Why Java threads use lightweight processes to implement threads

java used user-mode threads before 1.2. why did it change to lightweight processes to implement threads? Aren"t user-mode threads more efficient? Although the management and scheduling is a little complicated!

Mar.23,2021
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-64f5-1b31f4c-40363.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b31f4c-40363.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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