Rok Strniša
ABSTRACT
Java has no module system. Its packages only subdivide the class name space, allowing only a very limited form of component-level information hiding and reuse. Two Java Community Processes have started addressing this problem: one describes the runtime system and has reached an early draft stage, while the other considers the developer's view and only has a straw-man proposal. Both are natural language documents, which inevitably contain ambiguities.
In this work we design and formalize a core module system for Java. Where the JCP documents are complete, we follow them closely; elsewhere we make reasonable choices. We define the syntax, the type system, and the operational semantics of an LJAM language, defining these rigorously in the Isabelle/HOL automated proof assistant. Using this formalization, we identify various issues with the module system. We highlight the underlying design decisions, and discuss several alternatives and their benefits. Our Isabelle/HOL definitions should provide a basis for further consideration of the design alternatives, for reference implementations, and for proofs of soundness.
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Index Terms
- The java module system: core design and semantic definition
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Reviews
Alexandre Bergel
The use of packages and class loaders in Java does not help in managing several different versions of the same software unit in a satisfactory way. As expressed by the authors, this situation is traditionally referenced as dynamic-link library (DLL) or Java Archive (JAR) hell. Java Module System (JAM) is the latest of several attempts by the Java community to define a practical module system. The main feature of JAM is to add visibility restriction to packages using hierarchical information hiding, while being backward compatible with Java. JAM supports "super package," a new kind of compilation unit, to embody packages. Packages may be imported and exported to other super packages. A repository acts as a scoping mechanism by containing one module instance at most. A module definition (no matter what version) can be found from within the same repository, or any repository related to that repository through the "child" relation. The authors present a rigorous formalization and an extended discussion on various aspects of JAM. Other module systems, such as MZScheme's units [1] and Expanders [2], enable different class versions to be in use in the same executing application. JAM restricts one single module instance per repository, which implies that no more than one class version may be present at execution within the same control flow. One point that may leave a reader unsure has to do with interaction with legacy code. Recent major changes in Java, such as generics, show the importance of linking legacy code to new libraries that use the new features, and vice versa. The paper does not provide any convincing hints that go in that direction. With that said, this paper makes a strong and clear contribution, and provides an answer for a pressing need. Online Computing Reviews Service
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