Passar parametros com jsf

<h:outputText value="#{produtoBean.mensagemsucesso}" /
//formulario
//...
<h:commandButton value="Finalizar cadastro"  action="#{produtoBean.finalizarCadastro}" />

public String finalizarCadastro(){
       //Realiza o cadastro
       //...
       FacesContext.getCurrentInstance().getExternalContext().getRequestParameterMap().put("mensagem", "Cadastro realizado com sucesso"); // Erro nesta linha
       return "atualizarPagina";
}

private String mensagemsucesso = "";

public String getMensagemsucesso() {
		return FacesContext.getCurrentInstance().getExternalContext().getRequestParameterMap().get("mensagem");
	}

//...
root cause

javax.faces.el.EvaluationException: java.lang.UnsupportedOperationException
	javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:91)
	org.apache.myfaces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:54)
	javax.faces.component.UICommand.broadcast(UICommand.java:383)
	org.ajax4jsf.component.AjaxViewRoot.processEvents(AjaxViewRoot.java:321)
	org.ajax4jsf.component.AjaxViewRoot.broadcastEvents(AjaxViewRoot.java:296)
	org.ajax4jsf.component.AjaxViewRoot.processPhase(AjaxViewRoot.java:253)
	org.ajax4jsf.component.AjaxViewRoot.processApplication(AjaxViewRoot.java:466)
	com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:97)
	com.sun.faces.lifecycle.LifecycleImpl.phase(LifecycleImpl.java:251)
	com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:117)
	javax.faces.webapp.FacesServlet.service(FacesServlet.java:244)
	org.ajax4jsf.webapp.BaseXMLFilter.doXmlFilter(BaseXMLFilter.java:178)
	org.ajax4jsf.webapp.BaseFilter.handleRequest(BaseFilter.java:290)
	org.ajax4jsf.webapp.BaseFilter.processUploadsAndHandleRequest(BaseFilter.java:368)
	org.ajax4jsf.webapp.BaseFilter.doFilter(BaseFilter.java:495)
	org.apache.myfaces.webapp.filter.ExtensionsFilter.doFilter(ExtensionsFilter.java:147)

root cause

java.lang.UnsupportedOperationException
	java.util.Collections$UnmodifiableMap.put(Collections.java:1285)
	beans.ProdutoBean.adicionarProdutoPendente(ProdutoBean.java:170)
	sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	java.lang.reflect.Method.invoke(Method.java:597)
	org.apache.el.parser.AstValue.invoke(AstValue.java:172)
	org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:276)
	org.apache.jasper.el.JspMethodExpression.invoke(JspMethodExpression.java:68)
	javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:77)
	org.apache.myfaces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:54)

//...