What is the use of underlining the class name after it in java

github downloaded nginx-admin,java8 compiled, imported eclipse Times error, looked at the following code, there are several similar statements 

import com.jslsolucoes.nginx.admin.model.Nginx;
import com.jslsolucoes.nginx.admin.model.Nginx_;
In the

model directory, there are only files of Nginx.java, and no Nginx_.java, and the attributes in Nginx are private, but when calling the Nginx_ class, you call Nginx_.id, directly, so an error is reported. Would you like to ask, is this underlined java8 a new usage?

Java8 java
Apr.07,2021

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MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e425f6-44989.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e425f6-44989.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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