The resource is not on the build path of a java project

build a default normal project with maven. Open the project in eclipes and see a hollow J icon in front of the java file. Click on the code file and there is no error prompt for the code. After pressing Ctrl + Shift + O , the the resource is not on the build path of a java project warning pops up. Is there any other configuration that needs to be changed?

Nov.29,2021
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-64f5-7c14cc-163fc.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-7c14cc-163fc.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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