Support for Non-Null Types in Java

We implemented support for non-null types in Java according to the description in [1]. We built our implementation on top of the Polyglot Java compiler as a language extension. Our extension allows programmers to provide annotations specifying whether a variable is allowed to ever take on a null value and provides an augmented type system to enforce these guarantees. Once the separation between non-null and possibly null types is enforced, it is straightforward to modify Java’s typechecking rules to ensure possibly null variables are never used in an unsafe manner, such as dereferencing. In addition to improving software reliability, preventing null-related errors at compile time improves efficiency as superfluous runtime checks can be removed. A complication exists in that Java allows access to partially initialized objects during construction whose fields may not obey their nullity annotations. We address this issue by making special considerations for such “raw” types in a manner similar to the approach of Fähndrich and Leino. Nullity and rawness properties must be manually specified by programmers, but only about 5% of lines require annotations [1].