Re: Odd observation during debugging

Just set:
directive.if.tostring.nullcheck = false

See VELOCITY-731 and VELOCITY-692 and others...

On Wed, Jul 11, 2012 at 9:13 AM, Christopher Schultz
<[email protected]> wrote:
> All,
>
> I recently started getting intermittent OOMEs in my webapp that seem to
> be completely recoverable. The stack traces look like this:
>
> java.lang.OutOfMemoryError: Java heap space
>         at java.util.Arrays.copyOf(Arrays.java:2882)
>         at
> java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:100)
>         at
> java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:390
> )
>         at java.lang.StringBuilder.append(StringBuilder.java:119)
>         at java.lang.StringBuilder.append(StringBuilder.java:115)
>         at
> java.util.AbstractCollection.toString(AbstractCollection.java:422)
>         at
> org.apache.velocity.runtime.parser.node.ASTReference.evaluate(ASTReference.java:547)
>     [...]
>
> This happens so rarely, it's hard to catch, and it's fairly obvious what
> is happening: toString is being called on a really huge List. We try not
> to use really huge lists, but I suppose there is some potential danger
> in rare instances. I want to find those instances.
>
> Velocity 1.7, Tools 2.0 (plus some committed bugfixes that haven't made
> it into an official release, yet).
>
> So I thought I'd add some trickery to my VelocityContexts: We use our
> own subclasss of VelocityLayoutServlet for a number of things, and I
> have overridden the createContext() to wrap the Context the
> super.createContext() returns in a ChainedContext (btw, that's
> deprecated but doesn't document a replacement -- what should I use?). My
> wrapper wraps all List and Set objects in yet another set of wrappers
> which essentially pass-through everything except that their toString
> methods throw an exception: there should be no reason that a List or Set
> should have toString() called on it, because they should only be
> iterated over, for instance.
>
> But right on my login page, I see this exception after such instrumentation:
>
> SEVERE: Servlet.service() for servlet velocity threw exception
> org.apache.velocity.exception.VelocityException: Reference evaluation
> threw an exception at /layout/header.vm[line 18, column 5]
>         at
> org.apache.velocity.runtime.parser.node.ASTReference.evaluate(ASTRefe
> rence.java:551)
>         at
> org.apache.velocity.runtime.parser.node.ASTExpression.evaluate(ASTExp
> ression.java:62)
>         at
> org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfSt
> atement.java:85)
>         at
> org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.
> java:342)
>         at
> org.apache.velocity.runtime.directive.Parse.render(Parse.java:260)
>         at
> org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirect
> ive.java:207)
>         at
> org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.
> java:342)
>         at org.apache.velocity.Template.merge(Template.java:356)
>         [....]
> Caused by: java.lang.IllegalStateException: Request /myapp/login.vm,
> called toString on key 'extraStylesheets'
>         at
> com.chadis.web.servlet.VelocityLayoutServlet$WrappedList.toString(VelocityLayoutServlet.java:220)
>         at
> org.apache.velocity.runtime.parser.node.ASTReference.evaluate(ASTReference.java:547)
>
> If I look at everything that references the key 'extraStylesheets' in my
> login page (and the header, etc.), I get only these:
>
> login.vm only includes these references:
> #set($extraStylesheets = ['/includes/css/login.css'])
>
> I'm using VelocityLayoutServlet which is using my layouts/Default.vm (no
> references to 'extraStylesheets') which includes (via #parse)
> 'header.vm' which has this to say:
>
> #if($extraStylesheets)
>         #foreach($stylesheet in $extraStylesheets)
>     <link type="text/css" href="$link.setRelative($stylesheet)"
> rel="stylesheet" />
>         #end
> #end
>
> ...and that's it. Given the upper stack trace it seems that the #if() is
> evaluating the argument by converting it into a String for evaluation.
> Is that intentional? I always used "#if($foo)" to check to see if $foo
> had a value of any kind -- that is, if it was non-null (and non-false)
> but it appears that the value is being converted into a String for ...
> what purpose?
>
> I know a lot of changes have occurred over the years and that
> String-conversion is something that has seen some of that action. Is
> there a "better" way of determining if a List (or whatever) has a value?
> That is, without converting it to a String, first?
>
> One can argue that I shouldn't have Lists of objects so large that
> converting them to a String will cause an OOME, but the calling of
> toString on these Lists is certainly surprising.
>
> Thanks,
> -chris
>