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List:       japple-user
Subject:    Re: [japple-user] How to debug errors in jsp template files?
From:       Mark Deneen <mdeneen () japple ! org>
Date:       2002-02-25 19:56:06
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On Mon, 25 Feb 2002, Michael Schulz wrote:

> How am I to debug this?
> 
> 
> Saucon Server Exception
> An exception has occurred ...
> 
> Message: java.lang.NullPointerException - null
> Class:  japple.web.content.IncludedFileService
> Method:  include
> Included URL:  productListHandler.jsp
> 
> 
> java.lang.NullPointerException at
> org.apache.jsp.productListHandler$jsp._jspService(productListHandler$jsp.jav
> a:70) at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:107)
> at javax.servlet.http.HttpServlet.service(HttpServlet.java:853) at


You are referencing a null pointer in your productListHandler.jsp.  
Sometimes the stack trace of a jsp execution is not very clear.  I would 
take a look at productListHandler$jsp.java (in the 
tomcat/work directory), line 70.  This should give you a little more 
context, and you should be able to see what you referenced when it blew 
up.

Be careful of statements such as:

String str = data.get("ac.nothing").toString();

If data.get("ac.nothing") returns null, then the call to toString() will 
obviously fail.  To make this safer, use a call like this:

String str = data.get("ac.nothing", "default value").toString();

or, if you know it is a String object in client state:

String str = (String) data.get("ac.nothing");

and then

if( str != null ) {
    /* code follows */
    /* ...          */
}



Personally, I prefer the first method.  It involves somewhat more overhead 
of creating a new String object, it eliminates the null check later on, 
and makes the code a bit more compact.

-M


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