JCraft has kindly published the java source code for JSpringies under the GNU General Public License agreement. 

I have made several enhancements and bug-fixes to JSpringies. It's at the bottom of this page, give it a try! Source code is here.

Enhancements:

• Moved all controls onto applet panel like the original XSpringies layout
• Added a JSpringies logo
• Added a Instructions / Credits button
• Added a animation speed control to speed-up play (a feature adapted from the XSpringies C source)
• Changed the color spring feature to use a rainbow color palette
• Added a mouse cursor probe to allow interactive play
• Added a show springs option
• Use larger, filled oval masses

• Fixed deprecated Java API
• Corrected typo errors in physics code (example: bridge.xsp instabilty bug)
• Reset physics state on data file load (avoids psycho.xsp instabilty bug) 
• Reverse 'y' location and velocity when data file loads.