Java 1.3+ should display tamil without altering anything. but jre is
not configured for tamil officially till now. (Devanagari is added).
So you will see only boxes in applets or any other swing applications.
here is the procedure to see tamil characters.

It is considered you have Latha font installed.

Open java directory then go into ‘jre’ directory. Copy-Paste the
“fontconfig.properties.src” file, and rename it as
“fontconfig.properties”.This file then should be the one Java will use
by default. now open the “fontconfig.properties” file.

Find this line :

# Component Font Mappings

Then add :

allfonts.tamil=Latha

You must then find

sequence.allfonts=alphabetic

/default,dingbats,symbol

and add tamil: (am not sure, whether it is really to be done. it
worked without this entry)
sequence.allfonts=alphabetic/default,dingbats,symbol,tamil

add the line

sequence.allfonts.UTF-8.ta=alphabetic/1252,tamil,dingbats,symbol

restart if you are using any java applications. and restart the
browser (deleting temporary files may help if it dint reflect)

post a message, if anything dint work as expected.

-p.