How to get PID of process I've just started within java program?

I've started a process with following code

 ProcessBuilder pb = new ProcessBuilder("cmd", "/c", "path");
 try {
     Process p = pb.start();       
 } 
 catch (IOException ex) {}

Now I need to know the process's pid that I've just started.

Since Java 9 class Process has new method long pid(), so it is as simple as

ProcessBuilder pb = new ProcessBuilder("cmd", "/c", "path");
try {
    Process p = pb.start();
    long pid = p.pid();      
} catch (IOException ex) {
    // ...
}

There is no public API for this yet. see Sun Bug 4244896, Sun Bug 4250622

As a workaround:

Runtime.exec(...)

returns an Object of type

java.lang.Process

The Process class is abstract, and what you get back is some subclass of Process which is designed for your operating system. For example on Macs, it returns java.lang.UnixProcess which has a private field called pid. Using Reflection you can easily get the value of this field. This is admittedly a hack, but it might help. What do you need the PID for anyway?

This page has the HOWTO:

http://www.golesny.de/p/code/javagetpid

On Windows:

Runtime.exec(..)

Returns an instance of "java.lang.Win32Process") OR "java.lang.ProcessImpl"

Both have a private field "handle".

This is an OS handle for the process. You will have to use this + Win32 API to query PID. That page has details on how to do that.

In Unix System (Linux & Mac)

 public static synchronized long getPidOfProcess(Process p) {
    long pid = -1;

    try {
      if (p.getClass().getName().equals("java.lang.UNIXProcess")) {
        Field f = p.getClass().getDeclaredField("pid");
        f.setAccessible(true);
        pid = f.getLong(p);
        f.setAccessible(false);
      }
    } catch (Exception e) {
      pid = -1;
    }
    return pid;
  }

Include jna (both "JNA" and "JNA Platform") in your library and use this function:

import com.sun.jna.Pointer;
import com.sun.jna.platform.win32.Kernel32;
import com.sun.jna.platform.win32.WinNT;
import java.lang.reflect.Field;

public static long getProcessID(Process p)
    {
        long result = -1;
        try
        {
            //for windows
            if (p.getClass().getName().equals("java.lang.Win32Process") ||
                   p.getClass().getName().equals("java.lang.ProcessImpl")) 
            {
                Field f = p.getClass().getDeclaredField("handle");
                f.setAccessible(true);              
                long handl = f.getLong(p);
                Kernel32 kernel = Kernel32.INSTANCE;
                WinNT.HANDLE hand = new WinNT.HANDLE();
                hand.setPointer(Pointer.createConstant(handl));
                result = kernel.GetProcessId(hand);
                f.setAccessible(false);
            }
            //for unix based operating systems
            else if (p.getClass().getName().equals("java.lang.UNIXProcess")) 
            {
                Field f = p.getClass().getDeclaredField("pid");
                f.setAccessible(true);
                result = f.getLong(p);
                f.setAccessible(false);
            }
        }
        catch(Exception ex)
        {
            result = -1;
        }
        return result;
    }

You can also download JNA from here and JNA Platform from here.