The AWT Modality

This document, together with the API documentation for modality-related classes (such as java.awt.Dialog), briefly describes the new modality features and how to use them. It contains the following sections:

Definitions
Modality types
Show/hide blocking
Modal exclusion
Related AWT features
Security
Platform support
Compatibility
Examples

Definitions

Document - a window without an owner that, together with all its child hierarchy, may be operated on as a single self-contained document. Every window belongs to some document — its root can be found as the closest ancestor window without an owner.

Modal blocked window - a window, that:

doesn't receive any user input events
doesn't receive input focus
keeps its Z-order below the modal dialog that blocks it

Warning! Some window managers allow users to change the window Z-order in an arbitrary way — in that case the last requirement may not be met.

Modal dialog - a dialog that blocks some windows while it is visible. The blocked windows are determined according to the dialog's scope of blocking.

Modal excluded window - a window that stays unblocked while the modal dialog is visible. If a window is modal excluded then all its owned windows and child components are also excluded.

Scope of blocking (SB) - the set of windows (instances of java.awt.Window and all derived classes) that are blocked by the modal dialog while it is visible.

Note: Everywhere in this document the notion of "window" is equal to a top-level window in the Java programming language — in other words an instance of java.awt.Window or any descendant class.

Modality types

There are four supported modality types :

toolkit
application
document
modeless

A dialog is, by default, modeless. A modal dialog is, by default, application-modal.

Modeless dialogs
A modeless dialog doesn't block any windows while visible.
Document-modal dialogs
A document-modal dialog blocks all windows from the same document except those from its child hierarchy. The document root is determined as the closest ancestor window without an owner.
Application-modal dialogs
An application-modal dialog blocks all windows from the same application except for those from its child hierarchy. If there are several applets launched in a browser, they can be treated either as separate applications or a single application. This behavior is implementation-dependent.
Toolkit-modal dialogs
A toolkit-modal dialog blocks all windows that run in the same toolkit except those from its child hierarchy. If there are several applets launched all of them run with the same toolkit, so a toolkit-modal dialog shown from an applet may affect other applets and all windows of the browser instance which embeds the Java runtime environment for this toolkit. See the security section below.

Modality priority is arranged by the strength of blocking: modeless, document-modal, application-modal and toolkit-modal. This arrangement is used when determining what dialog should remain unblocked if two are visible and block each other. It naturally reflects the nesting of a dialog's scope of blocking (SB): a modeless dialog has an empty SB, a document-modal dialog's SB is complete in some applications, and all the applications are run in one toolkit.

Notes about owners:

Creating a document-modal dialog without an owner:
Since Dialog is a class derived from Window, a Dialog instance automatically becomes the root of the document if it has no owner. Thus, if such a dialog is document-modal, its scope of blocking is empty and it behaves the same way as a modeless dialog.
Creating an application-modal or toolkit-modal dialog with an owner:
The scope of blocking for an application- or toolkit-modal dialog, as opposed to a document-modal dialog, doesn't depend on its owner. Thus, in this case the only thing that the owner affects is the Z-order: the dialog always stays on top of its owner.

Implementation note: Changing the modality type for a visible dialog may have no effect until it is hidden and then shown again.

Show/hide blocking

Showing the window or modeless dialog: "F"
All the visible modal dialogs are looked through — if F is from the SB of one of them, it becomes blocked by it. If there are several such dialogs, the first shown is used. If no such dialogs exist, F remains unblocked.

Showing the modal dialog: "M"
When modal dialog M is shown, all the visible windows fall into one of three distinct groups:

Blockers of M (modal dialogs that block M and either are in M's child hierarchy, or are not blocked by M, or have a greater mode of modality, or block some other blocker of M)
Blocked by M (windows from M's SB that are not blockers and are not in child hierarchy of any blocker)
All other windows (windows or modeless dialogs outside M's SB and modal dialogs outside M's SB that do not block M).

After the modal dialog M is shown, it becomes blocked by the first shown dialog from the first group (if there are any), all the windows from the second one become blocked by M, and all the windows from the third group remain untouched.

In typical cases, when no child dialogs are shown before their owners, this rule can be simplified. (The following, simplified case, may leave out some details).

Showing the document-modal dialog: "M"
All the visible application- and toolkit-modal dialogs are looked through — if M is from the SB of one of them, it becomes blocked by it. If there are several such dialogs, the first shown is used. If no such dialogs exist, M remains unblocked.

Showing the application-modal dialog: "M"
All the visible toolkit-modal dialogs are looked through — if M is from the SB of one of them, it becomes blocked by it. If there are several such dialogs, the first shown is used. If no such dialogs exist, M remains unblocked.

Showing the toolkit-modal dialog: "M"
M remains unblocked.

The Standard Blocking Matrix
current/shown	frame & modeless	document	application	toolkit
-	-	-	-	-
document	blocked	-	-	-
application	blocked	blocked	-	-
toolkit	blocked	blocked	blocked	-

After the modal dialog is shown, all the windows from its SB are blocked, except those that block this modal dialog.

Hiding the window or modeless dialog: "F"
If F was blocked by any modal dialog M, it becomes unblocked and is removed from M's blocked windows list.

Hiding the modal dialog: "M"
If M was blocked by any other modal dialog, for example, "N", it becomes unblocked and is removed from N's blocked windows list. Then, all the windows and dialogs blocked by M become unblocked, and after that the same checks (as in Showing the modal dialog: "M") are performed for each of them in the order they were initially shown.

Modal exclusion

There are two modal exclusion types introduced as of JDK 6

Exclusion from blocking of toolkit-modal dialogs
Exclusion from blocking of application-modal dialogs

By default, a window's modal exclusion property is turned off.

Application-modal exclusion
If a window is application-modal excluded, it is not blocked by any application-modal dialogs. Also, it is not blocked by document-modal dialogs from outside of its child hierarchy.
Toolkit-modal exclusion
If a window is toolkit-modal excluded, it is not blocked by any application- or toolkit-modal dialogs. Also, it is not blocked by document-modal dialogs from outside of their child hierarchy.

Implementation note: Changing the modal exclusion type for a visible window may have no effect until it is hidden and then shown again.

Related AWT features

Always-On-Top
When a modal dialog that is not always-on-top blocks an always-on-top window, their relative Z-order is unspecified and platform-dependent.

The toFront() and toBack() methods
A modal dialog should always be above all its blocked windows. Thus, if a blocked window is brought to the front, its blocking dialog, if any, is also brought to the front and remains above the blocked window. Likewise, if a modal dialog is sent to the back, all of its blocked windows are sent to the back to keep them below the blocking dialog.

Minimizing, maximizing and closing blocked windows
When a modal dialog blocks a window, the user may not be able to maximize or minimize the blocked window— however, the actual behavior is unspecified and platform-dependent. In any case, the user can't close the blocked window interactively— but it can be closed programmatically by calling the setVisible(false) or dispose() methods on the blocked window.

Blocked windows activations
When the user selects a blocked window, it may be brought to the front, along with the blocking modal dialog which would then become the active window— however, the actual behavior is unspecified and platform-dependent.

Hiding a modal dialog
When the modal dialog that currently has focus is hidden, it is unspecified and platform-dependent, which other window will become the active window. Any of the following may become the active window:

The owner of the modal dialog - if the owner is unblocked.
The Window, which was active before this modal dialog gained focus - if the owner of the modal dialog is absent or is blocked.

If the modal dialog to be hidden does not have focus, the active window remains unchanged.

Security

A special AWTPermission, "toolkitModality", is required to show toolkit-modal dialogs. This would prevent, for example, blocking a browser or Java Web Start (JWS) by modal dialogs shown from applets.

The same permission is required to exclude a window from toolkit modality. This would prevent, for example, a dialog shown from an applet not to be blocked by a browser's or JWS's modal dialog.

Platform support

Two java.awt.Toolkit methods allow you to check whether the current platform supports specific modality features:

isModalityTypeSupported(modalityType)
Returns whether the specified modality type is supported on the current platform. If mode "M" is not supported and a dialog is set to M-modal, it behaves as modeless.
isModalExclusionTypeSupported(modalExclusionType)
Returns whether the given modal exclusion type is supported on the current platform. If exclusion type "E" is not supported and a window is marked as E-excluded, this has no effect.

Compatibility

The default modality type is application-modal. It is used by the API calls: Dialog.setModal(true), Dialog(owner, true), etc. Prior to JDK 6 the default type was toolkit-modal, but the only distinction between application- and toolkit-modality is for applets and applications launched from Java Web Start.

Examples

Frame "F" is shown Document-modal dialog "D_i" is shown F becomes blocked by D_i — it's in the same document Document-modal dialog "D_ii" is shown D_i becomes blocked by D_ii — it's in the same document
Frame "F" is shown Document-modal dialog "D_i" is shown F becomes blocked by D_i — it's in the same document Document-modal dialog "D_ii" is shown D_i becomes blocked by D_ii — it's in the same document D_i is hidden F becomes blocked by D_ii — it's in the same document
Frame "F" is shown Toolkit-modal dialog "D_i" is created, but not shown Document-modal dialog "D_ii" is shown F becomes blocked by D_ii — it's in the same document Application-modal dialog "D_iii" is shown D_ii becomes blocked by D_iii — it's in the same application D_i is shown D_i becomes blocked by D_ii — it's its owner D_iii remains unblocked — it blocks D_ii and D_ii blocks D_i
Frame "F" is shown Toolkit-modal dialog "D_i" is created, but not shown Document-modal dialog "D_ii" is shown F becomes blocked by D_ii — it's in the same document Application-modal dialog "D_iii" is shown D_ii becomes blocked by D_iii — it's in the same application D_i is shown D_iii becomes blocked by D_i — D_i is not blocked D_i remains unblocked