How to debug "java.lang.NullPointerException" in a db40 Java app?

Whe开发者_如何学编程n I run an application that uses db40 I get the following error.

Any ideas? Everything looks fine and Hashtable <String, db4o.Plant> l = db4o.db.getTheDatabase().getPlants(); shows OK in netbeans (all the methods were correctly written).

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.

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My first guess would be that db4o.db.getTheDatabase() returns null.