Jar made by gradle's jar task is not "fat" enough - kotlin

Question

...obviously some necessary things are not included from the dependencies.
Once it reaches a call to an external library, it breaks, either with ClassNotFoundException, or without a word.
I started with this skeleton project.
Relevant changes in build.gradle:
application {
mainClassName = 'net.laca.FoKt'
}
(my main function is in fo.kt)
dependencies {
//...
compile "com.sparkjava:spark-core:2.9.3"
implementation 'com.google.code.gson:gson:2.8.6'
implementation fileTree('libs') { include '*.jar' }
}
jar {
archiveBaseName = 'csira'
// Uncommend the last two lines to build a "fat" jar with `./gradlew jar`,
// and run it without Gradle's help: `java -jar build/libs/skeleton.jar`
manifest { attributes 'Main-Class': 'net.laca.FoKt' }
from { configurations.compile.collect { it.isDirectory() ? it : zipTree(it) } }
}
versions: Kotlin 1.4.20, Java 11, Gradle 6.7.1
Allegedly it should work this way. As it does if I start it with gradle run.
But when I start it with java -jar build/libs/csira.jar after gradle jar, it doesn't.
Relevant parts of fo.kt:
package net.laca
import spark.Spark.*
import com.google.gson.GsonBuilder
fun main(args: Array<String>) {
before("/*")
{ req, res ->
res.type("application/json")
println("hívás: ${req.requestMethod()} ${req.pathInfo()} " + req.queryString())
println(GsonBuilder().create().toJson(req.queryMap().toMap())) //line 14
//...
}
At GsonBuilder it breaks:
java.lang.NoClassDefFoundError: com/google/gson/GsonBuilder
at net.laca.FoKt$main$1.handle(fo.kt:14)
at spark.FilterImpl$1.handle(FilterImpl.java:73)
at spark.http.matching.BeforeFilters.execute(BeforeFilters.java:48)
at spark.http.matching.MatcherFilter.doFilter(MatcherFilter.java:133)
at ...
...
Caused by: java.lang.ClassNotFoundException: com.google.gson.GsonBuilder
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:602)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
... 19 more
And when I take/comment out the 14th line, and it reaches a call to my own jar in /libs:
get("/whatever")
{
println("before")
com.zz.app.MyScalaClass.apply().myFun()
println("after")
}
then the last thing I see is before, the rest is silence.

Answer 1

It happens because your jar task is configured incorrectly. To understand why, look at your dependencies:
dependencies {
//...
compile "com.sparkjava:spark-core:2.9.3"
implementation 'com.google.code.gson:gson:2.8.6'
implementation fileTree('libs') { include '*.jar' }
}
You are using both the compile and implementation configurations. The former is deprecated and should not be used by the way.
Then look at the jar task:
jar {
archiveBaseName = 'csira'
// Uncommend the last two lines to build a "fat" jar with `./gradlew jar`,
// and run it without Gradle's help: `java -jar build/libs/skeleton.jar`
manifest { attributes 'Main-Class': 'net.laca.FoKt' }
from { configurations.compile.collect { it.isDirectory() ? it : zipTree(it) } }
}
The from part instructs Gradle to collect all dependencies from the compile configuration only, and this will ignore the implementation configuration completely.
While you could change "compile" to "implementation" everywhere, the correct way to construct a fat jar is to actually collect from the runtimeClasspath configuration. This one extends other configurations like compile and implementation, but also runtimeOnly which you might find handy in the future.
There is actually also an example of how to do this in the Gradle user guide. To adapt it for your project, it should look like:
jar {
archiveBaseName = 'csira'
manifest { attributes 'Main-Class': 'net.laca.FoKt' }
dependsOn configurations.runtimeClasspath
from {
configurations.runtimeClasspath.findAll { it.name.endsWith('jar') }.collect { zipTree(it) }
}
}
The extra dependsOn line ensures that the runtimeClasspath configuration is completely resolved before trying to use it. Another difference is that it only collects jar files.