Is Java System Property "swing.defaultlaf" actually referenced in source?

[Alexey Ivanov]

Hi Mark,

The default look and feel is Metal on all OS except for macOS where the 
default is Aqua.

You can modify the default look and feel using the the 
|swing.defaultlaf| property, and it works for me as expected. The 
default look and feel of an application is initialised to the class in 
the |swing.defaultlaf| property.

I ran a simple Java program on Windows:

> java DefaultLookAndFeel.java
[*The Java(tm) Look and Feel* - javax.swing.plaf.metal.MetalLookAndFeel] 
com.sun.java.swing.plaf.windows.WindowsLookAndFeel 
javax.swing.plaf.metal.MetalLookAndFeel

> java 
-Dswing.defaultlaf=com.sun.java.swing.plaf.windows.WindowsLookAndFeel 
DefaultLookAndFeel.java
[*The Microsoft Windows Look and Feel* - 
com.sun.java.swing.plaf.windows.WindowsLookAndFeel] 
com.sun.java.swing.plaf.windows.WindowsLookAndFeel 
javax.swing.plaf.metal.MetalLookAndFeel


The |swing.defaultlaf| property is documented and is used in the 
tutorial How to Set the Look and Feel 
<https://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html>, 
specifically the section Specifying the Look and Feel: Command Line 
<https://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html#commandLine> 
has two examples how to modify the default look and feel.

If set the value to a class that doesn't exist, I'll get an exception:

> java -Dswing.defaultlaf=NimbusLookAndFeel DefaultLookAndFeel.java
Exception in thread "main" java.lang.Error: Cannot load 
NimbusLookAndFeel at 
java.desktop/javax.swing.UIManager.initializeDefaultLAF(UIManager.java:1390) 
at java.desktop/javax.swing.UIManager.initialize(UIManager.java:1501) at 
java.desktop/javax.swing.UIManager.maybeInitialize(UIManager.java:1467) 
at java.desktop/javax.swing.UIManager.getLookAndFeel(UIManager.java:492) 
at DefaultLookAndFeel.main(DefaultLookAndFeel.java:5)


You can use these line numbers to study how default look and feel is 
initialised.


Please note the client-libs-dev mailing list is not a support forum, it 
is place to discuss /development of client libraries/.

-- 
Regards,
Alexey

On 2025-06-19 19:04, Ludwig, Mark wrote:
>
> Greetings,
>
> We are in the process of (finally) completing a migration from 
> AWT-based windows and dialogs to Swing-based that was started in the 
> 1990s.
>
> Testing the external Swing controls on Linux, we find nothing seems to 
> happen as a result of setting the “swing.defaultlaf” System Property.  
> This is documented in multiple places – at least the 
> javax.swing.UIManager doc, and the *How to Set the Look and Feel* 
> tutorial (and is mentioned in 
> https://bugs.openjdk.org/browse/JDK-8159164).
>
> We find the “swing.systemlaf” System Property is effective in changing 
> the returned value from UIManager.getSystemLookAndFeelClassName(), but 
> does not by itself change the look-and-feel -- only with something 
> like 
> UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()), 
> which is documented.
>
> We would appreciate clarity on this discrepancy regarding the 
> “default” look-and-feel for Swing on Linux.
>
> Thanks,
>
> Mark Ludwig
>

The source of the sample app:

|import javax.swing.UIManager; public class DefaultLookAndFeel { public 
static void main(String[] args) { 
System.out.println(UIManager.getLookAndFeel()); 
System.out.println(UIManager.getSystemLookAndFeelClassName()); 
System.out.println(UIManager.getCrossPlatformLookAndFeelClassName()); } }|
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