Doc for version 11g :
Logging is done using java.util.logging package provided by Java SE.
To get log output, you must use the debug JAR files, which are indicated with a « _g » in the file name, like
ojdbc6_g.jar. The debug JAR file must be included in the
CLASSPATH environment variable.
This feature logs information about events that occur when JDBC driver code runs. Events can include user-visible events, such as SQL exceptions, running of SQL statements, and detailed JDBC internal events, such as entry to and exit from internal JDBC methods. Users can enable this feature to log specific events or all the events.