Looking for the way to validate XML file before file adapter pick up

We got "java null pointer" exception in our sender file adapter, so we need to handle it:

Validate the xml file adapter before file adapter pick it up, not after file is picked up, e.g. if the xml file is not valid (missing closing tag etc. ), file will not be picked up.

I searched the forum and most people do validation after file adapter picks the files

Thanks for your advise.