java.lang.ClassCastException: java.lang.String can not be cast to entities.User

I'm using the ViewPreparer class to prepare my modules, but I'm getting this error:

java.lang.ClassCastException: java.lang.String cannot be cast to entities.Usuario

The complete trace of the error is as follows:

java.lang.ClassCastException: java.lang.String cannot be cast to entities.Usuario
at pe.edu.udaff.util.ModuloPreparer.execute(ModuloPreparer.java:22)
at org.apache.tiles.impl.BasicTilesContainer.prepare(BasicTilesContainer.java:377)
at org.apache.tiles.impl.BasicTilesContainer.render(BasicTilesContainer.java:394)
at org.apache.tiles.impl.BasicTilesContainer.render(BasicTilesContainer.java:238)
at org.apache.tiles.impl.BasicTilesContainer.render(BasicTilesContainer.java:221)
at org.apache.tiles.renderer.DefinitionRenderer.render(DefinitionRenderer.java:59)
at org.springframework.web.servlet.view.tiles3.TilesView.renderMergedOutputModel(TilesView.java:114)
at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:267)
at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1221)
at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1005)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:952)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:870)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961)
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)

This is the ModuloPreparer class:

public class ModuloPreparer implements ViewPreparer {

@Autowired
private ModuloService moduloService;

@Override
public void execute(Request tilesContext, AttributeContext attributeContext) throws PreparerException {

    Usuario usuario = (Usuario) tilesContext.getContext("session").get("usuarioIniciado");

    if (usuario != null) {
        // Modulos de usuario en sesion
        List<Modulo> modulos = moduloService.listarPorPerfil(usuario.getUsuario());
        tilesContext.getContext("request").put("modulosPorPerfil", modulos);
    }
} }

And this is my class User:

@Entity
@Table(name = "usuario", catalog = "posbd")
public class Usuario implements java.io.Serializable {

private Integer idusuario;
private String nombre;
private String usuario;
private String password;
private String foto;
private Byte estado;
private Date ultimoLogin;
private Set<UsuarioPerfil> usuarioPerfils = new HashSet<UsuarioPerfil>(0);

public Usuario() {
}

public Usuario(String nombre, String usuario, String password, String foto, Byte estado, Date ultimoLogin,
        Set<UsuarioPerfil> usuarioPerfils) {
    this.nombre = nombre;
    this.usuario = usuario;
    this.password = password;
    this.foto = foto;
    this.estado = estado;
    this.ultimoLogin = ultimoLogin;
    this.usuarioPerfils = usuarioPerfils;
}

@Id
@GeneratedValue(strategy = IDENTITY)

@Column(name = "idusuario", unique = true, nullable = false)
public Integer getIdusuario() {
    return this.idusuario;
}

public void setIdusuario(Integer idusuario) {
    this.idusuario = idusuario;
} ...