fixed "endless recursion in font" exception in FlatLaf$ActiveFont.createValue() if UIManager.getFont() is invoked from multiple threads (issue #456)

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -1223,6 +1223,9 @@ public abstract class FlatLaf
 		}
 
 		private Object getValue( Object key ) {
+			// use local variable for getters to avoid potential multi-threading issues
+			List<Function<Object, Object>> uiDefaultsGetters = FlatLaf.this.uiDefaultsGetters;
+
 			if( uiDefaultsGetters == null )
 				return null;
 
@@ -1276,8 +1279,9 @@ public abstract class FlatLaf
 			this.scaleSize = scaleSize;
 		}
 
+		// using synchronized to avoid exception if invoked at the same time on multiple threads
 		@Override
-		public Object createValue( UIDefaults table ) {
+		public synchronized Object createValue( UIDefaults table ) {
 			if( inCreateValue )
 				throw new IllegalStateException( "FlatLaf: endless recursion in font" );
 

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatStressTest.java

@@ -22,7 +22,9 @@ import java.util.Random;
 import javax.swing.JComboBox;
 import javax.swing.JComponent;
 import javax.swing.JFrame;
+import javax.swing.JLabel;
 import javax.swing.SwingUtilities;
+import javax.swing.UIManager;
 import com.formdev.flatlaf.FlatClientProperties;
 import com.formdev.flatlaf.FlatLightLaf;
 
@@ -54,6 +56,7 @@ public class FlatStressTest
 
 	private JComponent createStressTest() {
 		return createComboBoxStressTest();
+//		return createGetFontStressTest();
 	}
 
 	// for https://github.com/JFormDesigner/FlatLaf/issues/432
@@ -80,4 +83,30 @@ public class FlatStressTest
 
 		return comboBox;
 	}
+
+	// for https://github.com/JFormDesigner/FlatLaf/issues/456
+	@SuppressWarnings( "unused" )
+	private JComponent createGetFontStressTest() {
+		JLabel label = new JLabel( "test" );
+
+		Runnable runnable = () -> {
+			for(;;) {
+				UIManager.getFont( "Label.font" );
+			}
+		};
+
+		Thread thread1 = new Thread( runnable);
+		thread1.setDaemon( true );
+		thread1.start();
+
+		Thread thread2 = new Thread( runnable);
+		thread2.setDaemon( true );
+		thread2.start();
+
+		Thread thread3 = new Thread( runnable);
+		thread3.setDaemon( true );
+		thread3.start();
+
+		return label;
+	}
 }