Java actions have supported microflows as parameters for a while now. What is missing is the option to check the microflow input and output parameters at build time.
It should be possible to declare in the java action the number and types of input arguments and the type of the output argument.
Such a thing is already possible for microflows called from widgets https://docs.mendix.com/howto/extensibility/use-xml-widget#5-2-7-microflow
After constraining the microflow’s input and output parameters Studio will check and enforce them at design time. This will make calling microflows from java actions much more secure as errors in the input and output types will be caught at design time instead of run time.