What is Java binary compatibility?

Separate compilation allows the decomposition of programs into units that may be compiled separately, and linked into an executable. Traditionally, separate compilation was equivalent to the compilation of all units together, and modi cation and re-compilation of one unit required re-compilation of all importing units. Java suggests a more exible framework, in which the linker checks the integrity of the binaries to be combined. Certain source code modi cations, such asaddition of methods to classes, are de ned as binary compatible. The language description guarantees that binaries of types (i.e. classes or interfaces) modi ed in binary compatible ways may be re-compiled and linked with the binaries of types that imported and were compiled using the earlier versions of the modi ed types. However, this is not always the case: some of the changes considered by Java as binary compatible do not guarantee successful linking and execution. In this paper we study the concepts around binary compatibility. We suggest a formalization of the requirement of safe linking and execution without re-compilation, investigate alternatives, demonstrate several of its properties, and propose a more restricted de nition of binary compatible changes. Finally, we prove for a substantial subset of Java, that this restricted de nition guarantees error-free linking and execution.