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https://issues.apache.org/jira/browse/VELOCITY-966?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17878307#comment-17878307
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Claude Brisson commented on VELOCITY-966:
-----------------------------------------
I admit that the API is far from perfect, mostly because of backward
compatibility issues.
But the RuntimeInstance.render() method is *not* meant for a reuse of the AST
tree. I will add it to the doc.
What you should do, instead, is rely on the Resource and Template API, as
follow:
{code}
final RuntimeInstance runtimeInstance = new RuntimeInstance();
final VelocityContext context = new VelocityContext();
context.put("user", "abc");
final Template aTemplate = new Template();
aTemplate.setName("a_template");
aTemplate.setRuntimeServices(runtimeInstance);
final SimpleNode parsedTemplate = runtimeInstance.parse(new
StringReader("Hello $user !!!"), aTemplate);
aTemplate.setData(parsedTemplate);
aTemplate.initDocument();
//first time using parsed template
final StringWriter writer = new StringWriter();
aTemplate.merge(context, writer);
System.out.println(writer);
//second time using parsed template
final StringWriter writer2 = new StringWriter();
aTemplate.merge(context, writer2);
System.out.println(writer2);
{code}
> RuntimeInstance.render throws null pointer exception when trying to render
> the same AST
> ----------------------------------------------------------------------------------------
>
> Key: VELOCITY-966
> URL: https://issues.apache.org/jira/browse/VELOCITY-966
> Project: Velocity
> Issue Type: Bug
> Affects Versions: 2.3
> Reporter: Alex
> Assignee: Claude Brisson
> Priority: Major
>
> The objective is to use
> *RuntimeInstance.parse* to get the parsed AST for some template and cache the
> AST.
> Then use *RuntimeInstance.render* multiple times to render the text.
>
> It appears that in version 2.3 {*}RuntimeInstance.{*}{*}render{*} invokes the
> Node.init call and at the end of the rendering clears the parser and tokens.
> On the next attempt to render init fails with the NullPointerException
> because tokens have been cleared.{*}{*}
>
> So you can not call *RuntimeInstance.render* on the same AST several times
> which seems to be the intent of this method...
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