JSP如何利用session在关闭浏览器时，自动清除缓存

<script type="text/javascript">
function window.onUnload(){
	var newWindow;
	if((window.screenLeft>=10000 && window.screenTop>=10000)||event.altKey){
		 newWindow=window.open('destorys.jsp','网页名称','width=0,height=0,top=4000,left=4000');
		 //新窗口将在视区之外打开
		  newWindow.opener=null; 
		  sleep(5000); 
		 newWindow.close();
		 //新窗口关闭
		  }}
function sleep(milisecond){
	var currentDate,beginDate=new Date();
	 var beginHour,beginMinute,beginSecond,beginMs; 
	 var hourGaps,minuteGaps,secondGaps,msGaps,gaps; 
	 beginHour=beginDate.getHours(); 
	 beginMinute=beginDate.getMinutes();
	  beginSecond=beginDate.getSeconds(); 
	  beginMs=beginDate.getMilliseconds(); 
	  do { 
		    currentDate=new Date();  
	   hourGaps=currentDate.getHours() - beginHour;  
	    minuteGaps=currentDate.getMinutes() - beginMinute;  
	     secondGaps=currentDate.getSeconds() - beginSecond;  
	      msGaps=currentDate.getMilliseconds() - beginMs;  
	       if(hourGaps<0) hourGaps+=24; 
	       //考虑进时进分进秒的特殊情况   
	       gaps=hourGaps*3600+ minuteGaps*60+ secondGaps;  
	        gaps=gaps*1000+msGaps; }
      while(gaps<milisecond); 
      }
</script>