The JDBC extensions provide Java objects corresponding to the internal PostGIS types. These objects can be used to write Java clients which query the PostGIS database and draw or do calculations on the GIS data in PostGIS.
The JDBC extensions require a PostgreSQL JDBC driver to be present in the current CLASSPATH during the build process.
PostgreSQL JDBC drivers can be downloaded from http://jdbc.postgresql.org or Sonatype Central Maven repository.