Fix ST_WRITE_TO_STATIC_FROM_INSTANCE_METHOD in MongoPersistenceProvider

Replace static field write from instance method with proper synchronization.
Changed from writing to static boolean field from constructor to using
synchronized block with volatile flag for thread-safe registration control.

Author: ic0ns

src/main/java/de/rub/nds/crawler/persistence/MongoPersistenceProvider.java

@@ -50,15 +50,18 @@ public class MongoPersistenceProvider implements IPersistenceProvider {
 
     private static final String BULK_SCAN_COLLECTION_NAME = "bulkScans";
 
-    private static boolean isInitialized = false;
     private static final Set<JsonSerializer<?>> serializers = new HashSet<>();
     private static final Set<Module> modules = new HashSet<>();
+    private static final Object registrationLock = new Object();
+    private static volatile boolean registrationClosed = false;
 
     public static void registerSerializer(JsonSerializer<?> serializer) {
-        if (isInitialized) {
-            throw new RuntimeException("Cannot register serializer after initialization");
+        synchronized (registrationLock) {
+            if (registrationClosed) {
+                throw new RuntimeException("Cannot register serializer after initialization");
+            }
+            serializers.add(serializer);
         }
-        serializers.add(serializer);
     }
 
     public static void registerSerializer(JsonSerializer<?>... serializers) {
@@ -68,10 +71,12 @@ public static void registerSerializer(JsonSerializer<?>... serializers) {
     }
 
     public static void registerModule(Module module) {
-        if (isInitialized) {
-            throw new RuntimeException("Cannot register module after initialization");
+        synchronized (registrationLock) {
+            if (registrationClosed) {
+                throw new RuntimeException("Cannot register module after initialization");
+            }
+            modules.add(module);
         }
-        modules.add(module);
     }
 
     public static void registerModule(Module... modules) {
@@ -148,7 +153,9 @@ private static ObjectMapper createMapper() {
      * @param mongoDbDelegate Mongodb command line configuration parameters
      */
     public MongoPersistenceProvider(MongoDbDelegate mongoDbDelegate) {
-        isInitialized = true;
+        synchronized (registrationLock) {
+            registrationClosed = true;
+        }
 
         mapper = createMapper();
         mongoClient = createMongoClient(mongoDbDelegate);