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());
        }
    }
}

If you want to get the number of ls /proc/my_pid/fd | wc -l in java, you could use JMX.

When you have MBeanServerConnection, you can get attribute "OpenFileDescriptorCount". this will give you the same result as the above ls..|wc -l.

also there is another attribute "MaxFileDescriptorCount", which tells your the max count allowed.

If you just want to know the value, but not getting it in your code, you could either do your ls..|wc -l way, or read from jconsole (with GUI).

NOTE

ls /proc/my_pid/fd | wc -l

this number indicates how many fd opened by your process(java application). e.g. your business files are counted, also those jars, libraries files are counted too. If you just want to get the count of your business files, you have to implement a counter by yourself. Or, say you want to do it with shellscript, grep something out then wc -l

EDIT

add code example, but it is just an example. not written in IDE, not tested with compiler. :)

ObjectName oName = new ObjectName("java.lang:type=OperatingSystem");
MBeanServerConnection conn ; // you should get the connection following the api, take a look the java api/ google some example
javax.management.AttributeList list = conn.getAttributes(oName, new String[]{"OpenFileDescriptorCount", "MaxFileDescriptorCount"});
for(Attribute attr: list.asList()){
    System.out.println(attr.getName() + ": " + attr.getValue());
}