Result Json recursive - Vraptor3 - CircularReferenceException

13:26:27,982 DEBUG [DefaultExceptionMapper] find for exception class com.thoughtworks.xstream.core.TreeMarshaller$CircularReferenceException
10/07/2012 13:26:27 org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet default threw exception
br.com.caelum.vraptor.InterceptionException: exception raised, check root cause for details: com.thoughtworks.xstream.core.TreeMarshaller$CircularReferenceException: Recursive reference to parent object
---- Debugging information ----
item-type           : br.com.nextrans.erp.modelo.bean.Equipamento
converter-type      : com.thoughtworks.xstream.converters.reflection.ReflectionConverter
[...]
Caused by: com.thoughtworks.xstream.core.TreeMarshaller$CircularReferenceException: Recursive reference to parent object
---- Debugging information ----
item-type           : br.com.nextrans.erp.modelo.bean.Equipamento
converter-type      : com.thoughtworks.xstream.converters.reflection.ReflectionConverter
-------------------------------

@Entity
public class OrdemServico {

	@Id @GeneratedValue
	private Long idOrdemServico;
	
	@Type(type = "org.joda.time.contrib.hibernate.PersistentLocalDateTime")
	private LocalDateTime inicio = new LocalDateTime();
	
	@Type(type = "org.joda.time.contrib.hibernate.PersistentLocalDateTime")
	private LocalDateTime previsaoConclusao = new LocalDateTime();
	
	@Type(type = "org.joda.time.contrib.hibernate.PersistentLocalDateTime")
	private LocalDateTime conclusao;
	
	@Enumerated(EnumType.STRING)
	private StatusOrdemServico status = StatusOrdemServico.ABERTO;
	
	private Double km;

	@Column(length = 400)
	private String obs;
	
	@OneToOne
	@JoinColumn
	private Equipamento equipamento;
	
	@OneToOne
	@JoinColumn
	private Filial filial;
	
	@Enumerated(EnumType.STRING)
	private TipoManutencao tipoManutencao;
	
	@Enumerated(EnumType.STRING)
	private TipoExecucaoServico tipoExecucao;
	
	@OneToOne
	@JoinColumn
	private Funcionario mecanico;
	
	@OneToOne
	@JoinColumn
	private Fornecedor fornecedor;
	
	@ManyToMany
	@JoinColumn
	private List<GrupoTarefa> grupos = new ArrayList<GrupoTarefa>();
	
	@OneToOne(cascade=CascadeType.ALL)
	@JoinColumn
	private Emissao emissao;
}