PIT contains an experimental feature to enable its use on very large codebases - incremental analysis.
If this option is activated, PIT will track changes to the code and tests and store the results from previous runs. It will then use this information to avoid re-running analysis when the results can be logically inferred.
A number of optimisations are possible.
With the exception of 4), all these optimisations introduce a degree of potential error into the analysis.
The main issue is that class behaviour is defined not only by its bytecode, but also by its dependencies (i.e the classes it interacts with and the graph of classes that they interact with). PIT will only consider the strongest of these dependencies - changes to super classes and outer classes, when deciding if a class’s behaviour might have changed.
So the incremental feature is based on the assumption that it will be relatively rare for changes in the dependencies of a class to change the status of a mutation. Although this assumption seems reasonable, it is currently unproven.
Optimisation 5) carries the additional risk that the mutations within the meta mutant might cancel each other out, leaving the behaviour of the class unchanged. Again it seems likely that this would be rare but this has not been quantified.
Incremental analysis is currently controlled by two parameters
or for maven
These point to the locations from which to read and write mutation analysis results. This can be the same location. If different locations are used you will need to implement some mechanism to swap the values between runs as PIT does itself does not currently provide a mechanism.