document DefaultFileSystemHolder

Author: zapster

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jdk/runtimeinit/FileSystemProviderRuntimeInitSupport.java

@@ -36,6 +36,7 @@
 import com.oracle.svm.core.annotate.InjectAccessors;
 import com.oracle.svm.core.annotate.TargetClass;
 import com.oracle.svm.core.jdk.JDKInitializedAtRunTime;
+import com.oracle.svm.core.util.BasedOnJDKFile;
 import com.oracle.svm.core.util.VMError;
 
 /**
@@ -71,8 +72,12 @@ final class Target_java_io_DefaultFileSystem_RunTime {
 
 /**
  * Holds the default java.io file system. Initialized at run time via
- * {@code JDKInitializationFeature}.
+ * {@code JDKInitializationFeature}. This cache is needed because
+ * {@link Target_java_io_DefaultFileSystem_RunTime#getFileSystem()} creates a new instance for every
+ * time. In the JDK, this method is called only once.
  */
+@BasedOnJDKFile("https://github.com/openjdk/jdk/blob/jdk-25+25/src/java.base/unix/classes/java/io/DefaultFileSystem.java#L39-L41")
+@BasedOnJDKFile("https://github.com/openjdk/jdk/blob/jdk-25+25/src/java.base/windows/classes/java/io/DefaultFileSystem.java#L39-L41")
 class DefaultFileSystemHolder {
     static final Target_java_io_FileSystem_RunTime FS;
     static {