[update] 单例添加私有构造方法

wj-common/src/main/java/net/wanji/common/utils/tool/JacksonUtils.java

@@ -9,6 +9,7 @@ import com.fasterxml.jackson.databind.ObjectMapper;
  */
 public class JacksonUtils {
     private static final ObjectMapper mapper = new ObjectMapper();
+    private JacksonUtils() {}
     public static ObjectMapper getInstance() {
         return mapper;
     }