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

On Unix, one way is using the ManagementFactory to get the OperatingSystemMxBean and if it is a UnixOperatingSystemMXBean, you can use the getOpenFileDescriptorCount() method.

Example code:

import java.lang.management.ManagementFactory;
import java.lang.management.OperatingSystemMXBean;
import com.sun.management.UnixOperatingSystemMXBean;

public class OpenFileCount{
    public static void main(String[] args){
        OperatingSystemMXBean os = ManagementFactory.getOperatingSystemMXBean();
        if(os instanceof UnixOperatingSystemMXBean){
            System.out.println("Number of open fd: " + ((UnixOperatingSystemMXBean) os).getOpenFileDescriptorCount());
        }
    }
}