Custom Gradle Plugin Exec task with extension does not use input properly - gradle

Question

I am following the Writing Custom Plugins section of the Gradle documentation, specifically the part about Getting input from the build. The following example provided by the documentation works exactly as expected:
apply plugin: GreetingPlugin
greeting.message = 'Hi from Gradle'
class GreetingPlugin implements Plugin<Project> {
void apply(Project project) {
// Add the 'greeting' extension object
project.extensions.create("greeting", GreetingPluginExtension)
// Add a task that uses the configuration
project.task('hello') << {
println project.greeting.message
}
}
}
class GreetingPluginExtension {
def String message = 'Hello from GreetingPlugin'
}
Output:
> gradle -q hello
Hi from Gradle
I'd like to have the custom plugin execute an external command (using the Exec task), but when changing the task to a type (including types other than Exec such as Copy), the input to the build stops working properly:
// previous and following sections omitted for brevity
project.task('hello', type: Exec) {
println project.greeting.message
}
Output:
> gradle -q hello
Hello from GreetingPlugin
Does anyone know what the issue could be?

Answer 1

It is not related to the type of the task, it's a typical << misunderstanding.
When you write
project.task('hello') << {
println project.greeting.message
}
and execute gradle hello, the following happens:
configuration phase
apply custom plugin
create task hello
set greeting.message = 'Hi from Gradle'
executon phase
run task with empty body
execute << closure { println project.greeting.message }
in this scenario output is Hi from Gradle
When you write
project.task('hello', type: Exec) {
println project.greeting.message
}
and execute gradle hello, the following happens
configuration phase
apply custom plugin
create exec task hello
execute task init closure println project.greeting.message
set greeting.message = 'Hi from Gradle' (too late, it was printed in step 3)
the rest of workflow does not matter.
So, small details matter. Here's the explanation of the same topic.
Solution:
void apply(Project project) {
project.afterEvaluate {
project.task('hello', type: Exec) {
println project.greeting.message
}
}
}