+Company
    -int companyId
    -String companyName
    -Set<Department> listOfDepartments = new HashSet<Department>();

    +Department
    -int departmentId
    -String departmentName
    -Set<Worker> listOfWorkers = new HashSet<Worker>();

    +Worker
    -int workerId
    -String workerName

@XmlRootElement(name="Company")
@XmlAccessorType(XmlAccessType.FIELD)
@Entity
public class Company {
    @XmlAttribute(name = "id")
    @Id @GeneratedValue(strategy = GenerationType.AUTO)
    private int companyId;
    @XmlElement(name = "companyName")
    private String companyName;

    @XmlElement(name="Department")
    @OneToMany(mappedBy = "company", cascade=CascadeType.PERSIST, fetch = FetchType.EAGER)
    private Set<Department> listOfDepartments = new HashSet<Department>();


@XmlRootElement(name="Department")
@XmlAccessorType(XmlAccessType.FIELD)
@Entity
public class Department {
    @XmlAttribute(name = "id")
    @Id @GeneratedValue(strategy = GenerationType.AUTO)
    private int idDepartment;

    @XmlElement(name = "departmentName")
    private String departmentName;

    @ManyToOne()
    @JoinColumn(name="companyId")
    private Company company;

    @XmlElement(name="Worker")
    @OneToMany(mappedBy = "department", cascade = CascadeType.PERSIST, fetch = FetchType.EAGER)
    private Set<Worker> listOfWorkers = new HashSet<Worker>();


@XmlRootElement(name="Worker")
@Entity
public class Worker {
    @Id @GeneratedValue(strategy = GenerationType.AUTO)
    private int idWorker;

    private String workerName;

    @ManyToOne
    @JoinColumn(name="departmentId")
    private Department department;

A cycle is detected in the object graph. This will cause infinitely deep XML: ru.eldarkaa.dto.Company@d1e43ed -> 
ru.eldarkaa.dto.Department@6e55f58 -> ru.eldarkaa.dto.Company@d1e43ed]