A method for modifying a class file to monitor data flow is provided. The method includes selecting (401) one or more methods in the class file and adding (206, 209) one or more lines of code to report incoming arguments and return values, if any, in the selected method. The incoming arguments and return values are reported (405, 410) to an instrumentation interface which may be an instrumentation class defining methods for each type of argument in the method. The one or more lines of code pass the incoming arguments and return values to the appropriate instrumentation class method. The method may produce a profile of data flow including a record of incoming arguments and return values, if any, for one or more methods in a class file. The record can include data describing the types and values of arguments and return values and their distribution.