Why do I need to specify 'from files'? - gradle

Question

I was trying to build a jar out of my first groovy script. My project structure is as follows:
- build.gradle
- src\main\groovy\app\Test.groovy
My original gradle script:
apply plugin: 'groovy'
apply plugin: 'java'
repositories {
mavenCentral()
}
dependencies {
compile 'org.codehaus.groovy:groovy-all:2.3.11'
testCompile group: 'junit', name: 'junit', version: '4.11'
}
sourceSets.main.groovy.srcDirs = ["src/main/groovy"]
jar {
manifest {
attributes('Main-Class': 'app.Test')
}
}
From the guides I read, this should create a runnable jar. When I try to run it though I always get the error
Error: Could not find or load main class app.Test
I found out now that I need to add these two lines to the jar task:
from files(sourceSets.main.output.classesDir)
from configurations.runtime.asFileTree.files.collect { zipTree(it) }
The weird thing is that if I replace the groovy script with a Test.java class (same content), I don't need those two extra lines to run the jar.
I couldn't find out why I need them or what exactly they do. Can anyone explain that, or offer a documentation link?
I'm new to SO, please help me with my mistakes.
EDIT
The code suggested by tim_yates is translated to test.jar with the following content:
META-INF/MANIFEST.MF
Manifest-Version: 1.0
Main-Class: app.Test
app/Test.class
package app;
import groovy.lang.GroovyObject;
import groovy.lang.MetaClass;
import org.codehaus.groovy.runtime.callsite.CallSite;
public class Test implements GroovyObject {
public Test() {
CallSite[] var1 = $getCallSiteArray();
MetaClass var2 = this.$getStaticMetaClass();
this.metaClass = var2;
}
public static void main(String... args) {
CallSite[] var1 = $getCallSiteArray();
var1[0].callStatic(Test.class, "Hi!");
}
}
I execute with the following statement:
java -jar test.jar
Which results in the error message stated above.

Answer 1

You've got to remember that this jar contains Groovy compiled classes. The answer is in your decompiled source that you showed in the beginning. It imports Groovy runtime classes.
If you just run java -jar test.jar those classes are not on the classpath.
Either include groovy on the classpath of your command line or use the gradle application plugin to build a fat JAR (which is probably better for runnable jars) that contain all your application dependencies.
apply plugin: 'groovy'
apply plugin: 'application'
mainClassName='app.Test'
repositories {
mavenCentral()
}
dependencies {
compile 'org.codehaus.groovy:groovy-all:2.4.7'
testCompile 'junit:junit:4.12'
}
task uberjar(type: Jar,dependsOn:[':compileJava',':compileGroovy']) {
from files(sourceSets.main.output.classesDir)
from configurations.runtime.asFileTree.files.collect { zipTree(it) }
manifest {
attributes 'Main-Class': mainClassName
}
}
Then build your jar with gradle uberjar

Answer 2

Assuming Test.groovy looks something like:
package app
class Test {
static main(args) {
println "Hi!"
}
}
Then you only need the following build script:
apply plugin: 'groovy'
repositories {
mavenCentral()
}
dependencies {
compile 'org.codehaus.groovy:groovy-all:2.4.7'
testCompile 'junit:junit:4.12'
}
jar {
manifest {
attributes('Main-Class': 'app.Test')
}
}