Linux: initialize font (#2)

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

@@ -201,7 +201,8 @@ public abstract class FlatLaf
 			uiFont = (font instanceof FontUIResource) ? (FontUIResource) font : new FontUIResource( font );
 
 		} else if( SystemInfo.IS_LINUX ) {
-			System.err.println( "WARNING: FlatLaf is not yet tested on Linux!" );
+			Font font = LinuxFontPolicy.getFont();
+			uiFont = (font instanceof FontUIResource) ? (FontUIResource) font : new FontUIResource( font );
 		}
 
 		if( uiFont == null )

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

@@ -0,0 +1,110 @@
+/*
+ * Copyright 2019 FormDev Software GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.formdev.flatlaf;
+
+import java.awt.Font;
+import java.awt.GraphicsEnvironment;
+import java.awt.Toolkit;
+import java.util.StringTokenizer;
+import javax.swing.text.StyleContext;
+
+/**
+ * @author Karl Tauber
+ */
+class LinuxFontPolicy
+{
+	static Font getFont() {
+		// see class com.sun.java.swing.plaf.gtk.PangoFonts background information
+
+		Object fontName = Toolkit.getDefaultToolkit().getDesktopProperty( "gnome.Gtk/FontName" );
+		if( !(fontName instanceof String) )
+			fontName = "sans 10";
+
+        String family = "";
+        int style = Font.PLAIN;
+        int size = 10;
+
+		StringTokenizer st = new StringTokenizer( (String) fontName );
+		while( st.hasMoreTokens() ) {
+			String word = st.nextToken();
+
+			if( word.equalsIgnoreCase( "italic" ) )
+				style |= Font.ITALIC;
+			else if( word.equalsIgnoreCase( "bold" ) )
+				style |= Font.BOLD;
+			else if( Character.isDigit( word.charAt( 0 ) ) ) {
+				try {
+					size = Integer.parseInt( word );
+				} catch( NumberFormatException ex ) {
+					// ignore
+				}
+			} else
+				family = family.isEmpty() ? word : (family + ' ' + word);
+		}
+
+		// scale font size
+		double dsize = size * getSystemFontScale();
+		size = (int) (dsize + 0.5);
+		if( size < 1 )
+			size = 1;
+
+		// handle logical font names
+		String logicalFamily = mapFcName( family.toLowerCase() );
+		if( logicalFamily != null )
+			family = logicalFamily;
+
+		// using StyleContext.getFont() here because it uses
+		// sun.font.FontUtilities.getCompositeFontUIResource()
+		Font font = new StyleContext().getFont( family, style, size );
+
+		// set font size in floating points
+		font = font.deriveFont( style, (float) dsize );
+
+		return font;
+	}
+
+	private static double getSystemFontScale() {
+		// see class com.sun.java.swing.plaf.gtk.PangoFonts background information
+
+		Object value = Toolkit.getDefaultToolkit().getDesktopProperty( "gnome.Xft/DPI" );
+		if( value instanceof Integer ) {
+			int dpi = ((Integer)value).intValue() / 1024;
+			if( dpi == -1 )
+				dpi = 96;
+			if( dpi < 50 )
+				dpi = 50;
+			return dpi / 72.0;
+		} else {
+			return GraphicsEnvironment.getLocalGraphicsEnvironment()
+				.getDefaultScreenDevice().getDefaultConfiguration()
+				.getNormalizingTransform().getScaleY();
+		}
+	}
+
+	/**
+	 * map GTK/fontconfig names to equivalent JDK logical font name
+	 */
+	private static String mapFcName( String name ) {
+		switch( name ) {
+			case "sans":		return "sansserif";
+			case "sans-serif":	return "sansserif";
+			case "serif":		return "serif";
+			case "monospace":	return "monospaced";
+		}
+		return null;
+	}
+}