Don't use "file.encoding", use Platform.getNativeEncoding() 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.getNativeEncoding() API that tries to provide
a suitable replacement compatible with Java 18+ and previous Java
versions.

See eclipse-platform#154

bundles/org.eclipse.core.resources/META-INF/MANIFEST.MF

@@ -27,7 +27,7 @@ Export-Package: org.eclipse.core.internal.dtree;x-internal:=true,
 Require-Bundle: org.eclipse.ant.core;bundle-version="[3.1.0,4.0.0)";resolution:=optional,
  org.eclipse.core.expressions;bundle-version="[3.2.0,4.0.0)",
  org.eclipse.core.filesystem;bundle-version="[1.3.0,2.0.0)",
- org.eclipse.core.runtime;bundle-version="[3.25.0,4.0.0)"
+ org.eclipse.core.runtime;bundle-version="[3.26.0,4.0.0)"
 Bundle-ActivationPolicy: lazy
 Service-Component: OSGI-INF/ResourceChangeListenerRegistrar.xml,
  OSGI-INF/org.eclipse.core.internal.resources.CheckMissingNaturesListener.xml

bundles/org.eclipse.core.resources/src/org/eclipse/core/resources/ResourcesPlugin.java

@@ -168,18 +168,19 @@ public final class ResourcesPlugin extends Plugin {
 	public static final Object FAMILY_MANUAL_REFRESH = new Object();
 
 	/**
-	 * Name of a preference indicating the encoding to use when reading text
-	 * files in the workspace.  The value is a string, and may
-	 * be the default empty string, indicating that the file system encoding should
-	 * be used instead.  The file system encoding can be retrieved using
-	 * <code>System.getProperty("file.encoding")</code>.
-	 * There is also a convenience method <code>getEncoding</code> which returns
-	 * the value of this preference, or the file system encoding if this
-	 * preference is not set.
+	 * Name of a preference indicating the encoding to use when reading text files
+	 * in the workspace. The value is a string, and may be the default empty string,
+	 * indicating that the file system encoding should be used instead. The file
+	 * system encoding can be retrieved using
+	 * <code>System.getProperty("file.encoding")</code> before Java 18, and using
+	 * <code>System.getProperty("native.encoding") with Java 18 and higher versions</code>.
+	 * There is also a convenience method <code>getEncoding</code> which returns the
+	 * value of this preference, or the file system encoding if this preference is
+	 * not set.
 	 * <p>
 	 * Note that there is no guarantee that the value is a supported encoding.
-	 * Callers should be prepared to handle <code>UnsupportedEncodingException</code>
-	 * where this encoding is used.
+	 * Callers should be prepared to handle
+	 * <code>UnsupportedEncodingException</code> where this encoding is used.
 	 * </p>
 	 *
 	 * @see #getEncoding()
@@ -372,10 +373,15 @@ public final class ResourcesPlugin extends Plugin {
 
 	/**
 	 * Returns the encoding to use when reading text files in the workspace. This is
-	 * the value of the <code>PREF_ENCODING</code> preference, or the file system
-	 * encoding (<code>System.getProperty("file.encoding")</code>) if the preference
-	 * is not set, the workspace is not ready yet or any other condition where it
-	 * could not be determined.
+	 * the value of the <code>PREF_ENCODING</code> preference. If the preference is
+	 * not set, the workspace is not ready yet or any other condition where it could
+	 * not be determined, returns native file system encoding, as specified by
+	 * <ul>
+	 * <li><code>System.getProperty("native.encoding")</code> on Java 18 and later
+	 * </li>
+	 * <li><code>System.getProperty("sun.jnu.encoding")</code> on previous Java
+	 * versions</li>
+	 * </ul>
 	 * <p>
 	 * Note that this method does not check whether the result is a supported
 	 * encoding. Callers should be prepared to handle
@@ -391,36 +397,43 @@ public final class ResourcesPlugin extends Plugin {
 	 * ServiceTracker, Blueprint, ...) to track the workspace</li>
 	 * <li>Calling workspace.getRoot().getDefaultCharset(false)</li>
 	 * <li>If <code>null</code> is returned take the appropriate action, e.g fall
-	 * back to <code>System.getProperty("file.encoding")</code> or even better
-	 * {@link Charset#defaultCharset()}</li>
+	 * back to {@link Platform#getNativeEncoding()}</li>
 	 * </ol>
 	 *
 	 * @return the encoding to use when reading text files in the workspace
 	 * @see java.io.UnsupportedEncodingException
+	 * @see #getSystemEncoding()
 	 */
 	public static String getEncoding() {
 		ResourcesPlugin resourcesPlugin = plugin;
 		if (resourcesPlugin == null) {
-			return getSystemEncoding();
+			return getSystemEncoding().name();
 		}
 		Workspace workspace = resourcesPlugin.workspaceInitCustomizer.workspace;
 		if (workspace == null) {
-			return getSystemEncoding();
+			return getSystemEncoding().name();
 		}
 		try {
 			String enc = workspace.getRoot().getDefaultCharset(false);
 			if (enc == null) {
-				return getSystemEncoding();
+				return getSystemEncoding().name();
 			}
 			return enc;
 		} catch (CoreException e) {
 			// should never happen here... even if we fall back to system encoding...
-			return getSystemEncoding();
+			return getSystemEncoding().name();
 		}
 	}
 
-	private static String getSystemEncoding() {
-		return System.getProperty("file.encoding"); //$NON-NLS-1$
+	/**
+	 * Retrieves the <b>native</b> system encoding ({@link Charset}) based on the
+	 * system locale.
+	 *
+	 * @return the {@link Charset}, never null
+	 * @see Platform#getNativeEncoding()
+	 */
+	private static Charset getSystemEncoding() {
+		return Platform.getNativeEncoding();
 	}
 
 	/**