Hello. I can't find the way how can I leave player all his experience after death. Any one knows how can I do this?

Not sure if this is what your looking for, because it also causes the player to keep inventory on death, but it does let you keep experience. 

 

The World class has a method to set "gamerules":

 

world.getGameRules().setOrCreateGameRule("keepInventory", "true")

 

It took me a while to figure this one out,  I played with alot of stuff to make just the experience stay.  You may want to look into the playerAPI, forge doesn't seem to hook into the player classes in the right places.