Post.java

@Entity
@Table(name = "post")
public class Post {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long id;

    @Column(name = "title")
    private String title;

    @ManyToMany
    @JoinTable(name = "post_tag",
            joinColumns = @JoinColumn(name = "post_id"),
            inverseJoinColumns = @JoinColumn(name = "tag_id")
    )
    private Set<Tag> tags = new HashSet<>();

    public Long getId() {
        return id;
    }

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

    public String getTitle() {
        return title;
    }

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

    public Set<Tag> getTags() {
        return tags;
    }

    public void setTags(Set<Tag> tags) {
        this.tags = tags;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Post tag = (Post) o;
        return Objects.equals(id, tag.id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }

}

Tag.java

@Entity
@Table(name = "tag")
public class Tag {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long id;

    @Column(name = "name")
    private String name;

    @ManyToMany(mappedBy = "tags")
    private Set<Post> posts = new HashSet<>();

    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Set<Post> getPosts() {
        return posts;
    }

    public void setPosts(Set<Post> posts) {
        this.posts = posts;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Tag tag = (Tag) o;
        return Objects.equals(id, tag.id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}