Don't use "file.encoding", use Platform.getSystemCharset() instead

With https://openjdk.java.net/jeps/400 implemented in Java 18,
"file.encoding" system property became meaningless and can't be used
anymore to determine system native encoding.

Instead, use new Platform.getSystemCharset() API that tries to provide
a suitable replacement compatible with Java 18+ and previous Java
versions.

Note: the proposed change makes sure that IF users have specified
encoding via command line arguments -Dfile.encoding=XYZ, it will be
still used.

This PR fixes
eclipse-platform/eclipse.platform.resources#154

bundles/org.eclipse.ui.workbench/Eclipse UI/org/eclipse/ui/WorkbenchEncoding.java

@@ -13,6 +13,7 @@
  *******************************************************************************/
 package org.eclipse.ui;
 
+import java.lang.management.ManagementFactory;
 import java.nio.charset.Charset;
 import java.nio.charset.IllegalCharsetNameException;
 import java.util.ArrayList;
@@ -37,7 +38,6 @@ public class WorkbenchEncoding {
 	private static class EncodingsRegistryReader extends RegistryReader {
 
 		private List<String> encodings;
-
 		/**
 		 * Create a new instance of the receiver.
 		 *
@@ -58,13 +58,32 @@ protected boolean readElement(IConfigurationElement element) {
 		}
 	}
 
+	private static String defaultEncoding;
+
 	/**
 	 * Get the default encoding from the virtual machine.
 	 *
 	 * @return String
 	 */
 	public static String getWorkbenchDefaultEncoding() {
-		return System.getProperty("file.encoding", "UTF-8");//$NON-NLS-1$ //$NON-NLS-2$
+		if (defaultEncoding == null) {
+			String encoding = null;
+			// Check if -Dfile.encoding= startup argument was given to Eclipse's JVM
+			List<String> commandLineArgs = ManagementFactory.getRuntimeMXBean().getInputArguments();
+			for (String arg : commandLineArgs) {
+				if (arg.startsWith("-Dfile.encoding=")) { //$NON-NLS-1$
+					encoding = arg.substring("-Dfile.encoding=".length()); //$NON-NLS-1$
+					// continue, it can be specified multiple times, last one wins.
+				}
+			}
+			// Now we are sure user didn't specified encoding, so we can use
+			// default native encoding
+			if (encoding == null || encoding.isBlank()) {
+				encoding = Platform.getSystemCharset().name();
+			}
+			defaultEncoding = encoding;
+		}
+		return defaultEncoding;
 	}
 
 	/**

bundles/org.eclipse.ui.workbench/META-INF/MANIFEST.MF

@@ -95,7 +95,7 @@ Export-Package: org.eclipse.e4.ui.workbench.addons.perspectiveswitcher;x-interna
  org.eclipse.ui.themes,
  org.eclipse.ui.views,
  org.eclipse.ui.wizards
-Require-Bundle: org.eclipse.core.runtime;bundle-version="[3.25.0,4.0.0)",
+Require-Bundle: org.eclipse.core.runtime;bundle-version="[3.26.0,4.0.0)",
  org.eclipse.help;bundle-version="[3.2.0,4.0.0)",
  org.eclipse.jface;bundle-version="[3.18.0,4.0.0)",
  org.eclipse.swt;bundle-version="[3.107.0,4.0.0)",