An exception occurred processing JSP page /business/look_buy/look_buy.jsp at line 42

Question

org.apache.jasper.JasperException: An exception occurred processing JSP page /business/look_buy/look_buy.jsp at line 42 39: tDAY1=zero_str.substring(0, 2-tDAY1.length())+tDAY1; 40: String start_time=tYEAR+"-"+tMON+"-"+tDAY+" 00:00:00"; 41: String end_time=tYEAR1+"-"+tMON1+"-"+tDAY1+" 00:00:00"; 42: String user_name=session.getValue("user_name").toString(); 45: String table_name= "buyinfomation001"; Stacktrace: org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:580) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:477) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339) javax.servlet.http.HttpServlet.service(HttpServlet.java:731) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) root cause java.lang.NullPointerException org.apache.jsp.business.look_005fbuy.look_005fbuy_jsp._jspService(look_005fbuy_jsp.java:133) org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) javax.servlet.http.HttpServlet.service(HttpServlet.java:731) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:439) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339) javax.servlet.http.HttpServlet.service(HttpServlet.java:731) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) note The full stack trace of the root cause is available in the Apache Tomcat/7.0.72 logs. 不知道怎么修改，谁知道的能不能告诉声， 42行怎么错了