  Automated static analysis tools (ASATs) have become a major part of the
software development workflow. Acting on the generated warnings, i.e., changing
the code indicated in the warning, should be part of, at latest, the code
review phase. Despite this being a best practice in software development, there
is still a lack of empirical research regarding the usage of ASATs in the wild.
In this work, we want to study ASAT warning trends in software via the example
of PMD as an ASAT and its usage in open source projects. We analyzed the commit
history of 54 projects (with 112,266 commits in total), taking into account 193
PMD rules and 61 PMD releases. We investigate trends of ASAT warnings over up
to 17 years for the selected study subjects regarding changes of warning types,
short and long term impact of ASAT use, and changes in warning severities. We
found that large global changes in ASAT warnings are mostly due to coding style
changes regarding braces and naming conventions. We also found that,
surprisingly, the influence of the presence of PMD in the build process of the
project on warning removal trends for the number of warnings per lines of code
is small and not statistically significant. Regardless, if we consider defect
density as a proxy for external quality, we see a positive effect if PMD is
present in the build configuration of our study subjects.
