NullAway: Practical Type-Based Null Safety for Java Subarno Banerjee Lazaro Clapp Manu Sridharan University of Michigan Uber Technologies, Inc. University of California, Riverside Ann Arbor, MI, USA San Francisco, CA, USA Riverside, CA, USA
[email protected] [email protected] [email protected] ABSTRACT app to users’ devices can take days to weeks, depending on the app NullPointerExceptions (NPEs) are a key source of crashes in mod- store release process and how often users install updates. Due to the ern Java programs. Previous work has shown how such errors can be severity of null-dereference errors, recent mainstream languages prevented at compile time via code annotations and pluggable type like Swift [13] and Kotlin [8] enforce null safety as part of type checking. However, such systems have been difficult to deploy on checking during compilation. large-scale software projects, due to significant build-time overhead Previous work has added type-based null safety to Java, via code and / or a high annotation burden. This paper presents NullAway, annotations and additional type checking [6, 23, 32]. With this a new type-based null safety checker for Java that overcomes these approach, developers use @Nullable and @NonNull code annotations issues. NullAway has been carefully engineered for low overhead, to indicate whether entities like fields, parameters, and return values so it can run as part of every build. Further, NullAway reduces may or may not be null. Given these annotations, a tool checks that annotation burden through targeted unsound assumptions, aiming the code is null safe, by ensuring, e.g., that @Nullable expressions are for no false negatives in practice on checked code.