Optional dependencies of external library added via Grab not available

Question

It is not possible to use external third party libaries with optional dependencies properly in shared libraries for Jenkins.

I have a shared library which uses Commons Configurations 2 to read varios configuration files, mostly written as YAML documents.

Commons configurations uses SnakeYAML to read YAML documents and the dependency to SnakeYAML is defined as optional as follows:

 <dependency>
      <groupId>org.yaml</groupId>
      <artifactId>snakeyaml</artifactId>
      <version>1.26</version>
      <optional>true</optional>
 </dependency>      

According to the documentation of Maven how optional dependencies work an optional dependency is not added by default to the classpath. If a person wants to the part of the library which depends on the optional dependency, it must add this dependency to his own POM.

As I was going to use Commons Configuration 2 in conjunction with SnakeYAML, I defined the following variable in vars/readConfig.groovy as follows:

@Grapes([
  @Grab(group = "org.apache.commons", module = "commons-configuration2", version = "2.7"),
  @Grab(group = "org.yaml", module = "snakeyaml", version = "1.26")
])
import org.apache.commons.configuration2.BaseConfiguration
import org.apache.commons.configuration2.Configuration
import org.apache.commons.configuration2.YAMLConfiguration

def call() {
    Configuration config = new BaseConfiguration();
    YAMLConfiguration yamlConfiguration = new YAMLConfiguration();  
}

Calling readConfig() from a shared Library results in a java.lang.ClassNotFoundException with the following message

java.lang.ClassNotFoundException: org.yaml.snakeyaml.DumperOptions
    at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1387)
    at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1342)
    at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1089)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:352)
    at java.lang.Class.getDeclaredMethods0(Native Method)
    at java.lang.Class.privateGetDeclaredMethods(Class.java:2701)
    at java.lang.Class.privateGetPublicMethods(Class.java:2902)
    at java.lang.Class.getMethods(Class.java:1615)
    at java.beans.Introspector.getPublicDeclaredMethods(Introspector.java:1336)
    at java.beans.Introspector.getTargetMethodInfo(Introspector.java:1197)
    at java.beans.Introspector.getBeanInfo(Introspector.java:426)
    at java.beans.Introspector.getBeanInfo(Introspector.java:173)
    at groovy.lang.MetaClassImpl$15.run(MetaClassImpl.java:3313)
    at java.security.AccessController.doPrivileged(Native Method)
    at groovy.lang.MetaClassImpl.addProperties(MetaClassImpl.java:3311)
    at groovy.lang.MetaClassImpl.initialize(MetaClassImpl.java:3288)
    at org.codehaus.groovy.reflection.ClassInfo.getMetaClassUnderLock(ClassInfo.java:260)
    at org.codehaus.groovy.reflection.ClassInfo.getMetaClass(ClassInfo.java:302)
    ...

I also checked the directory ~/.groovy/grapes for the presence of all needed jars and they are there.

jenkins@cb765137c926:~/.groovy$ find . -name "*.jar"
./grapes/commons-logging/commons-logging/jars/commons-logging-1.2.jar
./grapes/org.apache.commons/commons-configuration2/jars/commons-configuration2-2.7.jar
./grapes/org.apache.commons/commons-lang3/jars/commons-lang3-3.9.jar
./grapes/org.apache.commons/commons-text/jars/commons-text-1.8.jar
./grapes/org.yaml/snakeyaml/jars/snakeyaml-1.26.jar

To do a cross-check I wrote the following Groovy script and was able to execute it on my computer successfully.

@Grapes([
  @Grab(group = 'org.apache.commons', module = 'commons-configuration2', version = '2.7'),
  @Grab(group = 'org.yaml', module = 'snakeyaml', version = '1.26'),
  @GrabConfig(systemClassLoader = true)
])

import org.apache.commons.configuration2.*

println("Start")

YAMLConfiguration y = new YAMLConfiguration()

println y

So, I am not able to guess the reason for this problem, as I am not so familiar with the internals of Jenkins. But it would be great to know if there is a way to get it working as intended.

Answer 1

Grab is built upon Ivy. What we do in our Ivy config to get optional dependencies is to add optional to the element conf of the annotation.

@Grab(group='org.apache.commons', module='commons-configuration2', 
      version='2.7', conf='default,optional')