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<h1>Using the Multiplexing Look and Feel</h1>
<blockquote>
<hr>
<p>
<i>
This document is based on an article
originally published in
<a href="http://www.oracle.com/technetwork/java/javase/tech/articles-jsp-139072.html"
target="_top"><em>The Swing Connection</em></a>.
</i>
</p>
<hr>
</blockquote>
<p>
The Multiplexing look and feel lets
you supplement an ordinary look and feel
(called the <em>default</em> look and feel)
with one or more <em>auxiliary</em> look and feels.
For example, you could
simultaneously provide text-to-speech and Braille outputs,
in addition to the ordinary visual output that a Swing-based
application generates,
by adding
two auxiliary look and feels (one for text-to-speech,
the other for Braille)
to the default look and feel.
The default look and feel can be any ordinary look and feel --
the Java or Windows look and feel, for example --
and requires no modifications to work with auxiliary look and feels.
</p>
<p>
This document has the following sections:
<ul>
<li> <a href="#overview">Overview</a>
<li> <a href="#howtouse">How to Use Auxiliary Look and Feels</a>
<li> <a href="#howtowrite">Tips for Writing an Auxiliary Look and Feel</a>
<ul>
<li> <a href="#dosanddonts">Dos and Don'ts</a>
<li> <a href="#uidefaults">Extending UIDefaults</a>
<li> <a href="#defaultui">Examining Other UI Objects</a>
</ul>
<li> <a href="#implementation">How the Multiplexing Look and Feel is
Implemented</a>
<li> <a href="#custom">How to Provide a Custom Multiplexing
Look and Feel</a>
</ul>
<p>
Before reading further, you should be familiar
with the concept of pluggable look and feels.
For basic information, see
<a href="https://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html">
How to Set the Look and Feel</a>, a section in <em>The Java Tutorial</em>.
For architectural details, you can read
<a
href="http://www.oracle.com/technetwork/java/architecture-142923.html#pluggable">
Pluggable look-and-feel architecture</a>, a section within
a <em>Swing Connection</em> article.
</p>
<p>
<a id="overview"></a>
<hr>
<h2>Overview</h2>
<p>
The classes in the
<code>javax.swing.plaf.multi</code> package
implement a
<i>multiplexing look and feel</i>.
A multiplexing look and feel transparently creates -- and
simultaneously supports -- UI objects from several different look and feels
in response to a component requesting its UI object
(with the <code>getUI</code> method).
<p>
Without a multiplexing look and feel, a
developer who wanted to enhance a particular look and feel would
need to extend the classes supporting that look and feel. For example, to
add text-to-speech support to the Java look and feel without using a multiplexing
look and feel, the developer would need to create a group of
classes that extend those of
the Java look and feel, and add text-to-speech support to the new classes.
If the developer also wanted to add text-to-speech support to other look
and feels,
such as Motif or Windows, the developers would need to create subclasses
of those classes as well.
</p>
<p>
This approach has at least two shortcomings:
<ul>
<li>First, each subclass must use what is
essentially a copy of the same code, potentially creating a difficult
support situation for the developer.<br></li>
<li>Second, and more significantly for the
end user, some application developers might force a
particular look and feel to be used. When this approach is used,
the end user can't even use the enhanced look and feel.</li>
</ul>
<p>
A multiplexing look and feel
both these problems simultaneously because it allows multiple look
and feels to be combined.
The first problem (having to use what amounts to a second copy of the same
code) is solved because the developer can create a specialized look
and feel that can then be combined with other look and feels.
</p>
<p>
The second problem (having to force the use of
a particular look and feel) is solved because a specialized look and feel
can be used with whatever default look and feel the
application may have locked in place.
</p>
<p>
The default multiplexing look and feel implementation,
represented by the <code>MultiLookAndFeel</code> class
in the <code>javax.swing.plaf.multi</code> package,
is called (unsurprisingly)
the Multiplexing look and feel.
<p>
<a id="howtouse"></a>
<hr>
<h2>How to Use Auxiliary Look and Feels</h2>
<p>
It's easy to use auxiliary look and feels with Swing. To instruct
Swing to use the Multiplexing look and feel, all an application
has to do is modify the <code>$JDKHOME/conf/swing.properties</code>
file to include a definition of the <code>swing.auxiliarylaf</code>
property. Swing treats the <code>swing.auxiliarylaf</code>
property as a comma-separated list of <code>LookAndFeel</code>
subclasses that specify what auxiliary look and feels should be
used in addition to the default look and feel. If at least one valid
<code>LookAndFeel</code>
subclass is specified in the <code>swing.auxiliarylaf</code>
property, Swing automatically uses the Multiplexing look and feel
to load and support the default and auxiliary look and feels.
</p>
<p>
For example, let's assume that an application
makes use of a look and feel that supports text-to-speech feedback, and also
uses an look and feel that adds support for a device
that emits perfume.
Let's assume that the text-to-speech
look and feel is named <code>com.myco.TextTalkerLookAndFeel</code>,
and the look and feel that adds support for perfume
is named <code>com.smellco.OlfactoryLookAndFeel</code>.
</p>
<p>
To tell Swing to use both these look and feels
-- and to use a default look and feel at the same time -- your application
could simply add the following line to the <code>$JDKHOME/conf/swing.properties</code> file:
</p>
<p>
<code>
swing.auxiliarylaf=com.myco.TextTalkerLookAndFeel,<br>
com.smellco.OlfactoryLookAndFeel</code>
</p>
<p>
This statement tells Swing to obtain a component's UI from the Multiplexing
look and feel automatically, instead of obtaining it directly from
the default look and feel. The resulting multiplexing UI is a small
delegate that obtains and maintains UIs from the default and auxiliary
look and feels. As a result, when a method is invoked in a multiplexing
UI object, the multiplexing UI invokes the same method on each
of the UIs obtained from the default and auxiliary look and feels.
</p>
<p>
<a id="howtowrite"></a>
<hr>
<h2>Tips for Writing an Auxiliary Look and Feel</h2>
<p>
An auxiliary look and feel is like any other look and feel,
except that it doesn't have to provide the complete support
that a default look and feel must. For
example, an auxiliary look and feel that supports just text-to-speech feedback
doesn't need to provide any code for painting.
Also, it might not need to support all components --
<code>JSeparator</code>s, for example, might be ignored.
<p>
Auxiliary look and feels tend to be simple,
so developing one can be easier than developing a visual
look and feel.
The developer can concentrate solely
on providing the specialized functionality.
<p>
Because the primary purpose of an auxiliary look and feel is to enhance the
default look and feel, auxiliary look and feels tend
be nonvisual. Since an auxiliary look and feel is a genuine
look and feel, however, there is nothing to prevent it
from rendering information on the display.
</p>
<p>
Just like for any other look and feel, you
implement an auxiliary look and feel
by writing a subclass of <code>javax.swing.LookAndFeel</code>
and creating subclasses of the
<code><em>Foo</em>UI</code> classes defined in
the <code>javax.swing.plaf</code> package.
</p>
<a id="dosanddonts"></a>
<h3>Dos and Don'ts</h3>
<p>
The following paragraphs provide some general recommendations for developing
auxiliary look and feels.
</p>
<h4>Use the <code>installUI</code> method
to perform all initialization,
and the <code>uninstallUI</code> method
to perform all cleanup.</h4>
<blockquote>
The <code>installUI</code> and <code>uninstallUI</code>
methods are invoked when a component's look and feel is set.
The <code>installUI</code> method gives the new UI object
a chance to add listeners on the component and its data model.
Similarly, the <code>uninstallUI</code> method
lets the previous UI object remove its listeners.
</blockquote>
<h4><b>Don't extend visual look and feels.</b></h4>
<blockquote>
We recommended that you <i>don't</i> implement
UI classes of an auxiliary look and feel as subclasses of the
UI classes of a visual look and feel. Why not? Because they might
accidentally inherit code that installs listeners on a component
object or renders the component on the display. As a result,
your auxiliary look and feel would compete with the default look
and feel rather than cooperating with it.<br>
<br>
Instead, we recommend that the UI classes of an auxiliary look
and feel directly extend the abstract UI classes in the <code>javax.swing.plaf</code>
package. By using this strategy, the developer of an auxiliary
look and feel can avoid competing with the default look and feel.
</blockquote>
<h4><b>Override all UI-specific methods your UI classes inherit.</b></h4>
<blockquote>
We recommend that each UI class of
an auxiliary look and feel override the methods
defined in the <code>javax.swing.plaf</code>
UI classes it descends from
The reasons for this recommendation are similar
to those for not extending a visual look and feel.
For example, the <code>ComponentUI</code>
class, from which all UI classes descend,
provides a default implementation for the <code>update</code>
method. This default implementation paints on the display
if the
component is opaque. If a UI class from a non-visual auxiliary
look and feel does not override this method, all
opaque components appear as blank areas on the screen!
</blockquote>
<a id="uidefaults"></a>
<h3>Extending UIDefaults</h3>
<p>In many cases, you
might want an auxiliary look and feel to be "incomplete." That
is, you might not need to support the complete set
of components.
For example, an auxiliary look and feel might choose
to provide a <code>ButtonUI</code> subclass but no
<code>LabelUI</code> subclass.
This
option is allowed, and the multiplexing look and feel gracefully
handles such situations.</p>
<p>By default, however, Swing issues an error message when it asks
a look and feel for a UI object and the look and feel does not
support that UI. This message can be annoying, especially to auxiliary
look-and-feel developers who don't want to support a particular
component.</p>
<p>Fortunately, you can prevent this error
message by creating a subclass of the <code>UIDefaults</code>
class and returning an instance of it from the
<code>getDefaults</code> method
of your <code>LookAndFeel</code> class.
For example:
</p>
<p><code>public class MyAuxLookAndFeel
extends LookAndFeel {<br>
...<br>
public UIDefaults getDefaults() {<br>
<b>UIDefaults table = <br>
new MyAuxUIDefaults();<br>
</b> Object[] uiDefaults = {<br>
"ButtonUI", "MyAuxButtonUI",<br>
...<br>
}<br>
table.putDefaults(uiDefaults);<br>
return table;<br>
}<br>
}<br>
<br>
<b>class MyAuxUIDefaults extends UIDefaults {<br>
protected void getUIError(String msg) {<br>
//System.err.println<br>
// ("An
annoying message!");<br>
}<br>
}</b></code></p>
<p>
In the preceding example, an auxiliary look and feel named <code>MyAux</code>
creates a <code>UIDefaults</code> subclass
that overrides the <code>getUIError</code>
method. The <code>getUIError</code>
method is the method that is invoked when Swing cannot find a UI
object in a look and feel. By merely doing nothing in this method,
you can avoid the error message.</p>
<p>
<a id="defaultui"></a>
<h3>Examining Other UI Objects</h3>
<p>In
rare instances, a UI object from an auxiliary look and feel
may be interested in the default UI object used by the component. In
these cases, the UI object from auxiliary look and feel can obtain
the UI from a component by calling its <code>getUI</code>
method. The returned UI is an instance of one of the multiplexing
look and feel UI classes (for example, <code>MultiButtonUI</code>).
The UI object from the auxiliary look and feel can call the <code>getUIs</code>
method of the returned object to obtain an array containing a complete list
of all UI objects handled by the multiplexing UI. The first element
is guaranteed to be the UI created from the default look and feel.
</p>
<p>
<a id="implementation"></a>
<hr>
<h2>How the Multiplexing Look and Feel Is Implemented</h2>
<p>
The Multiplexing look and feel
(represented by
<code>javax.swing.plaf.multi.MultiLookAndFeel</code>)
is meant to be transparent to
all developers and users. It should "just work" -- and
it is used only when the user tells Swing to use an auxiliary look
and feel.</p>
<p>
When the Multiplexing look and
feel is in use, the type of the UI object
associated with each component
depends on whether
any of the auxiliary look and feels currently in use
support the component.
If so, the component's UI object is
an instance of a multiplexing UI.
If only the default look and feel supports the component,
then the component gets
a UI object from the default look and feel,
just as if no auxiliary look and feels were installed.
<p>
A multiplexing UI object
obtains and maintains UI objects
from the default and auxiliary look
and feels,
referring to these UIs in the following manner:
<ul>
<li> The UI object from the default look
and feel is always the first to be created. After that, a UI object
is created from each auxiliary look and feel in the order
they are specified in the <code>swing.auxiliarylaf</code>
property.<br><br></li>
<li> When a method that requests information
from a UI object is invoked, the multiplexing UI object
invokes the method on all the UI objects, but returns
only the results from the UI for the default look and feel.
For example, when the <code>getPreferredSize</code>
method is invoked on a multiplexing UI, the UI returns only the
results of invoking <code>getPreferredSize</code>
on the UI obtained from the default look and feel.
The <code>getPreferredSize</code> method
is also invoked on the UI object for each auxiliary look and feel,
but the return values are ignored.
<br><br></li>
<li> When a method that does not request information
from the UI object is invoked, the multiplexing UI object
invokes that method on all UIs --
on the UI object obtained from the default look
and feel
and on all the UIs obtained from the auxiliary look and feels,
as well.
For example, invoking the <code>installUI</code>
method on a multiplexing UI causes the multiplexing UI to invoke
<code>installUI</code>
on the UI obtained from the default look and feel and the UIs obtained from
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