How to investigate excessive java garbage collection

As to the actual problem: XML parsing can be very memory hogging when using a DOM based parser. Consider using a SAX or binary XML based parser (VTD-XML is a Java API based on that).

Actually, if the XML->JSON mapping is pure 1:1, then you can also consider to just read the XML and write the JSON realtime line by line using a little stack.

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Back to the question: I suggest to use VisualVM for this. You can find here a blog article how to get it to work with Tomcat.