Setting up a multi-project Gradle build - gradle

Question

I'm setting up a multi-module Gradle build for a legacy system at work (replacing the current Ant build). However, I'm new to Gradle, and I'm not sure what's the best way to do it. And I want to do it right, because this build script will be around for a long time. I have found a way to do things that works, but when I google around and read answers on StackOverflow, I see people using a different way, which --in my case-- doesn't work. But maybe I'm doing something wrong. I've also been reading in the Gradle in Action book, but haven't found this particular issue there.
I have a build.gradle file in the root of my project, with a bunch of subdirectories that each contain a sub-project. Most of these are regular Java projects, but there are some Wars and Ears in there, too, which require some special packaging love. Each sub-project has its own build.gradle file which, at this point, only contains a list of dependencies, nothing more.
The build.gradle file in the root of my projects looks something like this (I left out the War stuff for brevity):
configure(javaProjects()) {
jar.doFirst {
manifest {
...
}
}
}
configure(earProjects()) {
apply plugin: 'ear'
ear.doFirst {
manifest {
...
}
}
}
Set<String> javaProjects() {
subprojects - earProjects()
}
Set<String> earProjects() {
subprojects.findAll { it.name.endsWith(".ear") }
}
The only reason why I'm doing things this way, is because it was the first solution I tried that I could get to work in my situation. Now that the script is growing, though, it starts to feel a little clunky. Also, the doFirst thing seems a little awkward.
But when I look on StackOverflow, I see recommendations of using constructs like this:
allprojects {
tasks.withType(Jar) {
manifest {
...
}
}
tasks.withType(Ear) {
manifest {
...
}
}
}
This seems much nicer, but I don't seem to be able to rewrite my script in that way. I get errors like this one:
Cannot change configuration ':some.subproject:compile' after it has been resolved.
I don't know what to do about this error, and I can't seem to google it either, for some reason.
So, my question is: have I indeed been doing things the wrong way? Or rather: in a way that is not idiomatic Gradle? For the sake of maintainability, I'd like to do things as idiomatically as possible. And if so: what can I do about the error message?

Answer 1

In general you should do things like described in your second snippet:
allprojects {
tasks.withType(Jar) {
manifest {
...
}
}
}
But there are some limitations where this isn't sufficient. The error message you get means that you modify the compile configuration AFTER the configuration is already resolved. That happens for example when you do something like
configurations.compile.files.each...
during the configuration phase (e.g. in your manifest block like seen above) and in another place (e.g. in one of your subprojects build.gradle files):
dependencies{
compile "org.acme:somelib:1.2.3"
}
The other problem with this is, that you resolve the compile dependencies every time you invoke your build script, even when no jar task is triggered.
The suggested workaround is to replace
tasks.withType(Jar) {
manifest {
...
}
}
with
tasks.withType(Jar) {
doFirst{
manifest {
...
}
}
}
That means that resolving the configuration is postponed to the execution phase of gradle and really just triggered when needed.
When you configure a project in a multiproject build you can think of that each snippet that is part of the whole configuration. you're not configuring the project 'twice' but you configure different aspects of the project at different places.
This is a known limitation of the current gradle configuration model.
You can still use
configure(earProjects()) {
}
that doesn't matter here. IMO it is just a matter of personal preference. The gradle project itself uses 'configure'.
Personally I prefer to apply the plugins like Ear or war on the projects build.gradle file to mark a project as a ear/war project.
To share common configurations among all ear projects, you could have something like this in your root build.gradle file:
allprojects{
plugins.withType(EarPlugin){
// only applied if it is a ear project
// put ear specific logic here
}
}