How to find out number of files currently open by Java application?

Question

Suppose a lot of what your application does deals with reading contents of files. It goes without saying that files that are opened then closed and life is good unless ... n

Answer 1

For the sake of completing/adding to the answer:

Many people uses LSOF in Linux based system to monitor file descriptors and their details. However LSOF lists all kind of FDs(CWD,MEM) which is not returned by UnixOperatingSystemMXBean.getOpenFileDescriptorCount().

Details can be found in http://www.ibm.com/developerworks/aix/library/au-lsof.html This may cause some confusion.

To clarify UnixOperatingSystemMXBean.getOpenFileDescriptorCount() only shows application opened files descriptors. But LSOF (lsof -a -p ) lists other file descriptors opened by kernel on behalf of the process.

To list only application level FDS one can filter out other type of FDs

lsof -a -p yourprocid -d ^txt,^mem,^cwd,^rtd,^DEL