I am completely new to Groovy, trying to learn it, but stymied because I can't parse the syntax well enough to even know where to look in the documentation. I am using Groovy in Gradle. There are many places where examples are given, but no explanation on what it means, so I just need a few pointers.
publishing {
publications {
mavenJava(MavenPublication) {
groupId = 'com.xxx.yyy'
artifactId = 'zzz'
from components.java
}
}
repositories {
mavenLocal();
}
}
The main build code is referring to things on the project class. On that class, I can find a property called publishing, and it is a class PublishingExtension. It appears then that the curly brace starts a closure with code in it. The documentation says this syntax:
publishing { }
configures the PublishingExtension. What I want to understand is what it means (i.e. what is actually happening) when I specify what looks like a property and follow that with a Closure. In the Groovy documentation I could not find any syntax like this, nor explanation. I sure it is something simple but I don't know enough to even know what to look for.
If I visit the Project Class API Docs there is no method there named publishing. Nor is there a property defined by the method getPublishing. Apparently this magic capability is enabled by the publishing plugin. If I visit the Publishing Plugin API Doc there is no description of this publishing property either or how it modifies the base project.
Similarly, drilling down a little more, that closure starts with a symbol publications and in the documentation for the PublishingExtension I find a property which is of type PublicationContainer which is read only. I also find a method named publications which does not accept a closure, but instead a configuration of type Action<? super PublicationContainer>. Again, I don't know how the contents of the curly braces are converted to an Action class instance. How does this object get constructed? Action is an interface, and the only method is execute however it is completely unclear how this action gets constructed.
The block that defines the Action starts with symbol mavenJava that looks like a method, but actually that first symbol is declaring a name of a new object of type MavenPublication named mavenJava. Either this is magically constructed (I don't know the rules) or there is a method called, but which method? What is it about PublicationContainer that allows it to know that an arbitrary mavenJava command is supposed to create an object instance. Then again, the curly braces that follow this, is that a closure, a configuration, or something even more exotic?
So as you can see, I am missing a little info on how Groovy works. So far I can't find documentation that explains this syntax, however it might be there if I knew what to look for. Can anyone explain what the syntax is really doing, or refer me to a site that can explain it?
publishing is called to configure the PublishingExtension.
In PublishingExtension there is a publications method accepting an Action which is usually coerced from a Closure. In Groovy a Closure is automatically converted to an interface with a single method.
mavenJava is a non-existent DSL method, which is passed by Gradle DSL builder to the create method of PublicationContainer:
publishing.publications.create('mavenJava', MavenPublication) {
// Configure the maven publication here
}
groupId and artifactId are properties of MavenPublication and are being set here.
from is the from(component) of MavenPublication and is written using Groovy simplified method call literal without brackets.
In general Gradle uses a root DSL builder which calls the nested DSL builders provided by plugins. Hence sometimes it's difficuilt (also for the IDE) to find proper references of all parts of the build.gradle file.
Related
In the gradle doc: https://docs.gradle.org/current/userguide/custom_plugins.html#sec:custom_plugins_standalone_project
The code block is:
gradlePlugin {
plugins {
create("simplePlugin") {
id = "org.example.greeting"
implementationClass = "org.example.GreetingPlugin"
}
}
}
I noticed it calls create method. And I looked the source code. It says:
Creates a new item with the given name, adding it to this container,
then configuring it with the given action.
What does it mean? Is it actually used anywhere? Or it can be any name does not really matter?
gradlePlugin.plugins is a NamedDomainObjectContainer - which are used by Gradle to hold multiple objects, each with a name.
The documentation on plugin development goes into more detail on the usage of NamedDomainObjectContainers.
Sometimes you might want to expose a way for users to define multiple, named data objects of the same type.
[...]
It’s very common for a plugin to post-process the captured values within the plugin implementation e.g. to configure tasks.
Since the elements of a NamedDomainObjectContainer can be any type, there's no specific usage for one. Generally they are used to configure the Gradle project, for example creating specific tasks, configuring source code locations.
I'm trying to understand deeply how gradle is working.
Let's take the case of a basic dependencies {} declaration in a build.gradle script.
dependencies is a method on Project object.
DSL Project object = org.gradle.api.Project interface:
void dependencies(Closure configureClosure)
Configures the dependencies for this project.
This method executes the given closure against the DependencyHandler for this project. The
DependencyHandler is passed to the closure as the closure's delegate.
So: method receives as parameter a configuration closure and establish that closure's delegate object is DependencyHandler class, than execute the closure against it's delegate object.
Having this example:
dependencies {
// configurationName dependencyNotation
implementation 'commons-lang:commons-lang:2.6'
}
This is translated in a call of method add of DependencyHandler class:
Dependency add(String configurationName, Object dependencyNotation) Adds a dependency to the given configuration.
And now the question:
How exactly is done the translation of the line
implementation 'commons-lang:commons-lang:2.6'
into a class method call (e.g. DependencyHandler.add()) and who is responsable ?
My impression is that there is a missing explanation in the documentation, something like: default method on this delegate object DependencyHandler is add(...), so each configClosure's line, if matching notation configurationName dependencyNotation, will be translate into delegate's object default method.
But this is just a possible interpretation.
Ideea is: I give a closure with multiple lines, this is executed agains a delegation object which happens to have methods add(), and magically for each line this method is called ... how is this happening, based on what mechanism ? Is the Project.dependencies() method doing this, is the delegate object itself, or some other groovy specific mechanisms, etc.
Thank you.
If you want to get deeper insight on how Gradle (or its DSL) work under the hood, you can always check the actual source code. However, since this is not required to understand how to write build scripts, it is not included in the documentation.
Regarding your specific example, I have to admit that I do not exactly know how it is done, but I have a guess. If someone else has better insights, feel free to prove me wrong.
While Gradle indeed uses AST transformations to extend the regular Groovy syntax in some cases (e.g. the task definition syntax), I think they just rely on dynamic methods for dependency definitions.
Groovy is a dynamic language. This includes a method called methodMissing that may be defined by any class and will be called whenever a missing method is called on an object of that class:
class Example {
def methodMissing(String name, args) {
println name
}
}
def example = new Example()
example.method1()
You can find a more detailed example in Mr. Hakis blog.
Since Groovy allows omitting parentheses for method calls with arguments, your example implementation 'commons-lang:commons-lang:2.6' is basically nothing else but calling the method implementation with the dependency notation string as its argument.
Now Gradle could catch these calls via methodMissing and then call DependencyHandler.add() if the configuration actually exists. This allows you to dynamically add configurations in your build script:
configurations {
myConfig
}
dependencies {
myConfig 'commons-lang:commons-lang:2.6'
}
I have a multi-moduled Gradle project setup. I would like for a module to be able to accept an argument from a module that specifies it as a dependency. Can this be achieved?
Something like this in the parent module:
dependencies {
implementation project(':innerModule') {
arg "foo"
}
}
And the inner module would also have to declare the variable that gets assigned. def arg = ""? Really not sure on the correct syntax here.
EDIT:
What I am trying to do is create a build type variable, which I can then use for conditional logic in my project code. It is an Android project, but the one module it requires is non-Android. So I cannot use the buildTypes closure in that module, but I was hoping to feed a variable into the non-Android module once the parent Android module determines the build type. I feel like that should be possible
The answer to your specific question is no.
However, if you describe what you are trying to achieve, maybe Gradle will be able to help you.
What is this variable controlling?
What is the effect of the different values?
I am experimenting with some gradle at a new project, and in its settings.gradle, file I see these few lines that I am unable to make sense of as to what groovy structure or a language feature it is and what it does and how it works:
plugins {
id "com.gradle.build-scan" version "1.12.1"
id "cz.malohlava" version "1.0.3"
}
buildScan {
server = "some.host.com"
publishAlways()
}
I was suspecting it was either a a closure or an interface of some sort, but could not make head or tail of it.
Any help in understanding following will be a great help:
what it does?
How plugins and buildScan works here from the language's perspective?
From the language perspective, the closures are executed in the context of another objects than the build script. This is called delegation in Groovy.
http://groovy-lang.org/closures.html#_delegation_strategy
plugin delegates to https://docs.gradle.org/current/dsl/org.gradle.plugin.use.PluginDependenciesSpec.html
buildScan delegates to Build Scan Plugin's extension object which configures the plugin.
There may be some trickery here that I don't understand, particularly as I can't find either plugins() or buildScan() in the API docs. Nonetheless, the following is a reasonable reading of what the syntax means.
plugins {} and buildScan {} are both methods that take a closure (see other answers for explanation of this) as an argument.
Each closure has a delegate object of a particular type that's different depending on the method using the closure, i.e. the delegate of plugins() will be of a different type to the delegate of buildScan()
Within the closure, unqualified methods and properties will be executed against the delegate object. So for the plugins {} block, id(...).version(...) will be called against its delegate. For buildScan {}, you're setting the property server on the delegate and calling its publishAlways() method.
Honestly, I don't know how useful the above information is for using and understanding Gradle, but I think it's what you're asking for. Hope it helps!
I'm writing a Maven plugin and would like to convert a set of strings inputted as parameter excludes to a pattern before the plugin execution. I implemented the interface org.codehaus.plexus.personality.plexus.lifecycle.phase.Initializable but when I access in the initialize method the parameter excludes is null in the execute method is not null.
Which method is called to initialize a Maven plugin before its execution and has access to the parameters?
The best recommendation i can give is to take a deep look into the documentation which will give you the advice to define some kind of parameters and may be some default values for different parameters. The first method is usually the execute() method of the AbstractMojo class which you have to inherit from. If you need some kind of default values just check the parameters if they haven't been initialized (which means being null) and give the values you like for them.