Erro ao gerar Relatório em .xls

JRXlsExporter exporter = new JRXlsExporter();

type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: Servlet execution threw an exception

root cause

java.lang.NoClassDefFoundError: org/apache/poi/hssf/usermodel/HSSFCellStyle
	com.struts.action.RelComissaoAction.geraRelatorioExcel(RelComissaoAction.java:146)
	com.struts.action.RelComissaoAction.execute(RelComissaoAction.java:125)
	org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:484)
	org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
	org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
	org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

note The full stack trace of the root cause is available in the Apache Tomcat/5.5.20 logs.

OutputStream output = new FileOutputStream(new File(caminho)); 
	     ByteArrayOutputStream xlsReport = new ByteArrayOutputStream();
	     byte bytes[] = new byte[10];
	     
	     // exportação para excel
	     JRXlsExporter exporterXLS = new JRXlsExporter();
	     exporterXLS.setParameter(JRXlsExporterParameter.JASPER_PRINT, print);
	     exporterXLS.setParameter(JRXlsExporterParameter.OUTPUT_STREAM, xlsReport);
	     exporterXLS.setParameter(JRXlsExporterParameter.OUTPUT_FILE, caminho);
	     exporterXLS.setParameter(JRXlsExporterParameter.IS_ONE_PAGE_PER_SHEET, Boolean.FALSE);
	     exporterXLS.setParameter(JRXlsExporterParameter.IS_WHITE_PAGE_BACKGROUND, Boolean.TRUE);
	     exporterXLS.setParameter(JRXlsExporterParameter.IS_REMOVE_EMPTY_SPACE_BETWEEN_ROWS, Boolean.TRUE);
	     exporterXLS.exportReport();
	     
	     // converte para bytes
	     bytes = xlsReport.toByteArray();
	     xlsReport.close();
	     
	     // grava os bytes na saída
	     output.write( bytes, 0, bytes.length );
	     output.flush();
	     output.close();