GetPidJDK8.java

package io.github.barend.getpid;

import com.sun.jna.Library;
import com.sun.jna.Native;

/**
 * requires {@code jna.jar}.
 * http://stackoverflow.com/a/7303433/49489
 */
public class GetPid {

    public long getPid() throws UnsatisfiedLinkError {
        CLibrary libc;
        try {
            libc = (CLibrary) Native.loadLibrary("c", CLibrary.class);
        } catch (UnsatisfiedLinkError e) {
            libc = (CLibrary) Native.loadLibrary("System", CLibrary.class);
        }
        return libc.getpid();
    }

    private interface CLibrary extends Library {
        long getpid();
    }
}

GetPidJDK9.java

public class GetPid {
  public long getPid() {
    return ProcessHandle.current().pid()
   }
}