To load java classes into the database from a Java Archive (.jar), place the jarfile in a directory that can be reached from within the database (i.e. an Oracle directory has to be present that points to the physical directory).
Then issue the following command:
exec dbms_java.loadjava( ' -verbose -resolve -force -genmissing <physical directory>/myjarfile.jar);
Querying all existent database objects created from this jarfile shows a list of classes. Now, to drop the classes created by the loadjava-call one can issue:
exec dbms_java.dropjava('-verbose <physical directory>/myjarfile.jar')
The catch here is dat the jarfile has to be present in the directory. So even if you have it loaded in the database, keep it somewhere safe…