Using DecimalFormat in expression pattern ?

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When formatting a number you can choose to use an "expression pattern". It's useful because you can use a function to get the real pattern, but I found there is a problem with locale.
I want to set up my formatter locale-independent. If you use string pattern, it's not possibile, because, I suppose, Jasper creates DecimalFormat based on report locale.

Is there any mode to put a DecimalFormat object directly in "expression pattern" ? Is there an alternative using pattern ?