The pcap dumper example expands on the classic example and creates a PcapDumper object which facilitates copying of captured packets to a file. This is exactly how packet dumping is done with native libpcap library as well.

Download source from SVN:

• Pcap Dumper Example
• Pcap Dumper Example that uses Pcap.openDead, a dummy pcap object

package org.jnetpcap.examples;

import java.io.File;
import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.List;

import org.jnetpcap.Pcap;
import org.jnetpcap.PcapDumper;
import org.jnetpcap.PcapHandler;
import org.jnetpcap.PcapIf;


public class PcapDumperExample {
  public static void main(String[] args) {
    List<PcapIf> alldevs = new ArrayList<PcapIf>(); // Will be filled with NICs
    StringBuilder errbuf = new StringBuilder();     // For any error msgs

    /***************************************************************************
     * First get a list of devices on this system
     **************************************************************************/
    int r = Pcap.findAllDevs(alldevs, errbuf);
    if (r == Pcap.NOT_OK || alldevs.isEmpty()) {
      System.err.printf("Can't read list of devices, error is %s\n", 
        errbuf.toString());
      return;
    }
    PcapIf device = alldevs.get(1); // We know we have atleast 1 device

    /***************************************************************************
     * Second we open up the selected device
     **************************************************************************/
    int snaplen = 64 * 1024;           // Capture all packets, no trucation
    int flags = Pcap.MODE_PROMISCUOUS; // capture all packets
    int timeout = 10 * 1000;           // 10 seconds in millis
    Pcap pcap = Pcap.openLive(device.getName(), snaplen, flags, timeout, errbuf);
    if (pcap == null) {
      System.err.printf("Error while opening device for capture: %s\n", 
        errbuf.toString());
      return;
    }
		
    /***************************************************************************
     * Third we create a PcapDumper and associate it with the pcap capture
     ***************************************************************************/
    String ofile = "tmp-capture-file.cap";
    PcapDumper dumper = pcap.dumpOpen(ofile); // output file

    /***************************************************************************
     * Fouth we create a packet handler which receives packets and tells the 
     * dumper to write those packets to its output file
     **************************************************************************/
    PcapHandler<PcapDumper> dumpHandler = new PcapHandler<PcapDumper>() {

      public void nextPacket(PcapDumper dumper, long seconds, int useconds,
        int caplen, int len, ByteBuffer buffer) {

        dumper.dump(seconds, useconds, caplen, len, buffer);
      }
    };

    /***************************************************************************
     * Fifth we enter the loop and tell it to capture 10 packets. We pass
     * in the dumper created in step 3
     **************************************************************************/
    pcap.loop(10, dumpHandler, dumper);
		
    File file = new File(ofile);
    System.out.printf("%s file has %d bytes in it!\n", ofile, file.length());
		

    /***************************************************************************
     * Last thing to do is close the dumper and pcap handles
     **************************************************************************/
    dumper.close(); // Won't be able to delete without explicit close
    pcap.close();
		
    if (file.exists()) {
      file.delete(); // Cleanup
    }
  }
}