JavaScriptは新しいページを開きます.

<%@ page language="java" contentType="text/html; charset=utf-8"
	pageEncoding="utf-8"%>
<%@ taglib prefix="s" uri="/struts-tags"%>
<%
	String path = request.getContextPath();
	String basePath = request.getScheme() + "://"
			+ request.getServerName() + ":" + request.getServerPort()
			+ path + "/";
	request.setAttribute("basePath", basePath);
%>
<script type="text/javascript">
	window.open("<s:property value="#request.basePath"/>boss/admin_login.jsp","_top");
</script>