Environment

• Asciidoctorj-pdf 1.5.0-alpha.14 which depends on jruby-complete-9.1.8.0
• Spring Boot 2.1.2.RELEASE + other dependencies introduces SnakeYAML 1.23 into the classpath

Expected Behavior

• I expect to be able to call Asciidoctor.convertFile (or do anything with Asciidoctor for that matter) without errors.
• More specifically, the transitive SnakeYAML 1.23 dependency should not interfere with jruby-complete-9.1.8.0 because it bundles a compatible SnakeYAML jar.
• A similar issue is described here When using JRuby 9.x SnakeYaml needs to be on the classpath. asciidoctor/asciidoctorj-pdf#16. The user needs to include SnakeYAML manually in order to proceed. This should not be necessary.

Actual Behavior

• jruby-complete-9.1.8.0 embeds SnakeYAML 1.18 but it appears like it is being ignored in favour of SnakeYAML 1.23 and causing the error and exception below.
• If I downgrade SnakeYAML 1.19, I am able to proceed. This however is not desirable because there are backwards-incompatible changes between 1.19 and 1.23, and I don't want to risk runtime errors in production.
• Upgrading to SnakeYAML 1.23-compatible version of Asciidoctorj-pdf didn't work right away. However I don't know if that is related to the issues I am experiencing.

Error and Exception

uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:1:in `require':
It seems your ruby installation is missing psych (for YAML output).
To eliminate this warning, please install libyaml and reinstall your ruby.

org.jruby.exceptions.RaiseException: (LoadError) load error: psych -- java.lang.ExceptionInInitializerError: null
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:961)
at RUBY.require(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:55)
at RUBY.

(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/yaml.rb:6)
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:961)
at RUBY.(root)(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:1)
at RUBY.(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:55)
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:961)
at RUBY.(root)(uri:classloader:/gems/prawn-icon-1.3.0/lib/prawn/icon/font_data.rb:9)
at RUBY.(uri:classloader:/jruby/kernel/kernel.rb:1)
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:961)
at RUBY.require_relative(uri:classloader:/jruby/kernel/kernel.rb:13)
at RUBY.(uri:classloader:/gems/prawn-icon-1.3.0/lib/prawn/icon.rb:10)
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:961)
at RUBY.(root)(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:1)
at RUBY.(uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:55)
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:961)
at RUBY.(root)(uri:classloader:/gems/asciidoctor-pdf-1.5.0.alpha.14/lib/asciidoctor-pdf/prawn_ext/extensions.rb:3)
at RUBY.(uri:classloader:/jruby/kernel/kernel.rb:1)
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:961)
at RUBY.require_relative(uri:classloader:/jruby/kernel/kernel.rb:13)
at RUBY.(uri:classloader:/gems/asciidoctor-pdf-1.5.0.alpha.14/lib/asciidoctor-pdf/prawn_ext.rb:5)
at org.jruby.RubyKernel.require(org/jruby/RubyKernel.java:961)
at RUBY.(root)(uri:classloader:/jruby/kernel/kernel.rb:1)
at RUBY.(uri:classloader:/jruby/kernel/kernel.rb:13)