Book.java

@Entity
public class Book implements Serializable {

	…

	@ManyToOne
	@JoinColumn(name=”publisherid”)
	private Publisher publisher;

	…

	public Optional getPublisher() {
		return Optional.ofNullable(this.publisher);
	}

	public void setPublisher(final Publisher publisher) {
		this.publisher = publisher;
	}
}