custom sorter

[Siping Liu]

Hi,
I have requirements to place a document to a pre-determined  position for
special filter query values, for instance when filter query is
fq=(field1:"xyz") place document abc as first result (the rest of the
result set will be ordered by sort=field2). I guess I have to plug in my
Java code as a custom sorter. I'd appreciate it if someone can shed light
on this (how to add custom sorter, etc.)
TIA.