Like so many others, I have pretty much ignored project Jigsaw for some time now – assuming it would stay irrelevant for my work or slowly fade away and be gone for good. The repeated shifts in planned inclusion with the JDK seemed to confirm this course. Jigsaw started in 2009 – more than six years ago.
Jigsaw is about establishing a Java Module System deeply integrated with the Java language and core Java runtime specifications. Check out its goals on the project home page. It is important to note the fourth goal:
Make it easier for developers to construct and maintain libraries and large applications, for both the Java SE and EE Platforms.
Lately I have run into this mail thread: http://permalink.gmane.org/gmane.comp.java.openjdk.jigsaw/2492
In that mail thread Jürgen Höller (of Spring fame) notes that in order to map Spring’s module layout to Jigsaw modules, it would be required to support optional dependencies – dependencies that may or may not be satisfied given the presence of another module at runtime.
This is how Spring supports its set of adapter and support types for numerous other frameworks that you may or may not use in your application: Making use of Java’s late linking approach, it is possible to expose types that may not be usable without the presence of some dependent type but will not create a problem unless you start using them either. That is, optional dependencies would allow Spring to preserve its way of encapsulating the subject of „Spring support for many other libraries“ into one single module (or actually a jar file).
In case you do not understand the technical problem, it is sufficient to note that anybody who has been anywhere near Java class loading considerations as well as actual Java application construction in real live should know that Spring’s approach is absolutely common for Java infrastructure frameworks.
Do Jigsaw developers actually know or care about Java applications?
Who knows, maybe they simply forgot to fix their goals. I doubt it.
Module != JAR file
There is a deeper problem: The overloaded use of the term module and the believe of infrastructure developers in the magic of the term.
Considering use of the module term in programming languages, it typically denotes some encapsulation of code with some interface and rules on how to expose or require some other module. This is what Jigsaw focussed on and it is what OSGi focussed on. It is what somebody interested in programming language design would most likely do.
In Java this approach naturally leads using or extending the class loading mechanism to expose or hide types between modules for re-use (or information hiding resp.) which in turn means to invent descriptors that describe use relationships (meaning the ability to reference types in this case) and so on.
This is what Jigsaw does and this is what OSGi did for that matter.
It is not what application developers care about – most of the time.
There is an overlap in interest of course. Code modules are an important ingredient in application assembly. Problems of duplicate type definitions by the same name (think different library versions) and separation of API and implementation are essential to scalable, modular system design.
But knowing how to build a great wall is not the same as knowing how to build a great house.
From an application development perspective, a module is much rather a generic complexity management construct. A module encapsulates a responsibility and in particular should absolutely not be limited to code and is not particularly well-served by squeezing everything into the JAR form factor.
What we see here is a case of Application vs. Infrastructure culture clash in action (see for example Local vs. Distributed Complexity).
The focus on trying to find a particularly smart and technically elegant solution for the low-level modularization problem eventually hurts the usefulness of the result for the broader application development community (*).
Similarly, ignorance of runtime modularization leads to unmaintainable, growth-limited, badly deployable code bases as I tried to describe in Modularization is more than cutting it into pieces.
The truth is somewhere in between – which is necessarily less easily described and less universal in nature.
I believe that z2 is one suitable approach for a wide class of server-side applications. Other usage scenarios might demand other approaches.
I believe that Jigsaw will not deliver anything useful for application developers.
I wish you a happy new year 2016!
* One way of telling that the approach will be useless for developers is when discussions conclude that “tools will fix the complexity”. What that comes down to is that you need a compiler (the tool) to make use of the feature, which in turn means you need another input language. So who is going to design that language and why would that be easier?
* It is interesting to check out the history of SpringSource’s dm Server (later passed on to R.I.P. at Eclipse as project Virgo). See in particular the interview with Rod Johnson.