Underscore in model class on Spring Boot

I doing the maintenance of this code please can someone explain what is that notation Post_ in the code below? I know Post is a model that I created, but Post_ with underscore is not in files. I searched but didnt find any information about this. Its a pre generated Spring Boot file? And if so how to create it?

import java.util.ArrayList;
import java.util.Collection;
import java.util.Optional;

import javax.persistence.criteria.Expression;
import javax.persistence.criteria.Predicate;

import org.springframework.data.jpa.domain.Specification;

import com.example.demo.dto.filter.PostFilter;
import com.example.demo.model.Post;
import com.example.demo.model.Post_;

public class PostSpecs extends BaseSpecs {

    public static Specification<Post> specByFilter(Optional<PostFilter> filter) {
        return filter.isEmpty() ? null : (root, query, builder) -> {
            Collection<Predicate> predicates = new ArrayList<>();

            predicates.add(equal(builder, root.get(Post_.ID), filter.map(PostFilter::getId)));
            predicates.add(contains(builder, root.get(Post_.TITLE), filter.map(PostFilter::getTitle)));
            predicates.add(contains(builder, root.get(Post_.BODY), filter.map(PostFilter::getBody)));

            Expression<String> allCols = concatAll(builder, root.get(Post_.ID), root.get(Post_.TITLE),
                    root.get(Post_.BODY));
            predicates.add(contains(builder, allCols, filter.map(PostFilter::getAny)));
            return toAndArray(builder, predicates);
        };
    }
}

Post Class

package com.example.demo.model;

import java.time.LocalDateTime;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.EntityListeners;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.validation.constraints.NotBlank;
import org.springframework.data.annotation.CreatedBy;
import org.springframework.data.annotation.CreatedDate;
import org.springframework.data.annotation.LastModifiedDate;
import org.springframework.data.jpa.domain.support.AuditingEntityListener;

@Entity
@EntityListeners(AuditingEntityListener.class)
public class Post {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @CreatedDate
    private LocalDateTime createdAt;
    @LastModifiedDate
    private LocalDateTime updatedAt;
    @CreatedBy
    private Long createdBy;
    @NotBlank
    @Column(length = 100)
    private String title;
    @NotBlank
    @Column(length = 500)
    private String body;
    @ManyToOne
    private User user;

    public Post() {
    }

    public Post(Long id, String title, String body) {
        this.id = id;
        this.title = title;
        this.body = body;
    }

    public Post(String title, String body) {
        this.title = title;
        this.body = body;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public LocalDateTime getCreatedAt() {
        return createdAt;
    }

    public void setCreatedAt(LocalDateTime createdAt) {
        this.createdAt = createdAt;
    }

    public LocalDateTime getUpdatedAt() {
        return updatedAt;
    }

    public void setUpdatedAt(LocalDateTime updatedAt) {
        this.updatedAt = updatedAt;
    }

    public Long getCreatedBy() {
        return createdBy;
    }

    public void setCreatedBy(Long createdBy) {
        this.createdBy = createdBy;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getBody() {
        return body;
    }

    public void setBody(String body) {
        this.body = body;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }
}

Pom.XML

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.3.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>11</java.version>
        <springdoc.version>1.4.3</springdoc.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-validation</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <scope>runtime</scope>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        
        <dependency>
            <groupId>org.springdoc</groupId>
            <artifactId>springdoc-openapi-ui</artifactId>
            <version>${springdoc.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springdoc</groupId>
            <artifactId>springdoc-openapi-data-rest</artifactId>
            <version>${springdoc.version}</version>
        </dependency>
        
        <!-- Lorem Ipsum to generate test data -->
        <dependency>
            <groupId>com.thedeanda</groupId>
            <artifactId>lorem</artifactId>
            <version>2.1</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            
            <plugin>
                <groupId>org.bsc.maven</groupId>
                <artifactId>maven-processor-plugin</artifactId>
                <version>3.3.3</version>
                <executions>
                    <execution>
                        <id>process</id>
                        <goals>
                            <goal>process</goal>
                        </goals>
                        <phase>generate-sources</phase>
                        <configuration>
                            <processors>
                                <processor>org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor</processor>
                            </processors>
                        </configuration>
                    </execution>
                </executions>
                <dependencies>
                    <dependency>
                        <groupId>org.hibernate</groupId>
                        <artifactId>hibernate-jpamodelgen</artifactId>
                        <version>${hibernate.version}</version>
                    </dependency>
                </dependencies>
            </plugin>
        </plugins>
    </build>

</project>

Answer

That is the outcome of using Hibernate JPA 2 Metamodel Generator (see hibernate-jpamodelgen in your pom.xml).

You can find more information about it at https://docs.jboss.org/hibernate/stable/jpamodelgen/reference/en-US/html_single/.

Hibernate Static Metamodel Generator is an annotation processor based on the [Pluggable Annotation Processing API] with the task of creating JPA 2 static metamodel classes.

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Answer