Steps to reproduce

1. import and export any file
2. monitor the keikai web application with VisualVM
3. performant GC and get a heap dump on the server, search XSSFWorkbook

Current result

JVM garbage collector doesn't clear Workbook because it's referenced by an Exporter.
Since XSSFWorkbook consumes lots of memory, if many users perform exporting, many referenced Workbook will consume a lot of memory.

Expected result

JVM garbage collector can clear Workbook

Debug info

check if there are other variables that need to be nullified for the same reason