How to convert arbitrary string to Java identifier?

I need to convert any arbitrary string:

• string with spaces
• 100stringsstartswithnumber
• string€with%special†characters/\!
• [empty string]

to a valid Java identifier:

• string_with_spaces
• _100stringsstartswithnumber
• string_with_special_characters___
• _

Is there an existing tool for this task?

With so many Java source refactoring/generating frameworks one would think this should be quite common task.