The azi2pcore_<version>.jar is built directly from a subset of the I2P source - see https://geti2p.net/

Very minor changes have been made to enable I2P to run in the same JVM as Vuze. See the comments at the top of 

https://svn.vuze.com/public/plugins/networks/azi2phelper/org/parg/azureus/plugins/networks/i2p/I2PHelperPlugin.java