You are trying to modify your source code on the fly?

Read the file as one  great string. You can read it in a single
unbuffered i/o. See http://mindprod.com/jgloss/fileio.html
for how.

Now you indexOf to find the word.  Now write out the prefix. write the
word, write the suffix.

that saves creating a giant StringBuffer/String

Sounds like an awful lot of work to me! Wouldn't it make more sense to use
your text editor or IDE to alter your JSP source?

Rhino

Can you alter your JSP to read the value on the fly from a properties
file, or some dynamic value from the HttpRequest? Alternatively, how
about changing the jsp during your build process using an Ant <replace>
or <replaceregexp> task?

Combination of awk/sed should get you going.