If the request is denied for this reason, the
request is remembered and will be granted when the Window is later
focused by the user. Otherwise, the focus change request changes the
focused Window as well.
<p>
There is no way to determine synchronously whether a focus change
request has been granted. Instead, client code must install a
FocusListener on the Component and watch for the delivery of a
<code>FOCUS_GAINED</code> event. Client code must not assume that
the Component is the focus owner until it receives this event.
The event may or may not be delivered before <code>requestFocus()</code>
returns. Developers must not assume one behavior or the other.
<p>
The AWT supports type-ahead if all focus change requests are made on
the EventDispatchThread. If client code requests a focus change, and
the AWT determines that this request might be granted by the native
windowing system, then the AWT will notify the current
KeyboardFocusManager that is should enqueue all KeyEvents with a
timestamp later than the that of the event currently being handled.
These KeyEvents will not be dispatched until the new Component becomes
the focus owner. The AWT will cancel the delayed dispatching request
if the focus change does not succeed at the native level, if the
Component's peer is disposed, or if the focus change is vetoed by a
VetoableChangeListener. KeyboardFocusManagers are not required to
support type-ahead if a focus change request is made from a thread
other than the EventDispatchThread.
<p>
Because <code>Component.requestFocus()</code> cannot be implemented
consistently across platforms, developers are encouraged to use
<code>Component.requestFocusInWindow()</code> instead. This method
denies cross-Window focus transfers on all platforms automatically.
By eliminating the only platform-specific element of the focus transfer,
this method achieves consistent cross-platform behavior.
<p>
In addition, <code>requestFocusInWindow()</code> returns a boolean value.
If 'false' is returned, the request is guaranteed to fail. If 'true' is
returned, the request will succeed unless it is vetoed, or an
extraordinary event, such as disposal of the Component's peer, occurs
before the request can be granted by the native windowing
system. Again, while a return value of 'true' indicates that the
request is likely to succeed, developers must never assume that this
Component is the focus owner until this Component receives a
<code>FOCUS_GAINED</code> event.
<p>
If client code wants no Component in the application to be the focus
owner, it can call the method <code>KeyboardFocusManager</code>.
<code>clearGlobalFocusOwner()</code> on the current
<code>KeyboardFocusManager</code>. If there exists a focus owner
when this method is called, the focus owner will receive a permanent
<code>FOCUS_LOST</code> event. After this point, the AWT
focus implementation will discard all KeyEvents until the user or
client code explicitly sets focus to a Component.
<p>
The Component class also supports variants of <code>requestFocus</code> and
<code>requestFocusInWindow</code> that allow client code to specify
a temporary state.
See <a href="#TemporaryFocusEvents">Temporary FocusEvents</a>
<a id="FocusAndPropertyChangeListener"></a>
<h2>Focus and PropertyChangeListener</h2>
<p>
Client code can listen to changes in context-wide focus state, or to
changes in focus-related state in Components, via
PropertyChangeListeners.
<p>
The <code>KeyboardFocusManager</code> supports the following properties:
<ol>
<li><code>focusOwner</code>: the focus owner
<li><code>focusedWindow</code>: the focused Window
<li><code>activeWindow</code>: the active Window
<li><code>defaultFocusTraversalPolicy</code>: the default focus
traversal policy
<li><code>forwardDefaultFocusTraversalKeys</code>: the Set of default
<code>FORWARD_TRAVERSAL_KEYS</code>
<li><code>backwardDefaultFocusTraversalKeys</code>: the Set of default
<code>BACKWARD_TRAVERSAL_KEYS</code>
<li><code>upCycleDefaultFocusTraversalKeys</code>: the Set of default
<code>UP_CYCLE_TRAVERSAL_KEYS</code>
<li><code>downCycleDefaultFocusTraversalKeys</code>: the Set of default
<code>DOWN_CYCLE_TRAVERSAL_KEYS</code>
<li><code>currentFocusCycleRoot</code>: the current focus cycle root
</ol>
<p>
A <code>PropertyChangeListener</code> installed on the current
<code>KeyboardFocusManager</code> will only see these changes within
the <code>KeyboardFocusManager</code>'s context, even though the
focus owner, focused Window, active Window, and current focus cycle
root comprise the global focus state shared by all contexts.
We believe this is less intrusive than requiring client code to pass
a security check before installing a <code>PropertyChangeListener</code>.
<p>
Component supports the following focus-related properties:
<ol>
<li><code>focusable</code>: the Component's focusability
<li><code>focusTraversalKeysEnabled</code>: the Component's
focus traversal keys enabled state
<li><code>forwardFocusTraversalKeys</code>: the Component's Set of
<code>FORWARD_TRAVERSAL_KEYS</code>
<li><code>backwardFocusTraversalKeys</code>: the Component's Set of
<code>BACKWARD_TRAVERSAL_KEYS</code>
<li><code>upCycleFocusTraversalKeys</code>: the Component's Set of
<code>UP_CYCLE_TRAVERSAL_KEYS</code>
</ol>
<p>
In addition to the Component properties, Container supports the
following focus-related properties:
<ol>
<li><code>downCycleFocusTraversalKeys</code>: the Container's Set of
<code>DOWN_CYCLE_TRAVERSAL_KEYS</code>
<li><code>focusTraversalPolicy</code>: the Container's focus
traversal policy
<li><code>focusCycleRoot</code>: the Container's focus-cycle-root state
</ol>
<p>
In addition to the Container properties, Window supports the following
focus-related property:
<ol>
<li><code>focusableWindow</code>: the Window's focusable Window state
</ol>
<p>
Also note that a <code>PropertyChangeListener</code> installed on a
Window will never see a <code>PropertyChangeEvent</code> for the
<code>focusCycleRoot</code> property.
A Window is always a focus cycle root; this property cannot change.
<p>
<a id="FocusAndVetoableChangeListener"></a>
<h2>Focus and VetoableChangeListener</h2>
<p>
The <code>KeyboardFocusManager</code> also supports
<code>VetoableChangeListener</code>s for the following properties:
<ol>
<li>"focusOwner": the focus owner
<li>"focusedWindow": the focused Window
<li>"activeWindow": the active Window
</ol>
If a VetoableChangeListener vetoes a focus or activation change by
throwing a PropertyVetoException, the change is aborted. Any
VetoableChangeListeners which had already approved the change will
asynchronously receive PropertyChangeEvents indicating a reversion of
state to the previous value.
<p>
VetoableChangeListeners are notified of the state change before the
change is reflected in the KeyboardFocusManager. Conversely,
PropertyChangeListeners are notified after the change is reflected.
It follows that all VetoableChangeListeners will be notified before
any PropertyChangeListener.
<p>
VetoableChangeListeners must be idempotent, and must veto both loss
and gain events for a particular focus change (e.g., both
<code>FOCUS_LOST</code> and <code>FOCUS_GAINED</code>). For example,
if a <code>VetoableChangeListener</code> vetoes a <code>FOCUS_LOST</code>
event, a <code>KeyboardFocusManager</code> is not required to search the
<code>EventQueue</code> and remove the associated pending
<code>FOCUS_GAINED</code> event. Instead, the
<code>KeyboardFocusManager</code> is free to attempt to
dispatch this event and it is the responsibility of the
<code>VetoableChangeListener</code> to veto it as well. In addition,
during processing of the <code>FOCUS_GAINED</code> event, the
<code>KeyboardFocusManager</code> may attempt to resync the global
focus state by synthesizing another <code>FOCUS_LOST</code> event.
This event must be vetoed just as the first <code>FOCUS_LOST</code> event was.
<p>
A <code>KeyboardFocusManager</code> may not hold any locks while
notifying <code>PropertyChangeListener</code>s of a state change.
This requirement is relaxed for <code>VetoableChangeListeners</code>,
however. Therefore, client-definied <code>VetoableChangeListener</code>s
should avoid acquiring additional locks inside
<code>vetoableChange(PropertyChangeEvent)</code> as this may lead to deadlock.
If a focus or activation change is rejected, the KeyboardFocusManager
will initiate rejection recovery as follows:
<ul>
<li>If a focused or active Window change was rejected, then the
focused or active Window will be reset to the Window which was
previously the focused or active Window. If there is no such
Window, then the <code>KeyboardFocusManager</code> will clear
the global focus owner.
<li>If a focus owner change was rejected, then the focus owner will be
reset to the Component which was previously the focus owner. If
that is not possible, then it will be reset to the next Component
in the focus traversal cycle after the previous focus owner. If
that is also not possible, then the <code>KeyboardFocusManager</code>
will clear the global focus owner.
</ul>
<code>VetoableChangeListener</code>s must be careful to avoid vetoing focus
changes initiated as a result of veto rejection recovery. Failure
to anticipate this situation could lead to an infinite cycle of
vetoed focus changes and recovery attempts.
<a id="ZOrder"></a>
<h2>Z-Order</h2>
<p>
On some native windowing systems, the Z-order of a Window can affect
its focused or active (if applicable) state. On Microsoft Windows, the
top-most Window is naturally the focused Window as well. However, on
Solaris, many window managers use a point-to-focus model that ignores
Z-order in determining the focused Window.
When focusing or activating Windows, the AWT adheres to the UI
requirements of the native platform. Therefore, the focus behavior of
Z-order-related methods such as:
<ul>
<li><code>Window.toFront()</code>
<li><code>Window.toBack()</code>
<li><code>Window.show()</code>
<li><code>Window.hide()</code>
<li><code>Window.setVisible(boolean)</code>
<li><code>Window.dispose()</code>
<li><code>Frame.setState(int)</code>
</ul>
is platform-dependent. In JDK 1.4, the behavior of these methods on
Microsoft Windows and Solaris is as follows:
<ul>
<li><code>Window.toFront()</code>:<br>
<b>Microsoft Windows</b>: The Window is moved to front, if possible.
While we will always be able to move this Window in front of other
Windows in the same VM, Windows 98 and Windows 2000 do not allow an
application to bring any of its windows to the front unless one
of that application's windows is already in the foreground. In
this case, Windows will instead flash the Window's icon in the
taskbar. If the Window is moved to the front, it will be made
the focused and (if applicable) active Window.
<br>
<b>Solaris</b>: The Window is moved to front. In a point-to-focus
window manager, the Window will become the focused Window if it
is the top-most Window underneath the cursor. In a click-to-focus
window manager, the focused Window will remain unchanged.
<li><code>Window.toBack()</code>:<br>
<b>Microsoft Windows</b>: The Window is moved to back. Note however
that Microsoft Windows insists that an owned Window always be in
front of all of its recursive owners. Thus, after the completion of
this operation, the Window may not be the lowest Java Window in the
Z-order. If the Window, or any of its owners, was the focused Window,
then the focused Window is reset to the top-most Window in the VM.
<br>
<b>Solaris</b>: The Window is moved to back. Like Microsoft Windows,
some window managers insist than an owned Window always be in front
of all of its recursive owners. Thus, after the completion of this
operation, the Window may not be the lowest Java Window in the
Z-order. If the Window was the focused Window, it will lose
focus in a point-to-focus window manager if it is no longer the
top-most Window under the cursor. In a click-to-focus window
manager, the focused Window will remain unchanged.
<li><code>Window.show()/Window.setVisible(true)/Frame.setState(NORMAL)</code>:<br>
<b>Microsoft Windows</b>: The Window is moved to front and becomes the focused
Window.
<br>
<b>Solaris</b>: The Window is moved to front. In a point-to-focus focus
window manager, the Window will be focused if it is now the
top-most Window under the cursor. In a click-to-focus window
manager, the Window will become the focused Window.
<li><code>Window.hide()/Window.setVisible(false)/Window.dispose()/
Frame.setState(ICONIFIED)</code>:<br>
<b>Microsoft Windows</b>: If the Window was the focused Window, the focused
Window is reset to a window chosen by the OS, or to no window. The
window may be in a native application, or a Java application in
another VM.
<br>
<b>Solaris</b>: If the Window was the focused Window, in a point-to-
focus window manager, the top-most Window under the cursor will
become the focused Window. In a click-to-focus window manager,
the focused Window is reset to a window chosen by the window
manager. The window may be in a native application, or a Java
application in another VM.
</ul>
<a id="ReplacingDefaultKeyboardFocusManager"></a>
<h2>Replacing DefaultKeyboardFocusManager</h2>
<p>
<code>KeyboardFocusManager</code>s are pluggable at the browser context
level. Client code can subclass <code>KeyboardFocusManager</code> or
<code>DefaultKeyboardFocusManager</code> to modify the way that WindowEvents
related to focus, FocusEvents, and KeyEvents are handled and
dispatched, and to examine and modify the global focus state. A custom
<code>KeyboardFocusManager</code> can also reject focus changes at a more
fundamental level then a FocusListener or WindowListener ever could.
<p>
While giving a developer ultimate control over the focus model,
replacing the entire <code>KeyboardFocusManager</code> is a difficult process
requiring a thorough understanding of the peer focus layer.
Fortunately, most applications do not need this much control.
Developers are encouraged to use KeyEventDispatchers,
KeyEventPostProcessors, FocusTraversalPolicies,
VetoableChangeListeners, and other concepts discussed in this document
before resorting to a full replacement of the <code>KeyboardFocusManager</code>.
<p>
First note that, because unhindered access to Components in other
contexts represents a security hole, the SecurityManager must grant a
new permission, "replaceKeyboardFocusManager", before client code is
permitted to replace the <code>KeyboardFocusManager</code> with an arbitrary
subclass instance. Because of the security check, replacing the
<code>KeyboardFocusManager</code> is not an option for applications that will be
deployed in environments with a SecurityManager, such as applets in a
browser.
<p>
Once installed, a <code>KeyboardFocusManager</code> instance has
access to the global focus state via a set of protected functions.
The <code>KeyboardFocusManager</code> can only call these functions
if it is installed in the calling thread's context. This ensures
that malicious code cannot circumvent the security check in
<code>KeyboardFocusManager.setCurrentFocusManager</code>.
A <code>KeyboardFocusManager</code> should always work with
the global focus state instead of the context focus state.
Failure to do this will lead to incorrect behavior of the
<code>KeyboardFocusManager</code>.
<p>
The primary responsibility of a <code>KeyboardFocusManager</code>
is the dispatch of the following events:
<ul>
<li>all <code>KeyEvent</code>s
<li>all <code>FocusEvent</code>s
<li><code>WindowEvent.WINDOW_GAINED_FOCUS</code>
<li><code>WindowEvent.WINDOW_LOST_FOCUS</code>
<li><code>WindowEvent.WINDOW_ACTIVATED</code>
<li><code>WindowEvent.WINDOW_DEACTIVATED</code>
</ul>
The peer layer will provide the <code>KeyboardFocusManager</code>
with all of the above events except <code>WINDOW_ACTIVATED</code>
and <code>WINDOW_DEACTIVATED</code>. The <code>KeyboardFocusManager</code>
must synthesize <code>WINDOW_ACTIVATED</code> and
<code>WINDOW_DEACTIVATED</code> events when appropriate and target them
accordingly.
<p>
The <code>KeyboardFocusManager</code> may need to retarget the events
provided by the peer layer to its own notion of the focus owner or
focused Window:
<ul>
<li>A KeyEvent must be retargeted to the focus owner. Because the peer
layer is unaware of any lightweight Components, KeyEvents will
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