<p>My program receives large CSV files and transforms them to XML files. In order to have better performance I would like to split this files in smaller segments of (for example) 500 lines. What are the available Java libraries for splitting text files?</p>

What do you intend to do with those data ?
If it is just record by record processing then event oriented (SAX or StaX) parsing will be the way to go. For record by record processing, an existing "pipeline" toolkit may be applicable.
You can pre-process your file with a splitter function like
this one
or
this Splitter.java
.