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InfoQ Homepage News Can the Simple Module System save JSR294?

Can the Simple Module System save JSR294?

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Over the past month there has been a lot of debate on the current state of the Java Modularity working group (JSR 294). Although the JSR tries to find common ground between different module systems (notably Sun's Project Jigsaw and OSGi), the current set of proposals are overly complex and introduce the world's first concept of a meta-module system.

Much of the debate has centred on the difference between a language specification and its implementation. As it currently stands, the JSR 294 draft (which is disliked by most of the expert group) defines a meta-module system which delegates the concept of modules, versions, constraints and dependencies to a 'pluggable' external modularity provider. Unfortunately, this means that there can be no common ground for version numbers, dependencies, and ultimately leads to the fact that modules developed for one or other of the module systems will likely be incompatible for the other.

Such vagueness helps no-one in the Java community, which is in dire need of a module system which can work with either the OpenJDK jigsaw proposal or OSGi. Many have asked why OpenJDK can't reuse OSGi – but the simple answer is that OSGi is much more than just modularity, and although the core spec (which defines the module layer) is fairly small, licensing issues are still a factor in why this could not happen in any case.

Instead of developing a meta-module system with vaguely defined semantics and the concept of external providers dictating the meaning, a Simple Module System proposal has been posted to the mailing list group suggesting that now is the time to get rid of the meta-module system, and focus on the commonalities that both module systems can support. This would involve coming to an agreement on some key differences in the way that module versions are represented (a win for Java developers everywhere, in any case) and define module dependencies as much like the Maven module dependencies, where a module implicitly imports all the packages it needs whilst exporting all the packages that it defines.

This would allow Java developers to build a module that works in both Jigsaw and OSGi, and though both have more powerful extensions, this wouldn't be needed for the majority of developers who build Java libraries and need to provide modules. This would avoid the need for providers to choose which of two otherwise incompatible formats to use and instead focus on static modularity of Java libraries. More powerful dynamic modularity could then be used by using those modules within a dynamic module system; but for most use cases, static modularity would be enough.

The key points of the proposal are:

  • Simple Visibility Model – when a module requires another simple module, all the contents of this other module are made visible to the dependent module. There is no proscription against splitting packages across modules. This is what Jigsaw currently does and is what most people, unfamiliar with OSGi’s more expressive model, are likely expecting. It is very much what most people experience today with Maven based build systems and the current runtime classpath model. OSGi must update its specifications to provide support for this simple visibility model, which is almost identical to Require-Bundle on a bundle which exports all its packages.
  • Single Version Numbering Scheme – A single, shared version numbering scheme is necessary. The simple module system must define a version numbering scheme and Jigsaw must use it and OSGi must change to use it. The version numbering scheme must define a natural ordering so that compareTo() can be used to order versions. Currently, OSGi defines a 4-part version numbering scheme and a natural ordering over it. Sun has stated the OSGi scheme does not meet Jigsaw’s requirements. These requirements must be collected in order to design a version numbering scheme and its natural ordering which can be shared by the simple module system, OSGi and Jigsaw. OSGi must update its specifications to support this new version numbering scheme.
  • Module Membership – For the simple module system, the compiler must assume that module membership corresponds to the membership of the module artifact, for example a JAR file or directory. At runtime, each module artifact will have its own class loader that is associated with a single unique module. All types loaded from a module artifact will be loaded by this single class loader and made members of this single module.
  • No “black holes” – In order for this proposal to be truly valuable, it is paramount to have no “black holes” in the specification, where black holes are things in the source code which can not be understood without the assistance of a specific module system. The specification must specify the concrete syntax and semantics of the simple module system including version numbers, dependency expression, module membership, etc. The Java language specification does not allow one to define their own language keywords or operators. The same must be true for module information in module-info source files and for module membership rules.

Due to the way that the JSR process works, one or a few people are responsible for writing the specification and the expert group is there to debate, point out issues, or approve the draft. For some time, the meta-module system hasn't met with common approval for the members of the group, and a number of the expert group members (including a Sun employee) have felt the need to write their own proposal in order to highlight the issues with the current draft. This feels like a last effort to try and get to a standardised module system for Java, and it's hoped that both OpenJDK's Jigsaw requirements and OSGi's can be met by a simple module mechanism which is defined in its entirety in the language specification without appeal to an external implementation.

Perhaps the biggest win would be an agreement on the representation of version numbers. OSGi has long defined a major.minor.micro.qualifier format, which is sufficient for all Java libraries except Sun, who uses 1.major.minor.micro.qualifier. But the other significant difference is that the empty qualifier in OSGi is the 'bottom' value of the version number range, whereas in most Java developers common usage and in Jigsaw, the empty qualifier is the 'top' value. (In other words, 1.0.0 is bigger than 1.0.0.beta in Jigsaw, but has the opposite in OSGi.) Finding a common ground to solve this as a special case would be a good step towards the module system version requirements, and even if there's requirements for the addition of a fifth digit, it could be the case that a future version of OSGi would be able to support this.

What do you think of replacing the meta-module system with a simple module system for Java?

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