Tuesday, July 19, 2016

Gradle with "Service Provider" Pattern

I've started dusting off my old littleware project over the last month or so on a "dev" branch in github. As part of the cleanup I'm moving littleware from its old ANT-IVY build system to Gradle. When I started with ant-ivy it was a great alternative to Maven, and allowed me to extend the build setup put in place by a Netbeans IDE's project with transitive dependencies, jenkins integration, scala support - all that great stuff. The problem is that the ant-ivy setup is idiosyncratic, and Gradle and SBT are well designed, widely used systems. I wound up going with Gradle, because I've read a lot of good things about it, and I want to become more familiar with it, and I had used SBT a little in the past, and found SBT's DSL annoying and its extension mechanisms inscrutable, and of course Google is using Gradle to build Android, ...

Anyway - like I said, I'm dusting off the code, and trying to remember what the hell I was thinking with some of it, but so far the Gradle integration has gone well. One of the issues I'm dealing with is the IVY-centric mechanism littleware had setup to pull in optional run-time dependencies. With Gradle the default behavior of the java plugin is to include all compile-time dependencies at run-time - which is what most people expect. Littleware's ant-ivy setup excluded optional dependencies (like mysql-connector versus postgresql - something like that), and included add-on IVY configurations like "with_mysql", "with_postgres", "with_aws", ... that kind of thing, so a client project would have a dependency specification something like this:

<dependency org="littleware" name="littelware" ... conf="compile->compile;runtime->runtime,with_mysql" />

Of course nobody else works like that, so in the interest of clarity and doing what people expect - I'm going to try to rework things, so that littleware's base jar asset includes service provider interfaces (SPI's), that add-on modules implement, so a client that wants to hook up littleware's ability to wire in a MySQL DataSource to a Guice injector would add a dependency to the 'littleware_mysql_module' to their build - something like that. The Jackson JSON parser implements a module system like this, and the SPI pattern is all over java-EE (JDBC, Servlets, whatever); it's a good and widely understood pattern. We'll see how it goes.



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