How to make a task to call a main class - groovy

Question

What I am trying to do is make a task in build.gradle that will execute a main class (class with the main method), but I don't know how.
I made a test project to test how to do that. Here is the file structure layout:
testProject/
build.gradle
src/main/groovy/hello/world/HelloWorld.groovy
Here is the content of build.gradle:
apply plugin: 'groovy'
apply plugin: 'maven'
repositories {
mavenCentral()
}
dependencies {
compile 'org.codehaus.groovy:groovy-all:2.0.6'
}
task( hello, dependsOn: jar, type: JavaExec ) {
main = 'hello.world.HelloWorld'
}
Here is the content of HelloWorld.groovy:
package hello.world
class HelloWorld {
public static void main(String[] args) {
println "Hello World!"
}
}
Here is what I get from shell:
testProject>$ gradle hello
:compileJava UP-TO-DATE
:compileGroovy UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:jar UP-TO-DATE
:hello
Error: Could not find or load main class hello.world.HelloWorld
:hello FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':hello'.
> Process 'command '/Library/Java/JavaVirtualMachines/jdk1.7.0_25.jdk/Contents/Home/bin/java'' finished with non-zero exit value 1
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
BUILD FAILED
Total time: 4.232 secs
So, my question is: how can I make gradle hello work? Thank you very much.

Answer 1

After a bit of googling, I found a solution. The only thing I need to change is the task block. The working one is pasted below:
task( hello, dependsOn: jar, type: JavaExec ) {
main = 'hello.world.HelloWorld'
classpath = sourceSets.main.runtimeClasspath
}

Answer 2

There is an application plugin for doing this.
apply plugin: 'application'
mainClassName = 'hello.world.HelloWorld'
And then call
gradle run
Besides adding run task, applying application plugin will also change the behaviour of assemble task. Now it will produce a standalone application that can be run with a shell script.

Answer 3

Consider this build.gradle, which is a simplified version:
apply plugin: 'groovy'
task( hello, type: JavaExec ) {
main = 'hello.world.HelloWorld'
classpath = files('exampleDir/bin','jars/groovy-all-2.0.1.jar')
}
Note the 'classpath' argument to the JavaExec task. This uses subdirectories such as:
exampleDir/src/hello/world/HelloWorld.groovy
exampleDir/bin/hello/world/HelloWorld.class
jars/groovy-all-2.0.1.jar
where:
(a) groovy-all-2.0.1.jar copied from my GROOVY_HOME/embeddable
(b) HelloWorld.groovy is compiled via groovyc and is as follows:
package hello.world
class HelloWorld {
public static void main(String[] args) {
println "Hello World!"
}
}

Answer 4

Just specifying sourceSets.main.runtimeClasspath as a classpath might not be enough. If you see NoClassDefFoundError this might help:
task( hello, dependsOn: jar, type: JavaExec ) {
main = 'hello.world.HelloWorld'
classpath(sourceSets.main.runtimeClasspath, sourceSets.main.compileClasspath)
}