https://cds.sun.com/is-bin/INTERSHOP...-CDS_Developer

Select Linux (arm)

Then download
"Java SE for Embedded 6 Update 21 (Build 9) ARMv7 Linux - Headful, EABI, Hard Float (VFP), Little-Endian"

Extract it to /opt/

Then make symlinks:
In X Terminal, as root:

ln -s /opt/ejre1.6.0_21/bin/* /usr/bin/

You can start jar files from your N900, as user (unless if the java application require root rights, which is rare):

java -jar jarfile.jar