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  33       <h1>The AWT Focus Subsystem</h1>







  34 
  35     <p>
  36       Prior to Java 2 Standard Edition, JDK 1.4, the AWT focus subsystem
  37       was inadequate. It suffered from major design and API problems,
  38       as well as over a hundred open bugs. Many of these bugs were caused by
  39       platform inconsistencies, or incompatibilities between the native
  40       focus system for heavyweights and the Java focus system for
  41       lightweights.
  42     <p>
  43       The single worst problem with the AWT focus implementation was the
  44       inability to query for the currently focused Component. Not only was
  45       there no API for such a query, but also, because of an insufficient
  46       architecture, such information was not even maintained by the code.
  47     <p>
  48       Almost as bad was the inability of lightweight children of a Window
  49       (not a Frame or a Dialog) to receive keyboard input. This problem
  50       existed because Windows never received <code>WINDOW_ACTIVATED</code>
  51       events and thus could never be activated, and only active Windows
  52       could contain focused Components.
  53     <p>
  54       In addition, many developers noted that the APIs for FocusEvent and
  55       WindowEvent were insufficient because they did not provide a way for
  56       determining the "opposite" Component involved in the focus or
  57       activation change. For example, when a Component received a FOCUS_LOST
  58       event, it had no way of knowing which Component was gaining
  59       focus. Since Microsoft Windows provides this functionality for free,
  60       developers migrating from Microsoft Windows C/C++ or Visual Basic to
  61       Java had been frustrated by the omission.
  62     <p>
  63       To address these and other deficiencies, we have designed a new focus
  64       model for the AWT in JDK 1.4. The primary design changes were the
  65       construction of a new centralized KeyboardFocusManager class, and a
  66       lightweight focus architecture. The amount of focus-related,
  67       platform-dependent code has been minimized and replaced by fully
  68       pluggable and extensible public APIs in the AWT. While we have
  69       attempted to remain backward compatible with the existing
  70       implementation, we were forced to make minor incompatible changes in
  71       order to reach an elegant and workable conclusion. We anticipate that
  72       these incompatibilities will have only a trivial impact on existing
  73       applications.
  74     <p>
  75       This document is a formal specification both of the new APIs and of
  76       existing APIs which remain relevant in the new model. Combined with
  77       the javadoc for focus-related classes and methods, this document
  78       should enable developers to create substantial AWT and Swing
  79       applications with a focus behavior that is customized yet consistent
  80       across platforms.  This document has the following sections:
  81     <ul>
  82       <li><a href=#Overview>Overview of KeyboardFocusManager</a>
  83       <li><a href=#BrowserContexts>KeyboardFocusManager and Browser Contexts</a>
  84       <li><a href=#KeyEventDispatcher>KeyEventDispatcher</a>
  85       <li><a href=#FocusEventAndWindowEvent>FocusEvent and WindowEvent</a>
  86       <li><a href=#EventDelivery>Event Delivery</a>
  87       <li><a href=#OppositeComponents>Opposite Components and Windows</a>
  88       <li><a href=#TemporaryFocusEvents>Temporary FocusEvents</a>
  89       <li><a href=#FocusTraversal>Focus Traversal</a>
  90       <li><a href=#FocusTraversalPolicy>Focus Traversal Policy</a>
  91       <li><a href=#FocusTraversalPolicyProviders>Focus Traversal Policy Providers</a>
  92       <li><a href=#ProgrammaticTraversal>Programmatic Traversal</a>
  93       <li><a href=#Focusability>Focusability</a>
  94       <li><a href=#FocusableWindows>Focusable Windows</a>
  95       <li><a href=#RequestingFocus>Requesting Focus</a>
  96       <li><a href=#FocusAndPropertyChangeListener>Focus and PropertyChangeListener</a>
  97       <li><a href=#FocusAndVetoableChangeListener>Focus and VetoableChangeListener</a>
  98       <li><a href=#ZOrder>Z-Order</a>
  99       <li><a href=#ReplacingDefaultKeyboardFocusManager>Replacing DefaultKeyboardFocusManager</a>
 100       <li><a href=#Incompatibilities>Incompatibilities with Previous Releases</a>
 101      </ul>
 102 
 103       <a id="Overview"></a>
 104       <h3>Overview of KeyboardFocusManager</h3>
 105     <p>
 106       The focus model is centralized around a single class,
 107       KeyboardFocusManager, that provides a set of APIs for client code to
 108       inquire about the current focus state, initiate focus changes, and
 109       replace default focus event dispatching with a custom dispatcher.
 110       Clients can inquire about the focus state directly, or can register a
 111       PropertyChangeListener that will receive PropertyChangeEvents when a
 112       change to the focus state occurs.
 113     <p>
 114       KeyboardFocusManager introduces the following main concepts and their
 115       terminology:
 116     <ol>
 117       <li>The "focus owner" -- the Component which typically receives
 118           keyboard input.
 119       <li>The "permanent focus owner" -- the last Component to receive
 120           focus permanently. The "focus owner" and the "permanent focus
 121           owner" are equivalent unless a temporary focus change is
 122           currently in effect. In such a situation, the "permanent focus
 123           owner" will again be the "focus owner" when the temporary focus
 124           change ends.
 125       <li>The "focused Window" -- the Window which contains the "focus
 126           owner".
 127       <li>The "active Window" -- the Frame or Dialog that is either the
 128           "focused Window", or the first Frame or Dialog that is an owner
 129           of the "focused Window".
 130       <li>"Focus traversal" -- the user's ability to change the "focus
 131           owner" without moving the cursor. Typically, this is done using
 132           the keyboard (for example, by using the TAB key), or an
 133           equivalent device in an accessible environment. Client code can
 134           also initiate traversal programmatically. Normal focus traversal
 135           can be either "forward" to the "next" Component, or "backward" to
 136           the "previous" Component.
 137       <li>"Focus traversal cycle" -- a portion of the Component hierarchy,
 138           such that normal focus traversal "forward" (or "backward") will
 139           traverse through all of the Components in the focus cycle, but no
 140           other Components. This cycle provides a mapping from an arbitrary
 141           Component in the cycle to its "next" (forward traversal) and
 142           "previous" (backward traversal) Components.
 143       <li>"Traversable Component" -- Component that is in the focus traversal
 144           cycle.
 145       <li>"Non-traversable Component" -- Component that is not in the focus
 146           traversal cycle. Note that a non-traversable Component can nevertheless
 147           be focused in other way (e.g. by direct focus request).
 148       <li>"Focus cycle root" -- Container that is the root of the Component
 149           hierarchy for a particular "focus traversal cycle". When the
 150           "focus owner" is a Component inside a particular cycle, normal
 151           forward and backward focus traversal cannot move the "focus
 152           owner" above the focus cycle root in the Component hierarchy.
 153           Instead, two additional traversal operations, "up cycle" and
 154           "down cycle", are defined to allow keyboard and programmatic
 155           navigation up and down the focus traversal cycle hierarchy. </li>
 156       <li>"Focus traversal policy provider" - Container which has
 157           "FocusTraversalPolicyProvider" property as true. This Container will
 158           be used to acquire focus traversal policy. This container doesn't
 159           define new focus cycle but only modifies the order by which its
 160           children are traversed "forward" and "backward". Focus traversal
 161           policy provider can be set using
 162           <code>setFocusTraversalPolicyProvider</code> on the Container.
 163     </ol>
 164 
 165     <p>
 166       Every Window and JInternalFrame is, by default, a "focus cycle
 167       root". If it's the only focus cycle root, then all of its
 168       focusable descendants should be in its focus cycle, and its focus
 169       traversal policy should enforce that they are by making sure that
 170       all will be reached during normal forward (or backward)
 171       traversal. If, on the other hand, the Window or JInternalFrame
 172       has descendants that are also focus cycle roots, then each such
 173       descendant is a member of two focus cycles: the one that it is
 174       the root of, and the one of its nearest focus-cycle-root
 175       ancestor. In order to traverse the focusable components belonging
 176       to the focus cycle of such a "descendant" focus cycle root, one
 177       first traverses (forward or backward) to reach the descendant,
 178       and then uses the "down cycle" operation to reach, in turn, its
 179       descendants.
 180 
 181      <p>
 182      Here is an example:<br> <img src="FocusCycle.gif"
 183      alt="Three groups as described below: ABCF BDE and DGH. "><br>
 184 
 185      <p>Assume the following:
 186       <ul>
 187         <li><b>A</b> is a <code>Window</code>, which means that it
 188             must be a focus cycle root.
 189         <li><b>B</b> and <b>D</b> are <code>Container</code>s that
 190             are focus cycle roots.
 191         <li><b>C</b> is a <code>Container</code> that is not a focus cycle root.
 192         <li><b>G</b>, <b>H</b>, <b>E</b>, and <b>F</b> are all
 193             <code>Component</code>s.
 194       </ul>
 195 
 196      There are a total of three focus cycle roots in this example:
 197 
 198       <ol>
 199         <li><b>A</b> is a root, and <b>A</b>, <b>B</b>, <b>C</b>,
 200             and <b>F</b> are members of <b>A</b>'s cycle.
 201         <li><b>B</b> is a root, and <b>B</b>, <b>D</b>, and
 202             <b>E</b> are members of <b>B</b>'s cycle.
 203         <li><b>D</b> is a root, and <b>D</b>, <b>G</b>,
 204             and <b>H</b> are members of <b>D</b>'s cycle.
 205       </ol>
 206 
 207      Windows are the only Containers which, by default, are focus cycle
 208      roots.
 209 
 210 
 211 <code>KeyboardFocusManager</code> is an abstract class. AWT provides a default
 212 implementation in the <code>DefaultKeyboardFocusManager</code> class.
 213 
 214 
 215 <a id="BrowserContexts"></a>
 216 <h3>KeyboardFocusManager and Browser Contexts</h3>
 217 <p>
 218 Some browsers partition applets in different code bases into separate
 219 contexts, and establish walls between these contexts. Each thread and
 220 each Component is associated with a particular context and cannot
 221 interfere with threads or access Components in other contexts. In such
 222 a scenario, there will be one KeyboardFocusManager per context. Other
 223 browsers place all applets into the same context, implying that there
 224 will be only a single, global KeyboardFocusManager for all
 225 applets. This behavior is implementation-dependent. Consult your
 226 browser's documentation for more information. No matter how many
 227 contexts there may be, however, there can never be more than one focus
 228 owner, focused Window, or active Window, per ClassLoader.
 229 
 230 
 231 <a id="KeyEventDispatcher"></a>
 232 <h3>KeyEventDispatcher and KeyEventPostProcessor</h3>
 233 <p>
 234 While the user's KeyEvents should generally be delivered to the focus
 235 owner, there are rare cases where this is not desirable. An input
 236 method is an example of a specialized Component that should receive
 237 KeyEvents even though its associated text Component is and should
 238 remain the focus owner.
 239 <p>
 240 A KeyEventDispatcher is a lightweight interface that allows client
 241 code to pre-listen to all KeyEvents in a particular context. Instances
 242 of classes that implement the interface and are registered with the
 243 current KeyboardFocusManager will receive KeyEvents before they are
 244 dispatched to the focus owner, allowing the KeyEventDispatcher to
 245 retarget the event, consume it, dispatch it itself, or make other
 246 changes.
 247 <p>
 248 For consistency, KeyboardFocusManager itself is a
 249 KeyEventDispatcher. By default, the current KeyboardFocusManager will
 250 be the sink for all KeyEvents not dispatched by the registered
 251 KeyEventDispatchers. The current KeyboardFocusManager cannot be
 252 completely deregistered as a KeyEventDispatcher. However, if a
 253 KeyEventDispatcher reports that it dispatched the KeyEvent, regardless
 254 of whether it actually did so, the KeyboardFocusManager will take no
 255 further action with regard to the KeyEvent. (While it is possible for
 256 client code to register the current KeyboardFocusManager as a
 257 KeyEventDispatcher one or more times, there is no obvious reason why
 258 this would be necessary, and therefore it is not recommended.)
 259 <p>
 260 Client-code may also post-listen to KeyEvents in a particular context
 261 using the KeyEventPostProcessor interface. KeyEventPostProcessors
 262 registered with the current KeyboardFocusManager will receive
 263 KeyEvents after the KeyEvents have been dispatched to and handled by
 264 the focus owner. The KeyEventPostProcessors will also receive
 265 KeyEvents that would have been otherwise discarded because no
 266 Component in the application currently owns the focus. This will allow
 267 applications to implement features that require global KeyEvent post-
 268 handling, such as menu shortcuts.
 269 <p>
 270 Like KeyEventDispatcher, KeyboardFocusManager also implements
 271 KeyEventPostProcessor, and similar restrictions apply to its use in
 272 that capacity.
 273 
 274 <a id="FocusEventAndWindowEvent"></a>
 275 <h3>FocusEvent and WindowEvent</h3>
 276 <p>
 277 The AWT defines the following six event types central to the focus
 278 model in two different <code>java.awt.event</code> classes:
 279   <ol>
 280     <li><code>WindowEvent.WINDOW_ACTIVATED</code>: This event is
 281         dispatched to a Frame or Dialog (but never a Window which
 282         is not a Frame or Dialog) when it becomes the active Window.
 283     <li><code>WindowEvent.WINDOW_GAINED_FOCUS</code>: This event is
 284         dispatched to a Window when it becomes the focused Window.
 285         Only focusable Windows can receive this event.
 286     <li><code>FocusEvent.FOCUS_GAINED</code>: This event is dispatched
 287         to a Component when it becomes the focus owner. Only focusable
 288         Components can receive this event.
 289     <li><code>FocusEvent.FOCUS_LOST</code>: This event is dispatched
 290         to a Component when it is no longer the focus owner.
 291     <li><code>WindowEvent.WINDOW_LOST_FOCUS</code>: This event is
 292         dispatched to a Window when it is no longer the focused Window.
 293     <li><code>WindowEvent.WINDOW_DEACTIVATED</code>: This event is
 294         dispatched to a Frame or Dialog (but never a Window which is
 295         not a Frame or Dialog) when it is no longer the active Window.
 296   </ol>
 297 
 298 <a id="EventDelivery"></a>
 299 <h3>Event Delivery</h3>
 300 <p>
 301 If the focus is not in java application and the user clicks on a focusable
 302 child Component<b>a</b> of an inactive Frame <b>b</b>, the following events
 303 will be dispatched and handled in order:
 304 
 305   <ol>
 306     <li><b>b</b> will receive a <code>WINDOW_ACTIVATED</code> event.
 307     <li>Next, <b>b</b> will receive a <code>WINDOW_GAINED_FOCUS</code> event.
 308     <li>Finally, <b>a</b> will receive a <code>FOCUS_GAINED</code> event.
 309   </ol>
 310 
 311 If the user later clicks on a focusable child Component <b>c</b> of another
 312 Frame <b>d</b>, the following events will be dispatched and handled in
 313 order:
 314   <ol>
 315    <li><b>a</b> will receive a <code>FOCUS_LOST</code> event.
 316    <li><b>b</b> will receive a <code>WINDOW_LOST_FOCUS</code> event.
 317    <li><b>b</b> will receive a <code>WINDOW_DEACTIVATED</code> event.
 318    <li><b>d</b> will receive a <code>WINDOW_ACTIVATED</code> event.
 319    <li><b>d</b> will receive a <code>WINDOW_GAINED_FOCUS</code> event.
 320    <li><b>c</b> will receive a <code>FOCUS_GAINED</code> event.
 321   </ol>
 322 
 323 Note that each event will be fully handled before the next event is
 324 dispatched. This restriction will be enforced even if the Components
 325 are in different contexts and are handled on different event
 326 dispatching threads.
 327 <p>
 328 In addition, each event type will be dispatched in 1-to-1
 329 correspondence with its opposite event type. For example, if a
 330 Component receives a <code>FOCUS_GAINED</code> event, under no
 331 circumstances can it ever receive another <code>FOCUS_GAINED</code>
 332 event without an intervening <code>FOCUS_LOST</code> event.
 333 <p>
 334 Finally, it is important to note that these events are delivered for
 335 informational purposes only. It is impossible, for example, to prevent
 336 the delivery of a pending <code>FOCUS_GAINED</code> event by requesting
 337 focus back to the Component losing focus while handling the preceding
 338 <code>FOCUS_LOST</code> event. While client code may make such a request,
 339 the pending <code>FOCUS_GAINED</code> will still be delivered,
 340 followed later by the events transferring focus back to the original
 341 focus owner.
 342 <p>
 343 If it is absolutely necessary to suppress the <code>FOCUS_GAINED</code> event,
 344 client code can install a <code>VetoableChangeListener</code> which
 345 rejects the focus change. See <a href="#FocusAndVetoableChangeListener">Focus
 346 and VetoableChangeListener</a>.
 347 
 348 
 349 <a id="OppositeComponents"></a>
 350 <h3>Opposite Components and Windows</h3>
 351 <p>
 352 Each event includes information about the "opposite" Component or
 353 Window involved in the focus or activation change. For example, for a
 354 <code>FOCUS_GAINED</code> event, the opposite Component is the Component
 355 that lost focus. If the focus or activation change occurs with a native
 356 application, with a Java application in a different VM or context, or
 357 with no other Component, then the opposite Component or Window is
 358 null. This information is accessible using
 359 <code>FocusEvent.getOppositeComponent</code> or
 360 <code>WindowEvent.getOppositeWindow</code>.
 361 <p>
 362 On some platforms, it is not possible to discern the opposite
 363 Component or Window when the focus or activation change occurs between
 364 two different heavyweight Components. In these cases, the opposite
 365 Component or Window may be set to null on some platforms, and to a
 366 valid non-null value on other platforms. However, for a focus change
 367 between two lightweight Components which share the same heavyweight
 368 Container, the opposite Component will always be set correctly. Thus,
 369 a pure Swing application can ignore this platform restriction when
 370 using the opposite Component of a focus change that occurred within a
 371 top-level Window.
 372 
 373 <a id="TemporaryFocusEvents"></a>
 374 <h3>Temporary FocusEvents</h3>
 375 <p>
 376 <code>FOCUS_GAINED</code> and <code>FOCUS_LOST</code> events are
 377 marked as either temporary or permanent.
 378 <p>
 379 Temporary <code>FOCUS_LOST</code> events are sent when a Component is
 380 losing the focus, but will regain the focus shortly. These events
 381 can be useful when focus changes are used as triggers for validation
 382 of data. For instance, a text Component may want to commit its
 383 contents when the user begins interacting with another Component,
 384 and can accomplish this by responding to <code>FOCUS_LOST</code> events.
 385 However, if the <code>FocusEvent</code> received is temporary,
 386 the commit should not be done, since the text field will be receiving
 387 the focus again shortly.
 388 <p>
 389 A permanent focus transfer typically occurs as the result of a user
 390 clicking on a selectable, heavyweight Component, focus traversal with
 391 the keyboard or an equivalent input device, or from a call to
 392 <code>requestFocus()</code> or <code>requestFocusInWindow()</code>.
 393 <p>
 394 A temporary focus transfer typically occurs as the result of showing a
 395 Menu or PopupMenu, clicking or dragging a Scrollbar, moving a Window
 396 by dragging the title bar, or making another Window the focused
 397 Window. Note that on some platforms, these actions may not generate
 398 any FocusEvents at all. On others, temporary focus transfers will
 399 occur.
 400 <p>
 401 When a Component receives a temporary <code>FOCUS_LOST</code> event,
 402 the event's opposite Component (if any) may receive a temporary
 403 <code>FOCUS_GAINED</code> event, but could also receive a permanent
 404 <code>FOCUS_GAINED</code> event. Showing a Menu or PopupMenu, or
 405 clicking or dragging a Scrollbar, should generate a temporary
 406 <code>FOCUS_GAINED</code> event. Changing the focused Window,
 407 however, will yield a permanent <code>FOCUS_GAINED</code> event
 408 for the new focus owner.
 409 <p>
 410 The Component class includes variants of <code>requestFocus</code> and
 411 <code>requestFocusInWindow</code> which take a desired temporary state as a
 412 parameter. However, because specifying an arbitrary temporary state
 413 may not be implementable on all native windowing systems, correct
 414 behavior for this method can be guaranteed only for lightweight
 415 Components. This method is not intended for general use, but exists
 416 instead as a hook for lightweight Component libraries, such as Swing.
 417 
 418 <a id="FocusTraversal"></a>
 419 <h3>Focus Traversal</h3>
 420 <p>
 421 Each Component defines its own Set of focus traversal keys for a given
 422 focus traversal operation. Components support separate Sets of keys
 423 for forward and backward traversal, and also for traversal up one
 424 focus traversal cycle. Containers which are focus cycle roots also
 425 support a Set of keys for traversal down one focus traversal cycle. If
 426 a Set is not explicitly defined for a Component, that Component
 427 recursively inherits a Set from its parent, and ultimately from a
 428 context-wide default set on the current <code>KeyboardFocusManager</code>.
 429 <p>
 430 Using the <code>AWTKeyStroke</code> API, client code can specify
 431 on which of two specific KeyEvents, <code>KEY_PRESSED</code> or
 432 <code>KEY_RELEASED</code>, the focus traversal operation will occur.
 433 Regardless of which KeyEvent is specified, however, all KeyEvents
 434 related to the focus traversal key, including the associated
 435 <code>KEY_TYPED</code> event, will be consumed, and will not be
 436 dispatched to any Component. It is a runtime error to specify a
 437 <code>KEY_TYPED</code> event as mapping to a focus traversal operation,
 438 or to map the same event to multiple focus traversal operations for any
 439 particular Component or for a <code>KeyboardFocusManager</code>'s defaults.
 440 <p>
 441 The default focus traversal keys are implementation-dependent. Sun
 442 recommends that the all implementations for a particular native
 443 platform use the same keys. For Windows and Unix, the recommendations
 444 are:
 445 
 446   <ul>
 447      <li>traverse forward to the next Component:
 448       <br><i>TextAreas</i>: <code>CTRL-TAB</code> on <code>KEY_PRESSED</code>
 449       <br><i>All others</i>: <code>TAB</code> on <code>KEY_PRESSED</code> and
 450                        <code>CTRL-TAB</code> on <code>KEY_PRESSED</code>
 451      <li>traverse backward to the previous Component:
 452       <br><i>TextAreas</i>: <code>CTRL-SHIFT-TAB</code> on
 453                        <code>KEY_PRESSED</code>
 454       <br><i>All others</i>: <code>SHIFT-TAB</code> on <code>KEY_PRESSED</code>
 455                        and <code>CTRL-SHIFT-TAB</code> on
 456                        <code>KEY_PRESSED</code>
 457      <li>traverse up one focus traversal cycle : &lt;none&gt;
 458      <li>traverse down one focus traversal cycle : &lt;none&gt;
 459   </ul>
 460 <p>
 461 Components can enable and disable all of their focus traversal keys en
 462 masse using <code>Component.setFocusTraversalKeysEnabled</code>. When focus
 463 traversal keys are disabled, the Component receives all KeyEvents for
 464 those keys. When focus traversal keys are enabled, the Component never
 465 receives KeyEvents for traversal keys; instead, the KeyEvents are
 466 automatically mapped to focus traversal operations.
 467 <p>
 468 For normal forward and backward traversal, the AWT focus
 469 implementation determines which Component to focus next based on the
 470 <a href=#FocusTraversalPolicy><code>FocusTraversalPolicy</code></a> of
 471 the focus owner's focus cycle root or focus traversal policy provider. If the
 472 focus owner is a focus cycle root, then it may be ambiguous as to which
 473 Components represent the next and previous Components to focus during
 474 normal focus traversal. Thus, the current
 475 <code>KeyboardFocusManager</code> maintains a reference to the
 476 "current" focus cycle root, which is global across all contexts. The
 477 current focus cycle root is used to resolve the ambiguity.
 478 <p>
 479 For up-cycle traversal, the focus owner is set to the current focus
 480 owner's focus cycle root, and the current focus cycle root is set to
 481 the new focus owner's focus cycle root. If, however, the current focus
 482 owner's focus cycle root is a top-level window, then the focus owner
 483 is set to the focus cycle root's default component to focus, and the
 484 current focus cycle root is unchanged.
 485 <p>
 486 For down-cycle traversal, if the current focus owner is a focus cycle
 487 root, then the focus owner is set to the current focus owner's default
 488 component to focus, and the current focus cycle root is set to the
 489 current focus owner. If the current focus owner is not a focus cycle
 490 root, then no focus traversal operation occurs.
 491 
 492 
 493 <a id="FocusTraversalPolicy"></a>
 494 <h3>FocusTraversalPolicy</h3>
 495 <p>
 496 
 497 A <code>FocusTraversalPolicy</code> defines the order in which Components within
 498 a particular focus cycle root or focus traversal policy provider are
 499 traversed. Instances of <code>FocusTraversalPolicy</code> can be shared across
 500 Containers, allowing those Containers to implement the same traversal policy.
 501 FocusTraversalPolicies do not need to be reinitialized when the
 502 focus-traversal-cycle hierarchy changes.
 503 
 504 <p>
 505 Each <code>FocusTraversalPolicy</code> must define the following
 506 five algorithms:
 507 
 508   <ol>
 509     <li>Given a focus cycle root and a Component <b>a</b> in that cycle, the
 510         next Component after <b>a</b>.
 511     <li>Given a focus cycle root and a Component <b>a</b> in that cycle, the
 512         previous Component before <b>a</b>.
 513     <li>Given a focus cycle root, the "first" Component in that cycle.
 514         The "first" Component is the Component to focus when traversal
 515         wraps in the forward direction.
 516     <li>Given a focus cycle root, the "last" Component in that cycle.
 517         The "last" Component is the Component to focus when traversal
 518         wraps in the reverse direction.
 519     <li>Given a focus cycle root, the "default" Component in that cycle.
 520         The "default" Component will be the first to receive focus when
 521         traversing down into a new focus traversal cycle. This may be the
 522         same as the "first" Component, but need not be.
 523   </ol>
 524 
 525 <p>
 526 A <code>FocusTraversalPolicy</code> may optionally provide an
 527 algorithm for the following:
 528   <blockquote>
 529   Given a Window, the "initial" Component in that Window. The initial
 530   Component will be the first to receive focus when the Window is
 531   first made visible. By default, this is the same as the "default"
 532   Component.
 533   </blockquote>
 534 
 535 In addition, Swing provides a subclass of <code>FocusTraversalPolicy</code>,
 536 <code>InternalFrameFocusTraversalPolicy</code>, which allows developers
 537 to provide an algorithm for the following:
 538 
 539   <blockquote>
 540   Given a <code>JInternalFrame</code>, the "initial" Component in that
 541   <code>JInternalFrame</code>. The initial Component is the first to
 542   receive focus when the <code>JInternalFrame</code> is first selected.
 543   By default, this is the same as the <code>JInternalFrame</code>'s
 544   default Component to focus.
 545   </blockquote>
 546 
 547 A <code>FocusTraversalPolicy</code> is installed on a Container using
 548 Container.<code>setFocusTraversalPolicy</code>. If a policy is not explicitly
 549 set, then a Container inherits its policy from its nearest focus-cycle-root
 550 ancestor. Top-levels initialize their focus traversal policies using the context
 551 default policy. The context default policy is established by using
 552 KeyboardFocusManager.  <code>setDefaultFocusTraversalPolicy</code>.
 553 
 554 <p>
 555 AWT provides two standard <code>FocusTraversalPolicy</code>
 556 implementations for use by client code.
 557 
 558   <ol>
 559     <li><code>ContainerOrderFocusTraversalPolicy</code>: Iterates across the
 560         Components in a focus traversal cycle in the order they were added
 561         to their Containers. Each Component is tested for fitness using the
 562         accept(Component) method. By default, a Component is fit only if it
 563         is visible, displayable, enabled, and focusable.
 564     <li>By default, ContainerOrderFocusTraversalPolicy implicitly transfers
 565         focus down-cycle. That is, during normal forward focus traversal,
 566         the Component traversed after a focus cycle root will be the
 567         focus-cycle-root's default Component to focus, regardless of whether
 568         the focus cycle root is a traversable or non-traversable Container
 569         (see the pic.1,2 below). Such behavior provides backward compatibility
 570         with applications designed without the concepts of up- and down-cycle
 571         traversal.
 572     <li><code>DefaultFocusTraversalPolicy</code>: A subclass of
 573         <code>ContainerOrderFocusTraversalPolicy</code> which redefines
 574         the fitness test.  If client code has explicitly set the
 575         focusability of a Component by either overriding
 576         <code>Component.isFocusTraversable()</code> or
 577         <code>Component.isFocusable()</code>, or by calling
 578         <code>Component.setFocusable(boolean)</code>, then a
 579         <code>DefaultFocusTraversalPolicy</code> behaves exactly
 580         like a <code>ContainerOrderFocusTraversalPolicy</code>. If,
 581         however, the Component is relying on default focusability, then a
 582         <code>DefaultFocusTraversalPolicy</code> will reject all
 583         Components with non-focusable peers.
 584         <br>
 585         The focusability of a peer is implementation-dependent. Sun
 586         recommends that all implementations for a particular native platform
 587         construct peers with the same focusability. The recommendations for
 588         Windows and Unix are that Canvases, Labels, Panels, Scrollbars,
 589         ScrollPanes, Windows, and lightweight Components have non-focusable
 590         peers, and all other Components have focusable peers. These
 591         recommendations are used in the Sun AWT implementations. Note that
 592         the focusability of a Component's peer is different from, and does
 593         not impact, the focusability of the Component itself.
 594   </ol>
 595 <p>
 596 Swing provides two additional, standard FocusTraversalPolicy
 597 implementations for use by client code. Each implementation is an
 598 InternalFrameFocusTraversalPolicy.
 599 
 600   <ol>
 601     <li>SortingFocusTraversalPolicy: Determines traversal order by
 602         sorting the Components of a focus traversal cycle based on a given
 603         Comparator. Each Component is tested for fitness using the
 604         accept(Component) method. By default, a Component is fit only if it
 605         is visible, displayable, enabled, and focusable.
 606     <li>By default, SortingFocusTraversalPolicy implicitly transfers focus
 607         down-cycle. That is, during normal forward focus traversal, the
 608         Component traversed after a focus cycle root will be the
 609         focus-cycle-root's default Component to focus, regardless of
 610         whether the focus cycle root is a traversable or non-traversable
 611         Container (see the pic.1,2 below). Such behavior provides backward
 612         compatibility with applications designed without the concepts of
 613         up- and down-cycle traversal.
 614     <li>LayoutFocusTraversalPolicy: A subclass of
 615         SortingFocusTraversalPolicy which sorts Components based on their
 616         size, position, and orientation. Based on their size and position,
 617         Components are roughly categorized into rows and columns. For a
 618         Container with horizontal orientation, columns run left-to-right or
 619         right-to-left, and rows run top-to-bottom. For a Container with
 620         vertical orientation, columns run top-to-bottom and rows run
 621         left-to-right or right-to-left. All columns in a row are fully
 622         traversed before proceeding to the next row.
 623         <br>
 624         In addition, the fitness test is extended to exclude JComponents
 625         that have or inherit empty InputMaps.
 626   </ol>
 627 <p>
 628 The figure below shows an implicit focus transfer:
 629 <br><img src="ImplicitFocusTransfer.gif" alt="Implicit focus transfer."><br>
 630 
 631 Assume the following:
 632  <ul>
 633    <li><b>A</b>, <b>B</b> and <b>C</b> are components in some window (a container)
 634    <li><b>R</b> is a container in the window and it is a parent of <b>B</b> and <b>C</b>.
 635        Besides, <b>R</b> is a focus cycle root.
 636    <li><b>B</b> is the default component in the focul traversal cycle of <b>R</b>
 637    <li><b>R</b> is a traversable Container in the pic.1, and it is a non-traversable
 638        Container in the pic.2.
 639    <li>In such a case a forward traversal will look as follows:
 640    <ul>
 641      <li> pic.1 : <b>A</b> -> <b>R</b> -> <b>B</b> -> <b>C</b>
 642      <li> pic.2 : <b>A</b> -> <b>B</b> -> <b>C</b>
 643    </ul>
 644  </ul>
 645 
 646 <p>
 647 Swing applications, or mixed Swing/AWT applications, that use one of
 648 the standard look and feels, or any other look and feel derived from
 649 BasicLookAndFeel, will use LayoutFocusTraversalPolicy for all
 650 Containers by default.
 651 <p>
 652 All other applications, including pure AWT applications, will use
 653 <code>DefaultFocusTraversalPolicy</code> by default.
 654 
 655 <a id="FocusTraversalPolicyProviders"></a>
 656 <h3>Focus Traversal Policy Providers</h3>
 657 <p>
 658   A Container that isn't a focus cycle root has an option to provide a
 659   FocusTraversalPolicy of its own. To do so, one needs to set Container's focus
 660   traversal policy provider property to <code>true</code> with the call to
 661 
 662   <blockquote>
 663     <code>Container.setFocusTraversalPolicyProvider(boolean)</code>
 664   </blockquote>
 665 
 666   To determine whether a Container is a focus traversal policy provider, the
 667   following method should be used:
 668 
 669   <blockquote>
 670     <code>Container.isFocusTraversalPolicyProvider()</code>
 671   </blockquote>
 672 
 673   If focus traversal policy provider property is set on a focus cycle root, it
 674   isn't considered a focus traversal policy provider and behaves just like any
 675   other focus cycle root.
 676 
 677 <p>
 678   The main difference between focus cycle roots and focus traversal policy
 679   providers is that the latter allow focus to enter and leave them just as all other
 680   Containers. However, children inside focus traversal policy provider are
 681   traversed in the order determined by provider's FocusTraversalPolicy. In order
 682   to enable focus traversal policy providers to behave this way,
 683   FocusTraversalPolicies treat them in the following manner:
 684 
 685   <ul>
 686     <li> Focus traversal policy providers can be passed to FocusTraversalPolicy
 687         methods instead of focus cycle roots.
 688     <li> When calculating next or previous Component in
 689         <code>FocusTraversalPolicy.getComponentAfter</code> or
 690         <code>FocusTraversalPolicy.getComponentBefore</code>,
 691         <ul>
 692           <li>if a Component is a child of a focus traversal policy provider,
 693               the next and previous for this Component are determined using this
 694               focus traversal policy provider's FocusTraversalPolicy. However,
 695               in order for focus to leave the provider, the following rules are
 696               applied:
 697               <ul>
 698                 <li> if at some point the <code>next</code> found Component is
 699                     the <code>first</code> Component of focus traversal policy
 700                     provider, the Component after the focus traversal policy
 701                     provider is returned
 702                 <li> if at some point the <code>previous</code> found Component is
 703                     the <code>last</code> Component of focus traversal policy
 704                     provider, the Component before the focus traversal policy
 705                     provider is returned
 706               </ul>
 707           <li> When calculating the next Component in
 708               <code>FocusTraversalPolicy.getComponentAfter</code>,
 709               <ul>
 710                 <li> if an obtained Component is a non-traversable Container and
 711                      it is a focus traversal policy provider, then the default Component
 712                      of that provider is returned
 713                 <li> if the Component passed to the <code>FocusTraversalPolicy.getComponentAfter</code>
 714                      method is a traversable Container and it is a focus
 715                      traversal policy provider, then the default Component of this provider
 716                      is returned
 717               </ul>
 718           <li> When calculating the previous Component in
 719               <code>FocusTraversalPolicy.getComponentBefore</code>,
 720               <ul>
 721                 <li> if an obtained Component is a Container (traversable or
 722                      non-traversable) and it is a focus traversal policy provider, then
 723                      the last Component of that provider is returned
 724               </ul>
 725         </ul>
 726     <li> When calculating the first Component in FocusTraversalPolicy.getFirstComponent,
 727         <ul>
 728           <li> if an obtained Component is a non-traversable Container and it is a focus
 729                traversal policy provider, then the default Component of that provider is
 730                returned
 731           <li> if an obtained Component is a traversable Container and it is a focus traversal
 732                policy provider, then that Container itself is returned
 733         </ul>
 734     <li> When calculating the last Component in FocusTraversalPolicy.getLastComponent,
 735         <ul>
 736           <li> if an obtained Component is a Container (traversable or non-traversable)
 737                and it is a focus traversal policy provider, then the last Component of
 738                that provider is returned
 739         </ul>
 740   </ul>
 741 
 742 <a id="ProgrammaticTraversal"></a>
 743 <h3>Programmatic Traversal</h3>
 744 <p>
 745 In addition to user-initiated focus traversal, client code can
 746 initiate a focus traversal operation programmatically. To client code,
 747 programmatic traversals are indistinguishable from user-initiated
 748 traversals. The preferred way to initiate a programmatic traversal is
 749 to use one of the following methods on <code>KeyboardFocusManager</code>:
 750 
 751   <ul>
 752     <li><code>KeyboardFocusManager.focusNextComponent()</code>
 753     <li><code>KeyboardFocusManager.focusPreviousComponent()</code>
 754     <li><code>KeyboardFocusManager.upFocusCycle()</code>
 755     <li><code>KeyboardFocusManager.downFocusCycle()</code>
 756   </ul>
 757 
 758 <p>
 759 Each of these methods initiates the traversal operation with the
 760 current focus owner. If there is currently no focus owner, then no
 761 traversal operation occurs. In addition, if the focus owner is not a
 762 focus cycle root, then downFocusCycle() performs no traversal
 763 operation.
 764 <p>
 765 <code>KeyboardFocusManager</code> also supports the following variants
 766 of these methods:
 767 
 768   <ul>
 769     <li><code>KeyboardFocusManager.focusNextComponent(Component)</code>
 770     <li><code>KeyboardFocusManager.focusPreviousComponent(Component)</code>
 771     <li><code>KeyboardFocusManager.upFocusCycle(Component)</code>
 772     <li><code>KeyboardFocusManager.downFocusCycle(Container)</code>
 773   </ul>
 774 
 775 Each of these methods initiates the traversal operation with the
 776 specified Component rather than the focus owner. That is, the
 777 traversal occurs as though the specified Component is the focus owner,
 778 though it need not be.
 779 <p>
 780 Alternate, but equivalent, APIs are defined on the Component and
 781 Container classes themselves:
 782 
 783   <ul>
 784     <li><code>Component.transferFocus()</code>
 785     <li><code>Component.transferFocusBackward()</code>
 786     <li><code>Component.transferFocusUpCycle()</code>
 787     <li><code>Container.transferFocusDownCycle()</code>
 788   </ul>
 789 
 790 As with the <code>KeyboardFocusManager</code> variants, each of these methods
 791 initiates the traversal operation as though the Component is the focus
 792 owner, though it need not be.
 793 <p>
 794 Also note that hiding or disabling the focus owner, directly or
 795 indirectly via an ancestor, or making the focus owner non-displayable
 796 or non-focusable, initiates an automatic, forward focus traversal.
 797 While hiding any ancestor, lightweight or heavyweight, will always
 798 indirectly hide its children, only disabling a heavyweight ancestor
 799 will disable its children. Thus, disabling a lightweight ancestor of
 800 the focus owner does not automatically initiate a focus traversal.
 801 <p>
 802 If client code initiates a focus traversal, and there is no other
 803 Component to focus, then the focus owner remains unchanged. If client
 804 code initiates an automatic focus traversal by hiding the focus owner,
 805 directly or indirectly, or by making the focus owner non-displayable or
 806 non-focusable, and there is no other Component to focus, then the
 807 global focus owner is cleared. If client code initiates an automatic
 808 focus traversal by disabling the focus owner, directly or indirectly,
 809 and there is no other Component to focus, then the focus owner remains
 810 unchanged.
 811 
 812 
 813 <a id="Focusability"></a>
 814 <h3>Focusability</h3>
 815 <p>
 816 A focusable Component can become the focus owner ("focusability") and
 817 participates in keyboard focus traversal ("focus traversability") with
 818 a FocusTraversalPolicy. There is no separation of these two concepts;
 819 a Component must either be both focusable and focus traversable, or
 820 neither.
 821 
 822 A Component expresses this state via the isFocusable() method. By
 823 default, all Components return true from this method. Client code can
 824 change this default by calling Component.setFocusable(boolean).
 825 
 826 
 827 <a id="FocusableWindows"></a>
 828 <h3>Focusable Windows</h3>
 829 <p>
 830 To support palette windows and input methods, client code can prevent
 831 a Window from becoming the focused Window. By transitivity, this
 832 prevents the Window or any of its descendants from becoming the focus
 833 owner. Non-focusable Windows may still own Windows that are
 834 focusable. By default, every Frame and Dialog is focusable. Every
 835 Window which is not a Frame or Dialog, but whose nearest owning Frame
 836 or Dialog is showing on the screen, and which has at least one
 837 Component in its focus traversal cycle, is also focusable by
 838 default. To make a Window non-focusable, use
 839 Window.setFocusableWindowState(false).
 840 <p>
 841 If a Window is non-focusable, this restriction is enforced when the
 842 <code>KeyboardFocusManager</code> sees a <code>WINDOW_GAINED_FOCUS</code>
 843 event for the Window.  At this point, the focus change is rejected and
 844 focus is reset to a different Window. The rejection recovery scheme
 845 is the same as if a <code>VetoableChangeListener</code> rejected the
 846 focus change.  See <a href="#FocusAndVetoableChangeListener">Focus
 847 and VetoableChangeListener</a>.
 848 <p>
 849 Because the new focus implementation requires that KeyEvents intended
 850 for a Window or its descendants be proxied through a child of the
 851 Window's owner, and because this proxy must be mapped on X11 in order
 852 to receive events, a Window whose nearest owning Frame or Dialog is
 853 not showing could never receive KeyEvents on X11. To support this
 854 restriction, we have made a distinction between a Window's "window
 855 focusability" and its "window focusability state". A Window's
 856 focusability state is combined with the showing state of the Window's
 857 nearest owning Frame or Dialog to determine the Window's focusability.
 858 By default, all Windows have a focusability state of true. Setting a
 859 Window's focusability state to false ensures that it will not become
 860 the focused Window regardless of the showing state of its nearest
 861 owning Frame or Dialog.
 862 <p>
 863 Swing allows applications to create JWindows with null owners. Swing
 864 constructs all such JWindows so that they are owned by a private,
 865 hidden Frame. Because the showing state of this Frame will always be
 866 false, a JWindow constructed will a null owner can never be the
 867 focused Window, even if it has a Window focusability state of true.
 868 <p>
 869 If the focused Window is made non-focusable, then the AWT will attempt
 870 to focus the most recently focused Component of the Window's
 871 owner. The Window's owner will thus become the new focused Window. If
 872 the Window's owner is also a non-focusable Window, then the focus
 873 change request will proceed up the ownership hierarchy recursively.
 874 Since not all platforms support cross-Window focus changes (see
 875 <a href=#RequestingFocus>Requesting Focus</a>), it is possible that
 876 all such focus change requests will fail. In this case, the global
 877 focus owner will be cleared and the focused Window will remain unchanged.
 878 
 879 <a id="RequestingFocus"></a>
 880 <h3>Requesting Focus</h3>
 881 
 882 <p>
 883 A Component can request that it become the focus owner by calling
 884 <code>Component.requestFocus()</code>. This initiates a permanent
 885 focus transfer to the Component only if the Component is displayable,
 886 focusable, visible and all of its ancestors (with the exception of the
 887 top-level Window) are visible. The request will be immediately denied if
 888 any of these conditions is not met. A disabled Component may be
 889 the focus owner; however, in this case, all KeyEvents will be discarded.
 890 <p>
 891 The request will also be denied if the Component's top-level Window is
 892 not the focused Window and the platform does not support requesting
 893 focus across Windows. If the request is denied for this reason, the
 894 request is remembered and will be granted when the Window is later
 895 focused by the user. Otherwise, the focus change request changes the
 896 focused Window as well.
 897 <p>
 898 There is no way to determine synchronously whether a focus change
 899 request has been granted. Instead, client code must install a
 900 FocusListener on the Component and watch for the delivery of a
 901 <code>FOCUS_GAINED</code> event. Client code must not assume that
 902 the Component is the focus owner until it receives this event.
 903 The event may or may not be delivered before <code>requestFocus()</code>
 904 returns. Developers must not assume one behavior or the other.
 905 <p>
 906 The AWT supports type-ahead if all focus change requests are made on
 907 the EventDispatchThread. If client code requests a focus change, and
 908 the AWT determines that this request might be granted by the native
 909 windowing system, then the AWT will notify the current
 910 KeyboardFocusManager that is should enqueue all KeyEvents with a
 911 timestamp later than the that of the event currently being handled.
 912 These KeyEvents will not be dispatched until the new Component becomes
 913 the focus owner. The AWT will cancel the delayed dispatching request
 914 if the focus change does not succeed at the native level, if the
 915 Component's peer is disposed, or if the focus change is vetoed by a
 916 VetoableChangeListener. KeyboardFocusManagers are not required to
 917 support type-ahead if a focus change request is made from a thread
 918 other than the EventDispatchThread.
 919 <p>
 920 Because <code>Component.requestFocus()</code> cannot be implemented
 921 consistently across platforms, developers are encouraged to use
 922 <code>Component.requestFocusInWindow()</code> instead. This method
 923 denies cross-Window focus transfers on all platforms automatically.
 924 By eliminating the only platform-specific element of the focus transfer,
 925 this method achieves consistent cross-platform behavior.
 926 <p>
 927 In addition, <code>requestFocusInWindow()</code> returns a boolean value.
 928 If 'false' is returned, the request is guaranteed to fail. If 'true' is
 929 returned, the request will succeed unless it is vetoed, or an
 930 extraordinary event, such as disposal of the Component's peer, occurs
 931 before the request can be granted by the native windowing
 932 system. Again, while a return value of 'true' indicates that the
 933 request is likely to succeed, developers must never assume that this
 934 Component is the focus owner until this Component receives a
 935 <code>FOCUS_GAINED</code> event.
 936 <p>
 937 If client code wants no Component in the application to be the focus
 938 owner, it can call the method <code>KeyboardFocusManager</code>.
 939 <code>clearGlobalFocusOwner()</code> on the current
 940 <code>KeyboardFocusManager</code>. If there exists a focus owner
 941 when this method is called, the focus owner will receive a permanent
 942 <code>FOCUS_LOST</code> event. After this point, the AWT
 943 focus implementation will discard all KeyEvents until the user or
 944 client code explicitly sets focus to a Component.
 945 <p>
 946 The Component class also supports variants of <code>requestFocus</code> and
 947 <code>requestFocusInWindow</code> that allow client code to specify
 948 a temporary state.
 949 See <a href="#TemporaryFocusEvents">Temporary FocusEvents</a>
 950 
 951 <a id="FocusAndPropertyChangeListener"></a>
 952 <h3>Focus and PropertyChangeListener</h3>
 953 <p>
 954 Client code can listen to changes in context-wide focus state, or to
 955 changes in focus-related state in Components, via
 956 PropertyChangeListeners.
 957 <p>
 958 The <code>KeyboardFocusManager</code> supports the following properties:
 959 
 960   <ol>
 961     <li><code>focusOwner</code>: the focus owner
 962     <li><code>focusedWindow</code>: the focused Window
 963     <li><code>activeWindow</code>: the active Window
 964     <li><code>defaultFocusTraversalPolicy</code>: the default focus
 965         traversal policy
 966     <li><code>forwardDefaultFocusTraversalKeys</code>: the Set of default
 967         <code>FORWARD_TRAVERSAL_KEYS</code>
 968     <li><code>backwardDefaultFocusTraversalKeys</code>: the Set of default
 969         <code>BACKWARD_TRAVERSAL_KEYS</code>
 970     <li><code>upCycleDefaultFocusTraversalKeys</code>: the Set of default
 971         <code>UP_CYCLE_TRAVERSAL_KEYS</code>
 972     <li><code>downCycleDefaultFocusTraversalKeys</code>: the Set of default
 973         <code>DOWN_CYCLE_TRAVERSAL_KEYS</code>
 974     <li><code>currentFocusCycleRoot</code>: the current focus cycle root
 975   </ol>
 976 <p>
 977 A <code>PropertyChangeListener</code> installed on the current
 978 <code>KeyboardFocusManager</code> will only see these changes within
 979 the <code>KeyboardFocusManager</code>'s context, even though the
 980 focus owner, focused Window, active Window, and current focus cycle
 981 root comprise the global focus state shared by all contexts.
 982 We believe this is less intrusive than requiring client code to pass
 983 a security check before installing a <code>PropertyChangeListener</code>.
 984 <p>
 985 Component supports the following focus-related properties:
 986 
 987   <ol>
 988    <li><code>focusable</code>: the Component's focusability
 989    <li><code>focusTraversalKeysEnabled</code>: the Component's
 990        focus traversal keys enabled state
 991    <li><code>forwardFocusTraversalKeys</code>: the Component's Set of
 992        <code>FORWARD_TRAVERSAL_KEYS</code>
 993    <li><code>backwardFocusTraversalKeys</code>: the Component's Set of
 994        <code>BACKWARD_TRAVERSAL_KEYS</code>
 995    <li><code>upCycleFocusTraversalKeys</code>: the Component's Set of
 996        <code>UP_CYCLE_TRAVERSAL_KEYS</code>
 997   </ol>
 998 <p>
 999 In addition to the Component properties, Container supports the
1000 following focus-related properties:
1001 
1002   <ol>
1003     <li><code>downCycleFocusTraversalKeys</code>: the Container's Set of
1004         <code>DOWN_CYCLE_TRAVERSAL_KEYS</code>
1005     <li><code>focusTraversalPolicy</code>: the Container's focus
1006         traversal policy
1007     <li><code>focusCycleRoot</code>: the Container's focus-cycle-root state
1008   </ol>
1009 <p>
1010 In addition to the Container properties, Window supports the following
1011 focus-related property:
1012 
1013   <ol>
1014     <li><code>focusableWindow</code>: the Window's focusable Window state
1015   </ol>
1016 <p>
1017 Also note that a <code>PropertyChangeListener</code> installed on a
1018 Window will never see a <code>PropertyChangeEvent</code> for the
1019 <code>focusCycleRoot</code> property.
1020 A Window is always a focus cycle root; this property cannot change.
1021 <p>
1022 <a id="FocusAndVetoableChangeListener"></a>
1023 <h3>Focus and VetoableChangeListener</h3>
1024 <p>
1025 The <code>KeyboardFocusManager</code> also supports
1026 <code>VetoableChangeListener</code>s for the following properties:
1027 
1028   <ol>
1029     <li>"focusOwner": the focus owner
1030     <li>"focusedWindow": the focused Window
1031     <li>"activeWindow": the active Window
1032   </ol>
1033 
1034 If a VetoableChangeListener vetoes a focus or activation change by
1035 throwing a PropertyVetoException, the change is aborted. Any
1036 VetoableChangeListeners which had already approved the change will
1037 asynchronously receive PropertyChangeEvents indicating a reversion of
1038 state to the previous value.
1039 <p>
1040 VetoableChangeListeners are notified of the state change before the
1041 change is reflected in the KeyboardFocusManager. Conversely,
1042 PropertyChangeListeners are notified after the change is reflected.
1043 It follows that all VetoableChangeListeners will be notified before
1044 any PropertyChangeListener.
1045 <p>
1046 VetoableChangeListeners must be idempotent, and must veto both loss
1047 and gain events for a particular focus change (e.g., both
1048 <code>FOCUS_LOST</code> and <code>FOCUS_GAINED</code>). For example,
1049 if a <code>VetoableChangeListener</code> vetoes a <code>FOCUS_LOST</code>
1050 event, a <code>KeyboardFocusManager</code> is not required to search the
1051 <code>EventQueue</code> and remove the associated pending
1052 <code>FOCUS_GAINED</code> event. Instead, the
1053 <code>KeyboardFocusManager</code> is free to attempt to
1054 dispatch this event and it is the responsibility of the
1055 <code>VetoableChangeListener</code> to veto it as well. In addition,
1056 during processing of the <code>FOCUS_GAINED</code> event, the
1057 <code>KeyboardFocusManager</code> may attempt to resync the global
1058 focus state by synthesizing another <code>FOCUS_LOST</code> event.
1059 This event must be vetoed just as the first <code>FOCUS_LOST</code> event was.
1060 <p>
1061 A <code>KeyboardFocusManager</code> may not hold any locks while
1062 notifying <code>PropertyChangeListener</code>s of a state change.
1063 This requirement is relaxed for <code>VetoableChangeListeners</code>,
1064 however. Therefore, client-definied <code>VetoableChangeListener</code>s
1065 should avoid acquiring additional locks inside
1066 <code>vetoableChange(PropertyChangeEvent)</code> as this may lead to deadlock.
1067 
1068 If a focus or activation change is rejected, the KeyboardFocusManager
1069 will initiate rejection recovery as follows:
1070 
1071   <ul>
1072     <li>If a focused or active Window change was rejected, then the
1073         focused or active Window will be reset to the Window which was
1074         previously the focused or active Window. If there is no such
1075         Window, then the <code>KeyboardFocusManager</code> will clear
1076         the global focus owner.
1077     <li>If a focus owner change was rejected, then the focus owner will be
1078         reset to the Component which was previously the focus owner. If
1079         that is not possible, then it will be reset to the next Component
1080         in the focus traversal cycle after the previous focus owner. If
1081         that is also not possible, then the <code>KeyboardFocusManager</code>
1082         will clear the global focus owner.
1083   </ul>
1084 
1085 <code>VetoableChangeListener</code>s must be careful to avoid vetoing focus
1086 changes initiated as a result of veto rejection recovery. Failure
1087 to anticipate this situation could lead to an infinite cycle of
1088 vetoed focus changes and recovery attempts.
1089 
1090 
1091 <a id="ZOrder"></a>
1092 <h3>Z-Order</h3>
1093 <p>
1094 On some native windowing systems, the Z-order of a Window can affect
1095 its focused or active (if applicable) state. On Microsoft Windows, the
1096 top-most Window is naturally the focused Window as well. However, on
1097 Solaris, many window managers use a point-to-focus model that ignores
1098 Z-order in determining the focused Window.
1099 
1100 When focusing or activating Windows, the AWT adheres to the UI
1101 requirements of the native platform. Therefore, the focus behavior of
1102 Z-order-related methods such as:
1103   <ul>
1104     <li><code>Window.toFront()</code>
1105     <li><code>Window.toBack()</code>
1106     <li><code>Window.show()</code>
1107     <li><code>Window.hide()</code>
1108     <li><code>Window.setVisible(boolean)</code>
1109     <li><code>Window.dispose()</code>
1110     <li><code>Frame.setState(int)</code>
1111   </ul>
1112 is platform-dependent. In JDK 1.4, the behavior of these methods on
1113 Microsoft Windows and Solaris is as follows:
1114   <ul>
1115      <li><code>Window.toFront()</code>:<br>
1116         <b>Microsoft Windows</b>: The Window is moved to front, if possible.
1117         While we will always be able to move this Window in front of other
1118         Windows in the same VM, Windows 98 and Windows 2000 do not allow an
1119         application to bring any of its windows to the front unless one
1120         of that application's windows is already in the foreground. In
1121         this case, Windows will instead flash the Window's icon in the
1122         taskbar. If the Window is moved to the front, it will be made
1123         the focused and (if applicable) active Window.
1124       <br>
1125         <b>Solaris</b>: The Window is moved to front. In a point-to-focus
1126         window manager, the Window will become the focused Window if it
1127         is the top-most Window underneath the cursor. In a click-to-focus
1128         window manager, the focused Window will remain unchanged.
1129 
1130      <li><code>Window.toBack()</code>:<br>
1131         <b>Microsoft Windows</b>: The Window is moved to back. Note however
1132         that Microsoft Windows insists that an owned Window always be in
1133         front of all of its recursive owners. Thus, after the completion of
1134         this operation, the Window may not be the lowest Java Window in the
1135         Z-order. If the Window, or any of its owners, was the focused Window,
1136         then the focused Window is reset to the top-most Window in the VM.
1137       <br>
1138         <b>Solaris</b>: The Window is moved to back. Like Microsoft Windows,
1139         some window managers insist than an owned Window always be in front
1140         of all of its recursive owners. Thus, after the completion of this
1141         operation, the Window may not be the lowest Java Window in the
1142         Z-order. If the Window was the focused Window, it will lose
1143         focus in a point-to-focus window manager if it is no longer the
1144         top-most Window under the cursor. In a click-to-focus window
1145         manager, the focused Window will remain unchanged.
1146 
1147      <li><code>Window.show()/Window.setVisible(true)/Frame.setState(NORMAL)</code>:<br>
1148         <b>Microsoft Windows</b>: The Window is moved to front and becomes the focused
1149         Window.
1150       <br>
1151         <b>Solaris</b>: The Window is moved to front. In a point-to-focus focus
1152         window manager, the Window will be focused if it is now the
1153         top-most Window under the cursor. In a click-to-focus window
1154         manager, the Window will become the focused Window.
1155 
1156      <li><code>Window.hide()/Window.setVisible(false)/Window.dispose()/
1157     Frame.setState(ICONIFIED)</code>:<br>
1158         <b>Microsoft Windows</b>: If the Window was the focused Window, the focused
1159         Window is reset to a window chosen by the OS, or to no window. The
1160         window may be in a native application, or a Java application in
1161         another VM.
1162        <br>
1163         <b>Solaris</b>: If the Window was the focused Window, in a point-to-
1164         focus window manager, the top-most Window under the cursor will
1165         become the focused Window. In a click-to-focus window manager,
1166         the focused Window is reset to a window chosen by the window
1167         manager. The window may be in a native application, or a Java
1168         application in another VM.
1169   </ul>
1170 
1171 <a id="ReplacingDefaultKeyboardFocusManager"></a>
1172 <h3>Replacing DefaultKeyboardFocusManager</h3>
1173 <p>
1174 <code>KeyboardFocusManager</code>s are pluggable at the browser context
1175 level. Client code can subclass <code>KeyboardFocusManager</code> or
1176 <code>DefaultKeyboardFocusManager</code> to modify the way that WindowEvents
1177 related to focus, FocusEvents, and KeyEvents are handled and
1178 dispatched, and to examine and modify the global focus state. A custom
1179 <code>KeyboardFocusManager</code> can also reject focus changes at a more
1180 fundamental level then a FocusListener or WindowListener ever could.
1181 <p>
1182 While giving a developer ultimate control over the focus model,
1183 replacing the entire <code>KeyboardFocusManager</code> is a difficult process
1184 requiring a thorough understanding of the peer focus layer.
1185 Fortunately, most applications do not need this much control.
1186 Developers are encouraged to use KeyEventDispatchers,
1187 KeyEventPostProcessors, FocusTraversalPolicies,
1188 VetoableChangeListeners, and other concepts discussed in this document
1189 before resorting to a full replacement of the <code>KeyboardFocusManager</code>.
1190 <p>
1191 First note that, because unhindered access to Components in other
1192 contexts represents a security hole, the SecurityManager must grant a
1193 new permission, "replaceKeyboardFocusManager", before client code is
1194 permitted to replace the <code>KeyboardFocusManager</code> with an arbitrary
1195 subclass instance. Because of the security check, replacing the
1196 <code>KeyboardFocusManager</code> is not an option for applications that will be
1197 deployed in environments with a SecurityManager, such as applets in a
1198 browser.
1199 <p>
1200 Once installed, a <code>KeyboardFocusManager</code> instance has
1201 access to the global focus state via a set of protected functions.
1202 The <code>KeyboardFocusManager</code> can only call these functions
1203 if it is installed in the calling thread's context. This ensures
1204 that malicious code cannot circumvent the security check in
1205 <code>KeyboardFocusManager.setCurrentFocusManager</code>.
1206 A <code>KeyboardFocusManager</code> should always work with
1207 the global focus state instead of the context focus state.
1208 Failure to do this will lead to incorrect behavior of the
1209 <code>KeyboardFocusManager</code>.
1210 <p>
1211 The primary responsibility of a <code>KeyboardFocusManager</code>
1212 is the dispatch of the following events:
1213 
1214     <ul>
1215       <li>all <code>KeyEvent</code>s
1216       <li>all <code>FocusEvent</code>s
1217       <li><code>WindowEvent.WINDOW_GAINED_FOCUS</code>
1218       <li><code>WindowEvent.WINDOW_LOST_FOCUS</code>
1219       <li><code>WindowEvent.WINDOW_ACTIVATED</code>
1220       <li><code>WindowEvent.WINDOW_DEACTIVATED</code>
1221     </ul>
1222 
1223 The peer layer will provide the <code>KeyboardFocusManager</code>
1224 with all of the above events except <code>WINDOW_ACTIVATED</code>
1225 and <code>WINDOW_DEACTIVATED</code>. The <code>KeyboardFocusManager</code>
1226 must synthesize <code>WINDOW_ACTIVATED</code> and
1227 <code>WINDOW_DEACTIVATED</code> events when appropriate and target them
1228 accordingly.
1229 <p>
1230 The <code>KeyboardFocusManager</code> may need to retarget the events
1231 provided by the peer layer to its own notion of the focus owner or
1232 focused Window:
1233     <ul>
1234       <li>A KeyEvent must be retargeted to the focus owner. Because the peer
1235           layer is unaware of any lightweight Components, KeyEvents will
1236           arrive from the peer layer targeted to the focus owner's
1237           heavyweight Container, not the focus owner.
1238       <li>A <code>FOCUS_LOST</code> event must be retargeted to the focus
1239           owner. Again, this is necessary because the peer layer is
1240           unaware of lightweight Components.
1241       <li>A <code>WINDOW_LOST_FOCUS</code> event must be retargeted to
1242           the focused Window. The implementation of the Window class
1243           may cause the native focused Window to differ from the Java
1244           focused Window.
1245     </ul>
1246 <p>
1247 A <code>KeyboardFocusManager</code> must ensure proper event ordering,
1248 and a 1-to-1 correspondence between an event and its opposite event type.
1249 The peer layer does not make any of these guarantees. For example, it is
1250 possible for the peer layer to send a <code>FOCUS_GAINED</code>
1251 event before a <code>WINDOW_GAINED_FOCUS</code> event.
1252 The <code>KeyboardFocusManager</code> is responsible for
1253 ensuring that the <code>WINDOW_GAINED_FOCUS</code> event is dispatched
1254 before the <code>FOCUS_GAINED</code> event.
1255 <p>
1256 Before redispatching an event via <code>KeyboardFocusManager</code>.
1257 <code>redispatchEvent</code>, a <code>KeyboardFocusManager</code>
1258 must attempt to update the global focus state. Typically, this
1259 is done using one of the <code>KeyboardFocusManager.setGlobal*</code>
1260 methods; however, an implementation is free to implement its own methods.
1261 After attempting an update, the <code>KeyboardFocusManager</code>
1262 must verify that the global focus state change
1263 was not rejected. A rejection is detected when a call to the
1264 corresponding <code>getGlobal*</code> method returns a value different than the
1265 value just set. Rejections occur in three standard cases:
1266     <ul>
1267       <li>If the <code>KeyboardFocusManager</code> attempts
1268           to set the global focus owner to a non-focusable Component.
1269       <li>If the <code>KeyboardFocusManager</code> attempts
1270           to set the global focused Window to a non-focusable Window.
1271       <li>If the change is rejected by an installed
1272           <code>VetoableChangeListener</code>.
1273     </ul>
1274 <p>
1275 Client-defined implementations of <code>KeyboardFocusManager</code>
1276 can adjust the set of focus transfers which are rejected by overriding the
1277 accessor and mutator methods for the global focus state.
1278 <p>
1279 If a request to change the global focus state is rejected, the
1280 <code>KeyboardFocusManager</code> must discard the event which prompted
1281 the focus change request. The Component to which the event was targeted
1282 must not receive the event.
1283 <p>
1284 The <code>KeyboardFocusManager</code> is also expected to initiate rejection
1285 recovery as outlined in <a href="#FocusAndVetoableChangeListener">Focus
1286 and VetoableChangeListener</a>.
1287   <p>
1288      Finally, a KeyboardFocusManager must handle the following set of
1289      special cases:
1290     <ul>
1291       <li>When handling a <code>WINDOW_GAINED_FOCUS</code> event, the
1292           <code>KeyboardFocusManager</code> must set focus to the
1293           appropriate child Component of the Window. If a child
1294           Component of the Window previously requested focus,
1295           but the focus change was rejected because the platform
1296           does not support cross-Window focus change requests,
1297           then focus should be set to that child Component.
1298           Otherwise, if the Window has never been focused, focus should be
1299           set to the Window's initial Component to focus. If the Window was
1300           previously focused, focus should be set to the Window's most
1301           recent focus owner.
1302       <li>The <code>KeyboardFocusManager</code> must ensure that the
1303           opposite Component or Window are as accurate as the native
1304           windowing platform permits. For example, the
1305           <code>KeyboardFocusManager</code> may need to
1306           retarget the opposite Component to a lightweight child of the
1307           heavyweight initially specified by the peer layer.
1308           <br>
1309           If the peer layer states that the opposite Component or Window is
1310           <code>null</code>, it is acceptable for the
1311           <code>KeyboardFocusManager</code> to propagate
1312           this value. <code>null</code> indicates that it is highly
1313           probably that no other Component or Window was involved
1314           in the focus or activation change. Because of platform
1315           limitations, this computation may be
1316           subject to a heuristic and could be incorrect. Nevertheless, this
1317           heuristic will be the best possible guess which the peer layer
1318           could make.
1319        <li>Focus and activation changes in which a Component or Window loses
1320           focus or activation to itself must be discarded.
1321        <li>Events posted by the peer layer claiming that the active Window
1322           has lost focus to the focused Window must be discarded. The peer
1323           implementation of the Window class may generate these spurious
1324           events.
1325     </ul>
1326 
1327 <a id="Incompatibilities"></a>
1328 <h3>Incompatibilities with Previous Releases</h3>
1329   <p><b>Cross-platform changes:</b>
1330     <ol>
1331       <li>The default focus traversability for all Components is now
1332           'true'. Previously, some Components (in particular, all
1333           lightweights), had a default focus traversability of 'false'. Note
1334           that despite this change, however, the
1335           <code>DefaultFocusTraversalPolicy</code> for all AWT Containers
1336           will preserve the traversal order of previous releases.
1337       <li>A request to focus a non-focus traversable (i.e., non-focusable)
1338           Component will be denied. Previously, such requests were granted.
1339       <li><code>Window.toFront()</code> and <code>Window.toBack()</code>
1340           now perform no operation if the Window is not visible.
1341           Previously, the behavior was platform-dependent.
1342       <li>KeyListeners installed on <code>Component</code>s
1343           will no longer see <code>KeyEvent</code>s that map to focus
1344           traversal operations, and
1345           <code>Component.handleEvent()</code> will no longer be invoked
1346           for such events. Previously, AWT Components saw these events
1347           and had an opportunity to consume them before AWT
1348           initiated focus traversal. Code that requires this
1349           functionality should instead disable focus traversal keys on
1350           its <code>Component</code>s and handle focus traversal
1351           itself. Alternately, the code can use an
1352           <code>AWTEventListener</code> or
1353           <code>KeyEventDispatcher</code> to pre-listen to all
1354           <code>KeyEvent</code>s.
1355     </ol>
1356   <p><b>Changes specific to Microsoft Windows:</b>
1357     <ol>
1358       <li><code>Window.toBack()</code> changes the focused Window to
1359           the top-most Window after the Z-order change.
1360       <li><code>requestFocus()</code> now allows cross-Window focus
1361           change requests in all cases. Previously, requests were granted
1362           for heavyweights, but denied for lightweights.
1363     </ol>
1364 
1365 </body>
1366 </html>
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