On Wed, 25 Sep 2013, Brian Nesbitt wrote:
I want to cache Excel workbook data as close to having a Workbook object
as possible. I need it just before as there will be multiple threads
writing data to the cells of the workbook so I want to create a new
workbook object on each cache access.
That's not generally supported. The thread-safety contract in POI is that
multiple threads can safely write to their own objects without issue, but
not that multiple threads could write to the same object. For example, two
threads are safe to each write to their own HSSFWorkbook instances, but
things can go wrong if two threads are simultaniously writing to the
*same* Workbook object. Ditto for reading (some parts are lazy-parsed)
From my tests #2 is faster as I assume some pre-parsing is done by the
npiofs and opc classes. Are the NPOIFSFileSystem and OPCPackage
threadsafe? (Can I have multiple threads using the same instance in a call
to WorkbookFactory.create()).
You can pass the same byte array to two different NPOIFSFileSystem objects
in different threads and that'll be fine. If you open a NPOIFSFileSystem
object in one thread, I *think* you should be OK if two threads then read
from the same open NPOIFSFileSystem object. I'm fairly sure things will go
wrong if two threads try to write to the same NPOIFSFileSystem at the same
time, or if one thread tries to interact with a NPOIFSFileSystem object
that another one is only half way through opening. OPCPackage ought to be
broadly similar, but I don't know the details of the code of that so
closely.
Nick
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