如何在db40 Java應用程序中調試「java.lang.NullPointerException」？

Question

當我運行使用db40的應用程序時，出現以下錯誤。

任何想法？一切看起來都不錯，Hashtable <String, db4o.Plant> l = db4o.db.getTheDatabase().getPlants();在netbeans中顯示OK（所有方法都被正確寫入）。

type Exception report 

message 

description The server encountered an internal error() that prevented it from fulfilling this request. 
exception 

org.apache.jasper.JasperException: An exception occurred processing JSP page /plante.jsp at line 37 

    34: 
    35: 
    36: <% 
    37: Hashtable <String, db4o.Plant> l = db4o.db.getTheDatabase().getPlants(); 
    38: int i = 0; 
    39: 
    40: Iterator<String> iterator = l.keySet().iterator(); 


    Stacktrace: 
     org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:505) 
     org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:416) 


org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342) 
     org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267) 
     javax.servlet.http.HttpServlet.service(HttpServlet.java:717) 
     org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:390) 

    root cause 

java.lang.NullPointerException 
    org.apache.jsp.plante_jsp._jspService(plante_jsp.java:89) 
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) 
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717) 
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374) 
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342) 
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267) 
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717) 
    org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:390) 

note The full stack trace of the root cause is available in the Apache Tomcat/6.0.18 logs. 

Answer 1

我的第一個猜測是db4o.db.getTheDatabase（）返回null。