This document is based on an article originally published in The Swing Connection.
The Multiplexing look and feel lets you supplement an ordinary look and feel (called the default look and feel) with one or more auxiliary 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.
This document has the following sections:
Before reading further, you should be familiar with the concept of pluggable look and feels. For basic information, see How to Set the Look and Feel, a section in The Java Tutorial. For architectural details, you can read Pluggable look-and-feel architecture, a section within a Swing Connection article.
The classes in the 
javax.swing.plaf.multi package
implement a 
multiplexing look and feel.
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 getUI method).
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.
This approach has at least two shortcomings:
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.
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.
The default multiplexing look and feel implementation, 
represented by the MultiLookAndFeel class
in the javax.swing.plaf.multi package,
is called (unsurprisingly) 
the Multiplexing look and feel.
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 $JDKHOME/conf/swing.properties
file to include a definition of the swing.auxiliarylaf
property. Swing treats the swing.auxiliarylaf
property as a comma-separated list of LookAndFeel
subclasses that specify what auxiliary look and feels should be 
used in addition to the default look and feel. If at least one valid 
LookAndFeel
subclass is specified in the swing.auxiliarylaf
property, Swing automatically uses the Multiplexing look and feel 
to load and support the default and auxiliary look and feels.
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 com.myco.TextTalkerLookAndFeel, 
and the look and feel that adds support for perfume
is named com.smellco.OlfactoryLookAndFeel.
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 $JDKHOME/conf/swing.properties file:
    
              swing.auxiliarylaf=com.myco.TextTalkerLookAndFeel,
                      com.smellco.OlfactoryLookAndFeel
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.
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 --
JSeparators, for example, might be ignored.
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.
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.
Just like for any other look and feel, you
implement an auxiliary look and feel
by writing a subclass of javax.swing.LookAndFeel
and creating subclasses of the 
FooUI classes defined in
the javax.swing.plaf package.
The following paragraphs provide some general recommendations for developing auxiliary look and feels.
Use the installUI method
to perform all initialization,
and the uninstallUI method
to perform all cleanup.
TheinstallUIanduninstallUImethods are invoked when a component's look and feel is set. TheinstallUImethod gives the new UI object a chance to add listeners on the component and its data model. Similarly, theuninstallUImethod lets the previous UI object remove its listeners.
Don't extend visual look and feels.
We recommended that you don't 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.
Instead, we recommend that the UI classes of an auxiliary look and feel directly extend the abstract UI classes in thejavax.swing.plafpackage. By using this strategy, the developer of an auxiliary look and feel can avoid competing with the default look and feel.
Override all UI-specific methods your UI classes inherit.
We recommend that each UI class of an auxiliary look and feel override the methods defined in thejavax.swing.plafUI classes it descends from The reasons for this recommendation are similar to those for not extending a visual look and feel. For example, theComponentUIclass, from which all UI classes descend, provides a default implementation for theupdatemethod. 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!
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 ButtonUI subclass but no 
              LabelUI subclass.
	     This 
              option is allowed, and the multiplexing look and feel gracefully 
              handles such situations.
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.
Fortunately, you can prevent this error 
              message by creating a subclass of the UIDefaults 
              class and returning an instance of it from the 
	      getDefaults method
	      of your LookAndFeel class.
	      For example:
	      
public class MyAuxLookAndFeel 
              extends LookAndFeel {
                  ...
                  public UIDefaults getDefaults() {
                      UIDefaults table = 
                          
              new MyAuxUIDefaults();
                      Object[] uiDefaults = {
                        "ButtonUI", "MyAuxButtonUI",
                        ...
                      }
                      table.putDefaults(uiDefaults);
                      return table;
                  }
              }
              
              class MyAuxUIDefaults extends UIDefaults {
                  protected void getUIError(String msg) {
                      //System.err.println
                      //   ("An 
              annoying message!");
                  }
              }
In the preceding example, an auxiliary look and feel named MyAux 
              creates a UIDefaults subclass 
              that overrides the getUIError 
              method. The getUIError 
              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.
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 getUI
              method. The returned UI is an instance of one of the multiplexing 
              look and feel UI classes (for example, MultiButtonUI). 
              The UI object from the auxiliary look and feel can call the getUIs 
              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.
	      
The Multiplexing look and feel
(represented by
javax.swing.plaf.multi.MultiLookAndFeel)
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.
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.
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:
swing.auxiliarylaf 
                property.getPreferredSize 
                method is invoked on a multiplexing UI, the UI returns only the 
                results of invoking getPreferredSize 
                on the UI obtained from the default look and feel.
		The getPreferredSize method
		is also invoked on the UI object for each auxiliary look and feel,
		but the return values are ignored.
                installUI 
                method on a multiplexing UI causes the multiplexing UI to invoke 
                installUI 
                on the UI obtained from the default look and feel and the UIs obtained from 
                the auxiliary factories. In all cases, the UI object obtained from 
              the default look and feel is acted upon first, and then the auxiliary 
              look and feels are acted upon in the order they are specified in 
              the swing.auxiliarylaf 
              property.
While we hope the behavior of the Multiplexing look and feel is flexible enough not to require an alternative multiplexing look and feel, Swing allows the user to specify another multiplexing look and feel to use.
 To do that, all the user has to do is modify 
              the $JDKHOME/conf/swing.properties 
              file to include a definition of the swing.plaf.multiplexinglaf 
              property. Swing then treats the swing.plaf.multiplexinglaf 
              property as a LookAndFeel 
              subclass that supports multiplexing.
 For example, if a user has a multiplexing 
              look and feel represented by com.myco.SuperMultiLookAndFeel 
              that is a better match for their needs than the Multiplexing 
              look and feel 
	      (javax.swing.plaf.multi.MultiLookAndFeel), 
              the user could include the following line in $JDKHOME/conf/swing.properties:
swing.plaf.multiplexinglaf = com.myco.SuperMultiLookAndFeel
This statement instructs Swing to use com.myco.SuperMultiLookAndFeel 
instead of javax.swing.plaf.multi.MultiLookAndFeel. But 
if you use this kind of statement, be careful, because the suppliers 
of auxiliary look and feels will most likely have developed and 
tested against our Multiplexing look and feel.