Jpcap is a Java library for capturing and sending network packets.

Using Jpcap, you can develop applications to capture packets from
a network interface and visualize/analyze them in Java. You can
also develop Java applications to send arbitrary packets through a
network interface.

# Note: the jpcap at SourceForge is a bit outdated.

WWW: http://netresearch.ics.uci.edu/kfujii/jpcap/doc/index.html