GH-3001: Fuseki modules initialization fix

Author: afs

jena-fuseki2/jena-fuseki-main/src/main/java/org/apache/jena/fuseki/main/sys/FusekiAutoModules.java

@@ -43,41 +43,48 @@ public class FusekiAutoModules {
     // ServiceLoader
     private static ServiceLoader<FusekiAutoModule> serviceLoader = null;
 
+    /**
+     * Return the current (last loaded) Fuseki auto-modules.
+     */
+    static FusekiModules get() {
+        if ( currentLoadedModules != null )
+            return currentLoadedModules;
+        synchronized(lock) {
+            if ( currentLoadedModules == null )
+                load();
+            return currentLoadedModules;
+        }
+    }
 
     /**
      * Load FusekiAutoModules. This call reloads the modules every call.
      */
-    static FusekiModules load() {
-        if ( serviceLoader == null )
-            serviceLoader = createServiceLoader();
-        currentLoadedModules = loadAutoModules(serviceLoader);
-        return get();
+    private static FusekiModules load() {
+        synchronized (lock) {
+            if ( serviceLoader == null )
+                serviceLoader = createServiceLoader();
+            currentLoadedModules = loadAutoModules(serviceLoader);
+        }
+        return currentLoadedModules;
     }
 
     /**
-     * Return the current (last loaded) Fuseki auto-modules.
+     * For testing only.
      */
-    static FusekiModules get() {
-        if ( currentLoadedModules == null )
-            load();
-        return currentLoadedModules;
+    static void reset() {
+        currentLoadedModules = null;
     }
 
     // -- ServiceLoader machinery.
 
-//    // testing
-//    /*package*/ static void reset() {
-//        load();
-//    }
-
     /**
      * Discover FusekiModules via {@link java.util.ServiceLoader}.
      * This step does not create the module objects.
      */
     private static ServiceLoader<FusekiAutoModule> createServiceLoader() {
         Class<FusekiAutoModule> moduleClass = FusekiAutoModule.class;
         ServiceLoader<FusekiAutoModule> newServiceLoader = null;
-        synchronized (lock) {
+        synchronized (lock) { // Not necessary if createServiceLoader() only called from load(). But harmless.
             try {
                 newServiceLoader = ServiceLoader.load(moduleClass, FusekiAutoModules.class.getClassLoader());
             } catch (ServiceConfigurationError ex) {

jena-fuseki2/jena-fuseki-main/src/main/java/org/apache/jena/fuseki/main/sys/FusekiModules.java

@@ -26,26 +26,29 @@
  * A collection of {@linkplain FusekiModule Fuseki modules}.
  * <p>
  * There is one specific collection of modules - a system wide set of modules.
- * This collection defaults to the automatically discovered modules {@link FusekiAutoModules#load()}.
+ * This collection defaults to the automatically discovered modules {@link FusekiAutoModules#get()}.
  *
  * @see FusekiAutoModules
  */
 public class FusekiModules {
 
-    // Never null, maybe empty
-    private static FusekiModules systemFusekiModules = FusekiModules.create();
+    // Null means "not initialized".
+    // Null should never leave this class!
+    private static FusekiModules systemFusekiModules = null;
 
     /**
      * There is a system wide set of modules used when no other modules are indicated.
      * These default to the automatically discovered modules.
+     *
+     * Use {@link #resetSystemDefault} to cause a reload of Fuseki auto modules.
      */
     public static void setSystemDefault(FusekiModules fusekiModules) {
         systemFusekiModules = ( fusekiModules == null ) ? FusekiModules.create() : fusekiModules;
     }
 
     /** Restore the original setting of the system default collection. */
     public static void restoreSystemDefault() {
-        systemFusekiModules = FusekiModules.create();
+        systemFusekiModules = null;
     }
 
     public static FusekiModules getSystemModules() {
@@ -65,6 +68,7 @@ public static FusekiModules getSystemModules() {
     // Testing.
     /*package*/ static void resetSystemDefault() {
         // Reload, reset. Fresh objects.
+        FusekiAutoModules.reset();
         systemFusekiModules = FusekiAutoModules.get();
     }
 

jena-fuseki2/jena-fuseki-main/src/test/java/org/apache/jena/fuseki/main/sys/TestFusekiModules.java

@@ -100,7 +100,8 @@ private void lifecycle(FusekiServer.Builder builder, ModuleForTest module) {
         FusekiModules systemModules = FusekiModules.getSystemModules();
         ModuleByServiceLoader.reset();
         try  {
-            FusekiModules loadedModules = FusekiAutoModules.load();
+            FusekiModules.resetSystemDefault();
+            FusekiModules loadedModules = FusekiAutoModules.get();
             FusekiModules.setSystemDefault(loadedModules);
 
             // Reloaded by FusekiModules.resetSystemDefault