@Entity
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    private Long id=-1L;
    private String title;
    @OneToMany(cascade ={CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH}, fetch = FetchType.EAGER)
    private List<Creator> creator;
    private Lang lang;
    private Date date;
    @CollectionOfElements
    private List<String> keywords;
    @CollectionOfElements
    private List<String> categories;
    private String description;
    private String pdfLink;
    @OneToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH}, fetch = FetchType.EAGER)
    private User user;

    public Book() {
    }

    public Book(List<Creator> creator, String title, Lang lang, Date date, String description, List<String> keywords, List<String> categories, String pdfLink, User user) {
        this.creator = creator;
        this.title = title;
        this.lang = lang;
        this.date = date;
        this.description = description;
        this.keywords = keywords;
        this.categories = categories;
        this.pdfLink = pdfLink;
        this.user = user;
    }

    public Book(List<Creator> creator, String title, String pdfLink, User user) {
        this.creator = creator;
        this.title = title;
        this.pdfLink = pdfLink;
        this.user = user;
    }

    public Long getId() {
        return id;
    }

    ..getters + setters for every attribute

    @Column(name = "lang")
    @Enumerated(EnumType.ORDINAL)
    public Lang getLang() {
        return lang;
    }

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

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

@Entity
public class Creator {
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    private Long id;
    private String creator;

    public Creator() {
    }

    public Creator(String creator) {
        this.creator = creator;
    }

    public Long getId() {
        return id;
    }
   
    ..getters + setters

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null) return false;
        if (!(o instanceof Creator)) return false;
        final Creator other = (Creator) o;
        if (getCreator() == null) {
            if (other.getCreator() != null)
                return false;
        } else if (!getCreator().equals(other.getCreator()))
            return false;
        return true;
    }

    @Override
    public int hashCode() {
        return creator.hashCode();
    }
}

Hibernate: insert into Book_Creator (Book_id, creator_id) values (?, ?)
1286 [main] WARN org.hibernate.util.JDBCExceptionReporter - SQL Error: 0, SQLState: 23505
1287 [main] ERROR org.hibernate.util.JDBCExceptionReporter - Batch entry 0 insert into Book_Creator (Book_id, creator_id) values ('327680', '1') was aborted.  Call getNextException to see the cause.
1288 [main] WARN org.hibernate.util.JDBCExceptionReporter - SQL Error: 0, SQLState: 23505
1288 [main] ERROR org.hibernate.util.JDBCExceptionReporter - ERROR: duplicate key value violates unique constraint "book_creator_creator_id_key"
1289 [main] ERROR org.hibernate.event.def.AbstractFlushingEventListener - Could not synchronize database state with session
org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
   at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:94)

public Book editBook(Book book) {       
        getHibernateTemplate().saveOrUpdate(book);
        getHibernateTemplate().flush();
        return book;
    }