jpcap -- a network packet capture library for applications written in Java.

Jpcap development is hosted at on SourceForge Logo

The jpcap distribution includes both

The jpcap network capture tool performs real-time decomposition and visualization of network traffic.
Screenshots of the capture console and visualization component in action:

The jpcap API allows developers to create their own packet capture applications.
A portion of the capture interface is shown:

For more information visit jpcap at sourceforge or have a look at the jpcap javadoc.