org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot load JDBC driver class 'org.postgresql.Driver'

Bonjour je suis entrain de developper une application web , avec jsf et hibernate . j'ai utilisé PostgresSql commme base de donne , mon probleme c'est que quand je lance mon application depuis netbeans ça marche tres bien . mais quanf je prend le .war et je le deploie avec tomcat seul : la version 6.0.26 . ça marche mais pour certain pages il me donne une erreur comme quoi il ne trouve pas le driver postgres . je ne sais pas d'ou vien l'erreur sachant que j'ai le postgresql-8.4-701.jdbc4.jar dans mon repertoire lib , et aussi dans le repertoire lib de tomcat voici un apérçu du message d'erreur

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GRAVE: org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot load JDBC driver class 'org.postgresql.Driver'
java.lang.RuntimeException: org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot load JDBC driver class 'org.postgresql.Driver'
	at com.sun.data.provider.impl.CachedRowSetDataProvider.setCursorRow(CachedRowSetDataProvider.java:416)
	at com.sun.data.provider.impl.CachedRowSetDataProvider.setCursorIndex(CachedRowSetDataProvider.java:373)
	at com.sun.data.provider.impl.CachedRowSetDataProvider.getRowCount(CachedRowSetDataProvider.java:713)
	at com.sun.webui.jsf.faces.DataProviderELResolver$SelectItemsData.getSelectItems(DataProviderELResolver.java:702)
	at com.sun.webui.jsf.faces.DataProviderELResolver.getValue(DataProviderELResolver.java:166)
	at javax.el.CompositeELResolver.getValue(CompositeELResolver.java:54)
	at com.sun.faces.el.FacesCompositeELResolver.getValue(FacesCompositeELResolver.java:64)
	at org.apache.el.parser.AstValue.getValue(AstValue.java:123)
	at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:186)
	at org.apache.jasper.el.JspValueExpression.getValue(JspValueExpression.java:101)
	at com.sun.webui.jsf.component.Selector.getItems(Selector.java:577)
	at com.sun.webui.jsf.component.ListSelector.getOptions(ListSelector.java:212)
	at com.sun.webui.jsf.component.ListSelector.getListItems(ListSelector.java:140)
	at com.sun.webui.jsf.renderkit.widget.ListRendererBase.getListProperties(ListRendererBase.java:269)
	at com.sun.webui.jsf.renderkit.widget.ListRendererBase.getProperties(ListRendererBase.java:220)
	at com.sun.webui.jsf.renderkit.widget.DropDownRenderer.getProperties(DropDownRenderer.java:74)
	at com.sun.webui.jsf.renderkit.widget.RendererBase.encodeChildren(RendererBase.java:134)
	at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:812)
	at javax.faces.component.UIComponent.encodeAll(UIComponent.java:886)
	at javax.faces.component.UIComponent.encodeAll(UIComponent.java:892)
	at javax.faces.component.UIComponent.encodeAll(UIComponent.java:892)
	at javax.faces.component.UIComponent.encodeAll(UIComponent.java:892)
	at javax.faces.component.UIComponent.encodeAll(UIComponent.java:892)
	at javax.faces.component.UIComponent.encodeAll(UIComponent.java:892)
	at com.sun.faces.application.ViewHandlerImpl.doRenderView(ViewHandlerImpl.java:245)
	at com.sun.faces.application.ViewHandlerImpl.renderView(ViewHandlerImpl.java:176)
	at com.sun.rave.web.ui.appbase.faces.ViewHandlerImpl.renderView(ViewHandlerImpl.java:320)
	at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:106)
	at com.sun.faces.lifecycle.LifecycleImpl.phase(LifecycleImpl.java:251)
	at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:144)
	at com.sun.faces.extensions.avatar.lifecycle.PartialTraversalLifecycle.render(PartialTraversalLifecycle.java:106)
	at javax.faces.webapp.FacesServlet.service(FacesServlet.java:245)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at com.sun.webui.jsf.util.UploadFilter.doFilter(UploadFilter.java:267)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.connector.CoyoteAdapter.ser