Why does tomcat modify web.xml take effect without rebooting?

I always thought that in tomcat, modifying the web.xml file would not take effect until tomcat was restarted
but I tried it today. After modifying the web.xml file, the browser refreshed and the change took effect. Refresh it again and it took effect.
what"s the point? It takes effect without restarting tomcat
apache-tomcat-9.0.12

Sep.06,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1eb4f03-482dc.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1eb4f03-482dc.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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