When I search the web for inserting BLOBs into Oracle database with jdbc thin driver, most of the webpages suggest a 3-step approach:
empty_blob()
Provided the CLOB data is small enough to fit in your memory without blowing up, you can just create a prepared statement and simply call
ps.setString(1, yourString);
There may be other size limitations, but it seems to work for the sizes we're dealing with (500kB max).