Thank you for your request.
Basically you can implement InputDataHandler for GroupDocs.Annotation for Java, you can check our simple example for how it works here
But if you mean CustomHtmlCacheHandler , then yes, we are sorry for misunderstanding, for version 1.9.0 this class not implemented yet. But our product team are work on the new GroupDocs.Annotation which will be released soon. And this functionality will be included necessarily.
We apologize for the inconvenience.
Your Document Collaboration APIs
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