fix Solve the compatibility problem between fastjson1 and fastjson2 #3654

core/src/main/java/com/alibaba/fastjson2/JSONFactory.java

@@ -285,13 +285,23 @@ public static void setUseGsonAnnotation(boolean useGsonAnnotation) {
         JSONFactory.useGsonAnnotation = useGsonAnnotation;
     }
 
+    private static volatile boolean jsonFieldDefaultValueCompatMode;
+
+    public static boolean isJSONFieldDefaultValueCompatMode() {
+        return jsonFieldDefaultValueCompatMode;
+    }
+
+    public static void setJSONFieldDefaultValueCompatMode(boolean compatMode) {
+        jsonFieldDefaultValueCompatMode = compatMode;
+    }
+
     public static int getDefaultMaxLevel() {
         return defaultMaxLevel;
     }
 
     public static void setDefaultMaxLevel(int maxLevel) {
         if (maxLevel <= 0) {
-            throw new IllegalArgumentException("maxLevel must be positive");
+            throw new IllegalArgumentException("maxLevel must be positive, maxLevel " + maxLevel);
         }
         JSONFactory.defaultMaxLevel = maxLevel;
     }

core/src/main/java/com/alibaba/fastjson2/reader/ObjectReaderCreator.java

@@ -2317,13 +2317,19 @@ public <T> FieldReader createFieldReaderMethod(
         }
 
         if (defaultValue != null && defaultValue.getClass() != fieldClass) {
-            Function typeConvert = JSONFactory
-                    .getDefaultObjectReaderProvider()
-                    .getTypeConvert(defaultValue.getClass(), fieldType);
-            if (typeConvert != null) {
-                defaultValue = typeConvert.apply(defaultValue);
+            if (JSONFactory.isJSONFieldDefaultValueCompatMode()
+                    && defaultValue instanceof String
+                    && Date.class.isAssignableFrom(fieldClass)) {
+                        // For compatibility with JSONField defaultValue
             } else {
-                throw new JSONException("illegal defaultValue : " + defaultValue + ", class " + fieldClass.getName());
+                Function typeConvert = JSONFactory
+                        .getDefaultObjectReaderProvider()
+                        .getTypeConvert(defaultValue.getClass(), fieldType);
+                if (typeConvert != null) {
+                    defaultValue = typeConvert.apply(defaultValue);
+                } else {
+                    throw new JSONException("illegal defaultValue : " + defaultValue + ", class " + fieldClass.getName());
+                }
             }
         }