8331467: ImageReaderFactory can cause a ClassNotFoundException if the default FileSystemProvider is not the system-default provider

src/java.base/share/classes/java/lang/System.java

@@ -1968,6 +1968,21 @@ private static void initPhase3() {
 
         // system is fully initialized
         VM.initLevel(4);
+
+        // if the property java.nio.file.spi.DefaultFileSystemProvider is
+        // set then load the default provider (or a list)
+        String prop = "java.nio.file.spi.DefaultFileSystemProvider";
+        String propValue = System.getProperty(prop);
+        if (propValue != null) {
+            for (String cn: propValue.split(",")) {
+                try {
+                    Class.forName(cn, true, ClassLoader.getSystemClassLoader());
+                } catch (Exception x) {
+                    throw new Error(x);
+                }
+            }
+        }
+        VM.setCustomDefaultFileSystemProviderLoaded();
     }
 
     private static void setJavaLangAccess() {

src/java.base/share/classes/java/nio/file/FileSystems.java

@@ -171,7 +171,7 @@ private static FileSystemProvider getDefaultProvider() {
      * @return  the default file system
      */
     public static FileSystem getDefault() {
-        if (VM.isModuleSystemInited()) {
+        if (VM.isModuleSystemInited() && VM.isCustomDefaultFileSystemProviderLoaded()) {
             return DefaultFileSystemHolder.defaultFileSystem;
         } else {
             // always use the platform's default file system during startup

src/java.base/share/classes/jdk/internal/misc/VM.java

@@ -49,6 +49,7 @@ public class VM {
 
     // 0, 1, 2, ...
     private static volatile int initLevel;
+    private static volatile boolean isCustomDefaultFileSystemProviderLoaded = false;
     private static final Object lock = new Object();
 
     /**
@@ -67,6 +68,22 @@ public static void initLevel(int value) {
         }
     }
 
+    /**
+     * control the CustomDefaultFileSystemProvider loading status
+     *
+     * @see java.lang.System#initPhase3
+     */
+    public static void setCustomDefaultFileSystemProviderLoaded() {
+        isCustomDefaultFileSystemProviderLoaded = true;
+    }
+
+    /**
+     * Returns the current customDefaultFileSystemProvider loading status
+     */
+    public static boolean isCustomDefaultFileSystemProviderLoaded() {
+        return isCustomDefaultFileSystemProviderLoaded;
+    }
+
     /**
      * Returns the current init level.
      */