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