fix: return immutable maps from DefaultConverterLoader

Author: YangSiJun528

fesod/src/main/java/org/apache/fesod/sheet/converters/DefaultConverterLoader.java

@@ -19,6 +19,7 @@
 
 package org.apache.fesod.sheet.converters;
 
+import java.util.Collections;
 import java.util.Map;
 import org.apache.fesod.sheet.converters.ConverterKeyBuild.ConverterKey;
 import org.apache.fesod.sheet.converters.bigdecimal.BigDecimalBooleanConverter;
@@ -175,10 +176,10 @@ private static void initDefaultWriteConverter() {
     /**
      * Load default write converter
      *
-     * @return
+     * @return Unmodifiable map of default write converters
      */
     public static Map<ConverterKey, Converter<?>> loadDefaultWriteConverter() {
-        return defaultWriteConverter;
+        return Collections.unmodifiableMap(defaultWriteConverter);
     }
 
     private static void putWriteConverter(Converter<?> converter) {
@@ -193,7 +194,7 @@ private static void putWriteStringConverter(Converter<?> converter) {
     /**
      * Load default read converter
      *
-     * @return
+     * @return Unmodifiable map of default read converters
      */
     public static Map<ConverterKey, Converter<?>> loadDefaultReadConverter() {
         return loadAllConverter();
@@ -202,10 +203,10 @@ public static Map<ConverterKey, Converter<?>> loadDefaultReadConverter() {
     /**
      * Load all converter
      *
-     * @return
+     * @return Unmodifiable map of all converters
      */
     public static Map<ConverterKey, Converter<?>> loadAllConverter() {
-        return allConverter;
+        return Collections.unmodifiableMap(allConverter);
     }
 
     private static void putAllConverter(Converter<?> converter) {

fesod/src/main/java/org/apache/fesod/sheet/read/metadata/holder/AbstractReadHolder.java

@@ -114,7 +114,7 @@ public AbstractReadHolder(ReadBasicParameter readBasicParameter, AbstractReadHol
         }
 
         if (parentAbstractReadHolder == null) {
-            setConverterMap(DefaultConverterLoader.loadDefaultReadConverter());
+            setConverterMap(new HashMap<>(DefaultConverterLoader.loadDefaultReadConverter()));
         } else {
             setConverterMap(new HashMap<>(parentAbstractReadHolder.getConverterMap()));
         }

fesod/src/main/java/org/apache/fesod/sheet/write/metadata/holder/AbstractWriteHolder.java

@@ -255,7 +255,7 @@ public AbstractWriteHolder(WriteBasicParameter writeBasicParameter, AbstractWrit
 
         // Set converterMap
         if (parentAbstractWriteHolder == null) {
-            setConverterMap(DefaultConverterLoader.loadDefaultWriteConverter());
+            setConverterMap(new HashMap<>(DefaultConverterLoader.loadDefaultWriteConverter()));
         } else {
             setConverterMap(new HashMap<>(parentAbstractWriteHolder.getConverterMap()));
         }