Made the schemaRegionMap nonnull for SchemaEngine (apache#15775)

Author: Caideyipi

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/SchemaEngine.java

@@ -78,7 +78,7 @@ public class SchemaEngine {
   private final SchemaRegionLoader schemaRegionLoader;
 
   @SuppressWarnings("java:S3077")
-  private volatile Map<SchemaRegionId, ISchemaRegion> schemaRegionMap;
+  private final Map<SchemaRegionId, ISchemaRegion> schemaRegionMap = new ConcurrentHashMap<>();
 
   private ScheduledExecutorService timedForceMLogThread;
 
@@ -117,8 +117,6 @@ public void init() {
     // CacheMemoryManager
     schemaMetricManager = new SchemaMetricManager(schemaEngineStatistics);
 
-    schemaRegionMap = new ConcurrentHashMap<>();
-
     initSchemaRegion();
 
     if (!(config.getSchemaRegionConsensusProtocolClass().equals(ConsensusFactory.RATIS_CONSENSUS))
@@ -213,11 +211,8 @@ private void initSchemaEngineStatistics() {
   }
 
   public void forceMlog() {
-    Map<SchemaRegionId, ISchemaRegion> schemaRegionMap = this.schemaRegionMap;
-    if (schemaRegionMap != null) {
-      for (ISchemaRegion schemaRegion : schemaRegionMap.values()) {
-        schemaRegion.forceMlog();
-      }
+    for (ISchemaRegion schemaRegion : schemaRegionMap.values()) {
+      schemaRegion.forceMlog();
     }
   }
 
@@ -232,15 +227,13 @@ public synchronized void clear() {
       timedForceMLogThread = null;
     }
 
-    if (schemaRegionMap != null) {
-      // SchemaEngineStatistics will be clear after clear all schema region
-      for (ISchemaRegion schemaRegion : schemaRegionMap.values()) {
-        schemaRegion.clear();
-      }
-      schemaRegionMap.clear();
-      schemaRegionMap = null;
-      logger.info("clear schema region map.");
+    // SchemaEngineStatistics will be clear after clear all schema region
+    for (ISchemaRegion schemaRegion : schemaRegionMap.values()) {
+      schemaRegion.clear();
     }
+    schemaRegionMap.clear();
+    logger.info("clear schema region map.");
+
     // SchemaMetric should be cleared lastly
     if (schemaMetricManager != null) {
       schemaMetricManager.clear();
@@ -348,7 +341,7 @@ public synchronized void deleteSchemaRegion(SchemaRegionId schemaRegionId)
   }
 
   public int getSchemaRegionNumber() {
-    return schemaRegionMap == null ? 0 : schemaRegionMap.size();
+    return schemaRegionMap.size();
   }
 
   public Map<Integer, Long> countDeviceNumBySchemaRegion(final List<Integer> schemaIds) {